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Rule2024-09157

Hazardous and Solid Waste Management System: Disposal of Coal Combustion Residuals From Electric Utilities; Legacy CCR Surface Impoundments

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Published

May 8, 2024

Effective

November 4, 2024

Issuing agencies

Environmental Protection Agency

Abstract

On April 17, 2015, the Environmental Protection Agency (EPA or the Agency) promulgated national minimum criteria for existing and new coal combustion residuals (CCR) landfills and existing and new CCR surface impoundments. On August 21, 2018, the United States Court of Appeals for the District of Columbia Circuit vacated the exemption for inactive surface impoundments at inactive facilities (legacy CCR surface impoundments) and remanded the issue back to EPA to take further action consistent with its opinion in Utility Solid Waste Activities Group, et al. v. EPA. This action responds to that order and establishes regulatory requirements for legacy CCR surface impoundments. EPA is also establishing requirements for CCR management units at active CCR facilities and at inactive CCR facilities with a legacy CCR surface impoundment. Finally, EPA is making several technical corrections to the existing regulations, such as correcting certain citations and harmonizing definitions.

Full Text

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[Federal Register Volume 89, Number 90 (Wednesday, May 8, 2024)]
[Rules and Regulations]
[Pages 38950-39122]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2024-09157]

[[Page 38949]]

Vol. 89

Wednesday,

No. 90

May 8, 2024

Part II

Environmental Protection Agency

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40 CFR Parts 9 and 257

Hazardous and Solid Waste Management System: Disposal of Coal
Combustion Residuals From Electric Utilities; Legacy CCR Surface
Impoundments; Final Rule

Federal Register / Vol. 89, No. 90 / Wednesday, May 8, 2024 / Rules
and Regulations

[[Page 38950]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 9 and 257

[EPA-HQ-OLEM-2020-0107; FRL-7814-04-OLEM]
RIN 2050-AH14

Hazardous and Solid Waste Management System: Disposal of Coal
Combustion Residuals From Electric Utilities; Legacy CCR Surface
Impoundments

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: On April 17, 2015, the Environmental Protection Agency (EPA or
the Agency) promulgated national minimum criteria for existing and new
coal combustion residuals (CCR) landfills and existing and new CCR
surface impoundments. On August 21, 2018, the United States Court of
Appeals for the District of Columbia Circuit vacated the exemption for
inactive surface impoundments at inactive facilities (legacy CCR
surface impoundments) and remanded the issue back to EPA to take
further action consistent with its opinion in Utility Solid Waste
Activities Group, et al. v. EPA. This action responds to that order and
establishes regulatory requirements for legacy CCR surface
impoundments. EPA is also establishing requirements for CCR management
units at active CCR facilities and at inactive CCR facilities with a
legacy CCR surface impoundment. Finally, EPA is making several
technical corrections to the existing regulations, such as correcting
certain citations and harmonizing definitions.

DATES: This final rule is effective on November 4, 2024.

ADDRESSES: EPA has established a docket for this action under Docket ID
No. EPA-HQ-OLEM-2020-0107. All documents in the docket are listed on
the <a href="http://www.regulations.gov">http://www.regulations.gov</a> website. Although listed in the index,
some information is not publicly available, e.g., CBI or other
information whose disclosure is restricted by statute. Certain other
material, such as copyrighted material, is not placed on the internet
and will be publicly available only in hard copy form. Publicly
available docket materials are available electronically through <a href="http://www.regulations.gov">http://www.regulations.gov</a>.

FOR FURTHER INFORMATION CONTACT: For questions concerning this
proposal, contact Michelle Lloyd, Office of Resource Conservation and
Recovery, Materials Recovery and Waste Management Division,
Environmental Protection Agency, 1200 Pennsylvania Avenue NW, MC:
5304T, Washington, DC 20460; telephone number: (202) 566-0560; email
address: <a href="/cdn-cgi/l/email-protection#266a4a495f42086b4f454e434a4a436643564708414950">[email&#160;protected]</a>, or Taylor Holt, Office of Resource
Conservation and Recovery, Materials Recovery and Waste Management
Division, Environmental Protection Agency, 1200 Pennsylvania Avenue NW,
MC: 5304T, Washington, DC 20460; telephone number: (202) 566-1439;
email address: <a href="/cdn-cgi/l/email-protection#8ec6e1e2faa0daeff7e2e1fcceebfeefa0e9e1f8">[email&#160;protected]</a>. For more information on this
rulemaking, please visit <a href="https://www.epa.gov/coalash">https://www.epa.gov/coalash</a>.

SUPPLEMENTARY INFORMATION:

Table of Contents

I. General Information
A. Does this action apply to me?
B. What action is the Agency taking?
C. What is the Agency's authority for taking this action?
D. What are the incremental costs and benefits of this action?
II. Background
A. 2015 CCR Rule
B. 2018 USWAG Decision
C. 2020 Part B Proposed Rule
D. 2020 Advance Notice of Proposed Rulemaking
E. 2023 Proposed Rule and Comments
F. 2023 Notice of Data Availability
III. What is EPA finalizing?
A. Risks From Legacy CCR Surface Impoundments and CCR Management
Units
1. Summary of May 2023 Proposal
2. 2023 Draft Risk Assessment
3. Response to Comments on the Proposal and the NODA
4. 2024 Final Risk Assessment
B. Legacy CCR Surface Impoundment Requirements
1. Definition of a ``Legacy CCR Surface Impoundment''
2. Applicable Requirements for Legacy CCR Surface Impoundments
and Compliance Deadlines
C. CCR Management Unit Requirements
1. Damage Cases
2. Applicability and Definitions Related to CCR Management Units
3. Facility Evaluation for Identifying CCR Management Units
4. Applicable Requirements for CCR Management Units and
Compliance Deadlines
D. Closure of CCR Units by Removal of CCR
1. Background
2. March 2020 Proposed Rule
3. What is EPA Finalizing Related to the March 2020 Proposed
Rule
E. Technical Corrections
IV. Effect on State CCR Permit Programs
V. The Projected Economic Impact of This Action
A. Introduction
B. Affected Universe
C. Baseline Costs
D. Costs and Benefits of the Final Rule
VI. Statutory and Executive Order Reviews
Regulatory Text

List of Acronyms

ACM Assessment of Corrective Measures
ANPRM Advance Notice of Proposed Rulemaking
ARAR applicable or relevant and appropriate requirements
ASD alternative source demonstration
CAA Clean Air Act
CBI Confidential Business Information
CBR closure by removal
CCR coal combustion residuals
CCRMU coal combustion residuals management unit
CERCLA Comprehensive Environmental Response, Compensation, and
Liability Act
CIP closure in place
CFR Code of Federal Regulations
COALQUAL U.S. Geological Survey coal quality database
CWA Clean Water Act
DOE Department of Energy
EAP Emergency Action Plan
EIA Energy Information Administration
EIP Environmental Integrity Project
EJ environmental justice
ELG Effluent Limitation Guidelines
EPA Environmental Protection Agency
EPACMTP EPA Composite Model for Leachate Migration with
Transformation Products
EPRI Electric Power Research Institute
FER Facility Evaluation Report
FERC Federal Energy Regulatory Commission
FGD flue gas desulfurization
FR Federal Register
GWMCA groundwater monitoring and corrective action
GWPS groundwater protection standard
HQ hazard quotient
HSWA Hazardous and Solid Waste Amendments
ICR Information Collection Request
IRIS Integrated Risk Information System
LEAF Leaching Environmental Assessment Framework
MCL maximum contaminant level
MDE Maryland Department of the Environment
MNA monitored natural attenuation
MODFLOW-USG Modular Three-Dimension Finite-Difference Ground-Water
Flow Model
MSW Municipal Solid Waste
MW Megawatts
NAICS North American Industry Classification System
NERC North American Electric Reliability Corporation
NODA notice of data availability
NPDES National Pollution Discharge Elimination System
NPL National Priorities List
NTTAA National Technology Transfer and Advancement Act
OAFU Other Active Facilities
OLEM Office of Land and Emergency Management
OMB Office of Management and Budget
OSHA Occupational Safety and Health Administration

[[Page 38951]]

P.E. Professional Engineer
PM particulate matter
PRA Paperwork Reduction Act
PRG preliminary remediation goal
PUC Public Utility Commission
QA/QC quality assurance/quality control
RCRA Resource Conservation and Recovery Act
RIA Regulatory Impact Analysis
RME reasonable maximum exposure
RTO Regional Transmission Organizations
SMCL secondary maximum contaminant level
SSI statistically significant increase
SSL statistically significant level
TDS total dissolved solids
TSCA Toxic Substances Control Act
TSDF Transportation Storage and Disposal Facility
TVA Tennessee Valley Authority
UMRA Unfunded Mandates Reform Act
USGS U.S. Geological Survey
USWAG Utility Solid Waste Activities Group
WIIN Water Infrastructure Improvements for the Nation
WQC water quality criteria

I. General Information

A. Does this action apply to me?

This rule applies to and may affect all CCR generated by electric
utilities and independent power producers that fall within the North
American Industry Classification System (NAICS) code 221112. The
reference to NAICS code 221112 is not intended to be exhaustive, but
rather provides a guide for readers regarding entities likely to be
regulated by this action. This discussion lists the types of entities
that EPA is now aware could potentially be regulated by this action.
Other types of entities not described here could also be regulated. To
determine whether your entity is regulated by this action, you should
carefully examine the applicability criteria found in 40 CFR 257.50 of
title 40 of the Code of Federal Regulations. If you have questions
regarding the applicability of this action to a particular entity,
consult the person listed in the FOR FURTHER INFORMATION CONTACT
section.

B. What action is the Agency taking?

EPA is amending the regulations governing the disposal of CCR in
landfills and surface impoundments, codified in subpart D of part 257
of Title 40 of the Code of Federal Regulations (CFR) (CCR regulations).
Specifically, the Agency is establishing regulatory requirements for
inactive CCR surface impoundments at inactive utilities (``legacy CCR
surface impoundment'' or ``legacy impoundment''). This action is being
taken in response to the August 21, 2018, opinion by the U.S. Court of
Appeals for the District of Columbia Circuit in Utility Solid Waste
Activities Group v. EPA, 901 F.3d 414 (D.C. 2018) (``USWAG decision''
or ``USWAG'') that vacated and remanded the provision exempting legacy
impoundments from the CCR regulations. This action includes adding a
definition for legacy CCR surface impoundments and other terms relevant
to this rulemaking. It also requires that legacy CCR surface
impoundments comply with certain existing CCR regulations with tailored
compliance deadlines.
While this action is responsive to the D.C. Circuit's order, it is
also driven by the record, which clearly demonstrates that regulating
legacy CCR surface impoundments will have significant quantified and
unquantified public health and environmental benefits. As EPA concluded
in 2015, the risks posed by unlined CCR surface impoundments are
substantial, and the risks from legacy impoundments are at least as
significant. EPA's 2014 Risk Assessment concluded that the cancer risks
from unlined surface impoundments ranged from 3 x 10-\4\ for
trivalent arsenic to 4 x 10-\5\ for pentavalent arsenic.
Non-cancer risks from these same units also significantly exceeded
EPA's level of concern, with estimated Hazard Quotients (HQ) of two for
thallium, three for lithium, four for molybdenum and eight for
trivalent arsenic. In addition, as described in Unit III.A.1 of this
preamble, information obtained since 2015 indicates that the risks for
legacy CCR surface impoundments are likely to be greater than EPA
originally estimated. Finally, based on the demographic composition and
environmental conditions of communities within one and three miles of
legacy CCR surface impoundments, this final rule will reduce existing
disproportionate and adverse effects on economically vulnerable
communities, as well as those that currently face environmental
burdens. For example, in Illinois the population living within one mile
of legacy CCR surface impoundment sites is over three times as likely
compared to the State average to have less than a high school education
(35.66% compared to 10.10%, see Regulatory Impact Analysis (RIA)
exhibit ES.14), and that population already experiences higher than
average exposures to particulate matter, ozone, diesel emissions,
lifetime air toxics cancer risks, and proximity to traffic, Superfund
sites, Risk Management Plan sites, and hazardous waste facilities (see
RIA exhibit ES.15). Consistent with the directive in section 4004(a) to
ensure that the statutory standard is met at all regulated sites,
including the most vulnerable, this final rule will help EPA further
ensure that the communities and ecosystems closest to coal facilities
are sufficiently protected from harm from groundwater contamination,
surface water contamination, fugitive dust, floods and impoundment
overflows, and threats to wildlife.
EPA is also establishing requirements to address the risks from
currently exempt solid waste management that involves the direct
placement of CCR on the land. EPA is extending a subset of the existing
requirements in 40 CFR part 257, subpart D to CCR surface impoundments
and landfills that closed prior to the effective date of the 2015 CCR
Rule, inactive CCR landfills, and other areas where CCR is managed
directly on the land. In this action, EPA refers to these as CCR
management units, or CCRMU. The final rule expands the CCRMU
requirements to a set of active facilities that were not regulated by
the 2015 CCR rule because they had ceased disposing of CCR in their on-
site disposal units, and they did not have an inactive surface
impoundment. Accordingly, this rule applies to all CCRMU at active CCR
facilities and inactive facilities with a legacy CCR surface
impoundment.
EPA is also finalizing alternative closure provisions to allow a
facility to complete the closure by removal in two stages: first, by
completing all removal and decontamination procedures; and second, by
completing all groundwater remediation in a separate post closure care
period.
Finally, EPA is making a number of technical corrections to the
existing regulations, such as correcting certain citations and
harmonizing definitions.
EPA intends the provisions of the rule to be severable. In the
event that any individual provision or part of the rule is invalidated,
EPA intends that this would not render the entire rule invalid, and
that any individual provisions that can continue to operate will be
left in place. For example, EPA intends that the provisions governing
each class of facilities--legacy CCR inactive surface impoundments, CCR
management units, other active facility units, and regulated CCR
landfills containing waste in contact with groundwater--to be
independently severable from one another as each set of requirements
operates independently from the other.
Likewise, the provisions regulating existing units at active
facilities, including those units at non-fossil-fuel-fired facilities
generating energy, are severable from the other substantive
requirements--each provision may continue operating even if one of the
others is invalidated. EPA also intends

[[Page 38952]]

that, within each set of provisions for legacy CCR surface impoundments
and for CCR management units, the substantive requirements be severable
from each other. For example, if any of the closure requirements were
to be set aside (e.g., the requirement that CCRMU initiate closure
within 48 months of publication), the groundwater monitoring and
corrective action requirements can continue to fully and effectively
operate. These requirements function independently from each other,
address environmental concerns through different means, and are not
dependent on the others; they are therefore severable from each other.
Lastly, as set forth below, EPA has deferred the dates by when some
units in some circumstances must comply with the substantive standards
governing legacy CCR surface impoundments and CCR management units. If
any of the deferrals were to be set aside, EPA intends that the
substantive standards would remain in place because the rationale for
and effectiveness of each set of substantive standards is not dependent
on any of the deferrals.
For the reader's convenience, EPA has provided a background
description of existing requirements in several places throughout this
preamble.

C. What is the Agency's authority for taking this action?

EPA is publishing this notice under the authority of sections
1008(a), 2002(a), 3007, 4004, and 4005(a) and (d) of the Solid Waste
Disposal Act of 1970, as amended by the Resource Conservation and
Recovery Act of 1976 (RCRA), as amended by the Hazardous and Solid
Waste Amendments of 1984 (HSWA) and the Water Infrastructure
Improvements for the Nation (WIIN) Act of 2016, 42 U.S.C. 6907(a),
6912(a), 6927, 6944, 6945(a) and (d).
RCRA section 1008(a) authorizes EPA to publish ``suggested
guidelines for solid waste management.'' 42 U.S.C. 6907(a). RCRA
defines solid waste management as ``the systematic administration of
activities which provide for the collection, source separation,
storage, transportation, transfer, processing, treatment, and disposal
of solid waste.'' 42 U.S.C. 6903(28).
Pursuant to section 1008(a)(3), the guidelines are to include the
minimum criteria to be used by the States to define the solid waste
management practices that constitute the open dumping of solid waste or
hazardous waste and are prohibited as ``open dumping'' under section
4005. Only those requirements promulgated under the authority of
section 1008(a)(3) are enforceable under section 7002 of RCRA.
RCRA section 4004(a) generally requires EPA to promulgate
regulations containing criteria distinguishing ``sanitary landfills,''
which may continue to operate, from ``open dumps,'' which are
prohibited. 42 U.S.C. 6944(a); see id. 6903(14), (26); 6945(a). The
statute directs that, ``at a minimum, the criteria are to ensure that
units are classified as sanitary landfills only if there is no
reasonable probability of adverse effects on health or the environment
from disposal of solid wastes at such facility.'' 42 U.S.C. 6944(a).
RCRA section 4005(a), entitled ``Closing or upgrading of existing
open dumps,'' prohibits any solid waste management practices or
disposal of solid waste that does not comply with EPA regulations
issued under RCRA section 1008(a) and 4004(a). 42 U.S.C. 6945(a). See
also 42 U.S.C. 6903(14) (definition of ``open dump''). This prohibition
takes effect ``upon promulgation'' of any rules issued under section
1008(a)(3) and is enforceable either through a citizen suit brought
pursuant to section 7002, or through an EPA enforcement action brought
pursuant to section 4005(d)(4)(A). See 42 U.S.C. 6945(a), (d)(4)(A)
(authorizing EPA to use the authority under RCRA section 3008(a) to
enforce the open dumping prohibition for CCR). RCRA section 4005(a)
also directs that open dumps (i.e., facilities out of compliance with
EPA's criteria), must be closed or upgraded. See 42 U.S.C. 6945(a).
RCRA section 4005(d)(3) specifies that the regulations in 40 CFR
part 257, subpart D ``(or successor regulations promulgated pursuant to
sections 6907(a)(3) and 6944(a) of this title), shall apply to each CCR
unit'' unless a permit issued by an approved State or by EPA is in
effect. Similarly, section 4005(d)(6) \1\ provides that:
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\1\ 42 U.S.C. 6945(d)(6).

a CCR unit shall be considered to be a sanitary landfill for
purposes of this chapter, including subsection (a), only if the coal
combustion residuals unit is operating in accordance with [a permit
issued by EPA or an approved State] or the applicable criteria for
coal combustion residuals units under part 257 of title 40, Code of
Federal Regulations (or successor regulations promulgated pursuant
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to sections 6907(a)(3) and 6944(a) of this title).

1. Regulation of Solid Wastes Under RCRA Subtitle D
Solid wastes that are neither a listed nor characteristic hazardous
waste are subject to the requirements of RCRA subtitle D. Subtitle D of
RCRA establishes a framework for Federal, State, and local government
cooperation in controlling the management of nonhazardous solid waste.
The Federal role is to establish the overall regulatory direction by
providing minimum nationwide standards that will protect human health
and the environment. States may, but are not required to, adopt these
requirements into their State programs.
Under RCRA section 4005(a), upon promulgation of criteria under
section 1008(a)(3), any solid waste management practice or disposal of
solid waste that constitutes the ``open dumping'' of solid waste is
prohibited. The Federal standards apply directly to the facility (are
self-implementing) and facilities are directly responsible for ensuring
that their operations comply with these requirements.
RCRA section 4005(d) establishes an additional regulatory
structure, applicable exclusively to the solid waste management of CCR,
that builds on the provisions in sections 1008(a)(3), 4004, and
4005(a), without restricting the scope of EPA's authority under those
sections. See, 42 U.S.C. 6945 (d)(7). Under 4005(d), States may seek
EPA approval of a State permitting program under which individualized
facility permits would ``operate in lieu of [EPA] regulation of coal
combustion residuals units in the State.'' 42 U.S.C. 6945(d)(1)(A). EPA
is also directed to ``implement a permit program,'' which would operate
in absence of an approved State program. 42 U.S.C. 6945(d)(2). However,
the statute makes clear that facilities must continue to comply with
the Federal regulations until a permit issued by either EPA or an
approved State is in effect. 42 U.S.C. 6945(d)(3), (6).
RCRA sections 1008(a)(3) and 4004(a) delegate broad authority to
EPA to establish regulations governing the management of solid waste.
Under section 4004(a) EPA is charged with establishing requirements to
ensure that facilities will be classified as sanitary landfills and not
an open dump ``only if there is no reasonable probability of adverse
effects on health or the environment from the disposal of solid waste''
at the facility. Or in other words, under section 4004(a) EPA is
charged with issuing regulations to address all ``reasonable
probabilities of adverse effects'' (i.e., all reasonably anticipated
risks) to health and the environment from the disposal of solid waste.
Section 1008(a)(3) expands EPA's authority to address the risks from
any of the activities identified as ``solid waste management'' in RCRA
section

[[Page 38953]]

1004(28). Specifically, EPA is authorized to establish requirements
applicable to ``storage, transportation, transfer, processing,
treatment, and disposal of solid waste.'' (42 U.S.C. 6907(a),
6903(28)). Under RCRA, EPA sets these requirements without taking cost
into account as a factor. See USWAG et al. v. EPA, 901 F.3d at 448-49
(citing RCRA section 4004(a)).
The statute is clear that EPA is authorized to issue regulations to
address the current risks from previous solid waste management
activities. EPA explained at length the basis for this conclusion as
part of the Agency's rationale for regulating inactive impoundments.
See, 80 FR 21344--21347. See also USWAG, 901 F.3d at 440. Among other
provisions, the statutory definition of an ``open dump'' conclusively
resolves the question. RCRA defines an ``open dump'' as ``any facility
or site where solid waste is disposed of . . . .'' 42 U.S.C. 6903(14).
As the D.C. Circuit explained,

Importantly, while the ``is'' retains its active present tense,
the ``disposal'' takes the form of a past participle (``disposed'').
In this way, the disposal itself can exist (it ``is''), even if the
act of disposal took place at some prior time . . . . Properly
translated then, an open dump includes any facility (other than a
sanitary landfill or hazardous waste disposal facility), where solid
waste still ``is deposited,'' ``is dumped,'' ``is spilled,'' ``is
leaked,'' or ``is placed,'' regardless of when it might have
originally been dropped off. See 42 U.S.C. 6903(3), (14). In other
words, the waste in inactive impoundments ``is disposed of'' at a
site no longer receiving new waste in just the same way that it ``is
disposed of'' at a site that is still operating.

901 F.3d at 440. See also In re Consolidated Consol. Land Disposal
Regulation Litig., 938 F.2d 1386, 1389 (D.C. Cir. 1991) (EPA's reading
of the term ``disposal'' in RCRA's subtitle C, 42 U.S.C. 6924, to
include ``the continuing presence of waste'' was reasonable); USWAG,
901 F.3d at 453-54 (Henderson, J., concurring) (same). By the same
logic, these provisions would authorize EPA to regulate closed units
that continue to pose risks to health or the environment, for example
by requiring the owners or operators of such units to remediate any
contamination from these units, or to take action to prevent such
contamination.
The 2016 amendments further confirm EPA's authority over these
activities. In section 4005, Congress referenced the 2015 regulations
in the statute, and expressly stated that the amendments in 4005(d)
were not intended to limit or restrict the authority already provided
under sections 1008(a)(3) and 4004(a). See, 42 U.S.C. 6945(d)(3), (6),
(7). By incorporating the rule into the statute without modification,
Congress has affirmed the Agency's authority to impose the kind of
requirements established in part 257 (e.g., corrective action to
remediate groundwater contamination). Moreover, Congress made clear
that EPA retains the authority to modify or expand these requirements
as necessary to ensure that the standard in section 4004(a) will
continue to be met. See, e.g., 42 U.S.C. 6945(d)(1)(A)(i), (3), (6)
(referencing ``or successor regulations promulgated pursuant to
sections 6907(a)(3) and 6944(a) of this title'').
EPA interprets the standard in section 4004(a) to apply equally to
criteria issued under sections 1008(a)(3) and 4004(a); namely that the
criteria must ensure that a facility is to be classified as a sanitary
landfill, and thus allowed to continue to operate, ``only if there is
no reasonable probability of adverse effects on health or the
environment'' from either the disposal or other solid waste management
practices at the facility. Thus, under the combined authority conferred
by sections 1008(a)(3) and 4004(a), a facility is an ``open dump'' if
it engages in any activity involving the management of solid waste that
does not meet the standard in section 4004(a); or in other words, any
activity involved with the management of solid waste that presents a
reasonable probability of causing adverse effects on health or the
environment. EPA also interprets these provisions to authorize the
establishment of criteria that define the manner in which facilities
upgrade or close, consistent with the standard in section 4004(a), to
ensure there will be no reasonable probability of adverse effects on
health or the environment.

D. What are the incremental costs and benefits of this action?

As noted previously, EPA establishes the requirements under RCRA
sections 1008(a)(3) and 4004(a) without taking cost into account. See,
USWAG, 901 F.3d at 448-49. The following cost estimates are presented
in the RIA and summarized in this preamble for compliance with OMB
Circular A-4, E.O. 12866, and E.O. 14094. The requirements in this rule
do not rely on these cost estimates.\2\
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\2\ Although EPA did not consider costs in developing this rule,
if the Agency had considered costs, the final rule would not have
been different. As discussed in greater detail later in this
preamble and in the RIA, the monetized benefits are based on only a
subset of adverse health effects from a single constituent. EPA
monetized the benefit from two additional human health endpoints
associated with that single constituent in a sensitivity analysis
and estimated an additional $19 million per year when discounting at
2% from that single contaminant. The RIA also describes a number of
important benefits that cannot currently be quantified or monetized
due to data limitations or limitations in current methodologies.
Based on these estimates EPA believes that after considering all
unquantified and distributional effects, the public health and
welfare gains that will result from the proposed alternative would
justify the rule's costs.
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The RIA estimates that the annualized monetized costs of this
action will be approximately $214-$240 million per year when
discounting at 2%. Of this, $123-$135 million is attributable to the
requirements for legacy CCR surface impoundments, which are subject to
the D.C. Circuit's order in USWAG, $79-$92 million is attributable to
the requirements for CCRMU, an additional $8-$9 million is attributable
to the requirements for CCRMU at Other Active Facilities (OAFUs) (a
term used in the RIA) that are discussed in Unit III.C.2.e of the
preamble, and $4 million is attributable to requirements for landfills.
The costs of this final rule are discussed further in the RIA and
include the costs of unit closure, corrective action, fugitive dust
controls, structural integrity inspections, and recordkeeping and
reporting.
The RIA estimates that the annualized monetized benefits
attributable to this action will be approximately $53-$80 million per
year when discounting at 2%. Of this, $43-$57 million is attributable
to the requirements for legacy CCR surface impoundments, $9-$21 million
is attributable to the requirements for CCRMU, $1-$2 million is
attributable to the requirements for CCRMU at ``other active
facilities,'' or OAFUs. Requirements for landfills account for a de
minimis amount of benefits.
In addition to monetized benefits, the RIA describes ten categories
of non-monetized benefits. These include human health effects from lead
exposure such as ADHD, cardiovascular mortality, and increased cancer
risk. They also include ecosystem benefits from avoided exposure to the
heavy metals in CCR effluent. The RIA describes several property-
related benefits including increased property values near closed and
remediated CCR units, and option values for remediated land. The RIA
also contextualizes the final rule within EPA's broader efforts to
regulate air and surface water pollution from coal fired power plants.
Further information on the economic effects of this action can be
found in Unit V of this preamble.

[[Page 38954]]

II. Background

A. 2015 CCR Rule

On April 17, 2015, EPA finalized national minimum criteria for the
disposal of CCR as solid waste under Subtitle D of RCRA titled,
``Hazardous and Solid Waste Management System; Disposal of Coal
Combustion Residuals from Electric Utilities'' (80 FR 21302) (2015 CCR
Rule). The 2015 CCR Rule, codified in 40 CFR part 257, subpart D,
established regulations for existing and new CCR landfills, as well as
existing and new CCR surface impoundments (including all lateral
expansions of CCR units). The criteria consist of location
restrictions, design and operating criteria, groundwater monitoring and
corrective action requirements, closure and post-closure care
requirements, recordkeeping, notification, and internet posting
requirements.
The 2015 CCR Rule also imposed requirements on inactive surface
impoundments at active facilities. A CCR surface impoundment is a
natural topographic depression, man-made excavation, or diked area,
which is designed to hold an accumulation of CCR and liquids, and the
unit treats, stores, or disposes of CCR. The 2015 CCR Rule defined an
``inactive CCR surface impoundment'' as ``a CCR surface impoundment
that no longer receives CCR on or after October 19, 2015, and still
contains both CCR and liquids on or after October 19, 2015.'' 40 CFR
257.53. The rule defined ``active facility or active electric utilities
or independent power producers'' as ``any facility subject to the
requirements of this subpart that is in operation on October 19, 2015.
An electric utility or independent power producer is in operation if it
is generating electricity that is provided to electric power
transmission systems or to electric power distribution systems on or
after October 19, 2015. An off-site disposal facility is in operation
if it is accepting or managing CCR on or after October 19, 2015.'' 40
CFR 257.53.
The 2015 CCR Rule did not impose any requirements on inactive
facilities. EPA explained that this was consistent with past decisions
under RCRA subtitle C. See, 80 FR 21344 (April 17, 2015). EPA further
raised concerns that it would be difficult to identify the owners or
other parties responsible for such facilities, as well as concerns that
the present owner of the land on which an inactive facility was located
might have no connection (other than present ownership of the land)
with the prior disposal activities. Id. Consequently, EPA exempted
those units at Sec. 257.50(e).

B. 2018 USWAG Decision

The 2015 CCR Rule was challenged by several parties, including
coalitions of regulated entities and environmental organizations
(``Environmental Petitioners''). See USWAG et al. v. EPA, 901 F.3d 414
(D.C. Cir. 2018). Environmental Petitioners raised two challenges that
are relevant to this final rule. First, they challenged the provision
at Sec. 257.101(a)(1) that allowed existing, unlined surface
impoundments to continue to operate until they exceeded the groundwater
protection standard. They contended that EPA failed to show how
continued operation of unlined impoundments met RCRA's baseline
requirement that any solid waste disposal site pose ``no reasonable
probability of adverse effects on health or the environment.'' 42
U.S.C. 6944(a). Second, Environmental Petitioners challenged the
exemption at Sec. 257.50(e) for inactive surface impoundments at
inactive power plants (i.e., ``legacy ponds''). Environmental
Petitioners argued that legacy ponds are at risk of unmonitored leaks
and catastrophic structural failures.
On August 21, 2018, the U.S. Court of Appeals for the D.C. Circuit
upheld most of the 2015 CCR Rule but decided in favor of Environmental
Petitioners on these two claims. The Court held that EPA acted
``arbitrarily and capriciously and contrary to RCRA'' in failing to
require the closure of unlined surface impoundments \3\ and in
exempting inactive surface impoundments at inactive power plants from
regulation. The Court vacated these provisions and remanded the matter
back to the Agency for further action consistent with its opinion.
---------------------------------------------------------------------------

\3\ After the Court's ruling, the closure of unlined CCR surface
impoundments was addressed in a separate regulatory action that was
published on August 28, 2020 (85 FR 53516).
---------------------------------------------------------------------------

In overturning the exemption for legacy ponds, the Court evaluated
the evidence in the rulemaking record and reached specific conclusions
about the risks that legacy ponds pose based on the record for the 2015
CCR Rule. The Court pointed to evidence that legacy ponds are most
likely to be unlined and unmonitored and that such units have been
shown to be more likely to leak than units at utilities still in
operation. 901 F.3d at 432. The Court also determined that legacy
ponds:

. . . pose the same substantial threats to human health and the
environment as the riskiest Coal Residuals disposal methods,
compounded by diminished preventative and remediation oversight due
to the absence of an onsite owner and daily monitoring. See 80 FR at
21343 through 21344 (finding that the greatest disposal risks are
``primarily driven by the older existing units, which are generally
unlined''). Notably, this very Rule was prompted by a catastrophic
legacy pond failure that resulted in a ``massive'' spill of 39,000
tons of coal ash and 27 million gallons of wastewater into North
Carolina's Dan River. . . .
[T]here is no gainsaying the dangers that unregulated legacy
ponds present. The EPA itself acknowledges the vital importance of
regulating inactive impoundments at active facilities. That is
because, if not properly closed, those impoundments will
``significant[ly]'' threaten ``human health and the environment
through catastrophic failure'' for many years to come. 75 FR at
35,177; see also 80 FR at 21,344 n. 40.
The risks posed by legacy ponds are at least as substantial as
inactive impoundments at active facilities. See 80 FR at 21,343-21,
344 (finding ``no [ ] measurabl[e] differen[ce]'' in risk of
catastrophic events between active and inactive impoundments). And
the threat is very real. Legacy ponds caused multiple human and
environmental disasters in the years leading up to the Rule's
promulgation. See 75 FR at 35,147 (proposed rule discusses multiple
serious incidents). For example, a pipe break at a legacy pond at
the Widows Creek plant in Alabama caused 6.1 million gallons of
toxic slurry to deluge local waterways. Id. Another legacy pond in
Gambrills, Maryland caused the heavy metal contamination of local
drinking water. Id. And the preamble to the Rule itself specifically
points to the catastrophic spill at the Dan River legacy pond in
North Carolina. 80 FR at 21,393-21,394.

Id. at 432-433. Relying on this evidence, the Court concluded there was
no logical basis for distinguishing between the inactive impoundments
at active facilities that were regulated and the legacy impoundments
that were exempt. Id. at 434. Consequently, the Court vacated the
provision of the 2015 CCR Rule (Sec. 257.50(e)) that specifically
exempted inactive impoundments at inactive facilities from regulation
and remanded the matter back to EPA for further action consistent with
its opinion. Notwithstanding the vacatur of Sec. 257.50(e), until EPA
amended the regulations to effectuate the Court's order, facilities
were not legally obliged to take any action to comply with the Federal
CCR regulations. This is because, as originally drafted, legacy CCR
surface impoundments did not fall within the scope of the rule, as
defined in Sec. 257.50. The specific provision in Sec. 257.50(e)
exempting legacy impoundments merely identified the units that were not
covered by Sec. 257.50(b). Because the vacatur of Sec. 257.50(e) did
not amend Sec. 257.50(b), legacy impoundments remained exempt.

[[Page 38955]]

C. 2020 Part B Proposed Rule

In the March 3, 2020 proposed rule, Hazardous and Solid Waste
Management System: Disposal of CCR; A Holistic Approach to Closure Part
B: Alternate Demonstration for Unlined Surface Impoundments;
Implementation of Closure (85 FR 12456), EPA proposed revisions to the
2015 CCR Rule, including: procedures to allow facilities to request
approval to use an alternate liner for CCR surface impoundments; two
alternative proposed options to allow the use of CCR during unit
closure; an additional closure option for CCR units being closed by
removal of CCR; and requirements for annual closure progress reports.
On November 12, 2020, EPA finalized the procedures to allow facilities
to request approval to use an alternate liner for CCR surface
impoundments. 85 FR 72506. In this final rule, the Agency is taking
final action on the proposed closure option for units being closed by
removal of CCR, which action is discussed in Unit III.D of this
preamble. EPA is still considering provisions from the proposed rule
that are not addressed in this rule and may be addressed in a
subsequent action.

D. 2020 Advance Notice of Proposed Rulemaking

On October 14, 2020, EPA published an Advance Notice of Proposed
Rulemaking (ANPRM) (85 FR 65015). In that action, EPA requested
information related to legacy CCR surface impoundments to inform a
future rulemaking. The Agency requested input on its regulatory
authority, input on a potential definition of a legacy CCR surface
impoundment and specific information on the types of inactive surface
impoundments at inactive facilities that might be considered legacy CCR
surface impoundments. Specifically, EPA requested information on how
many of these units exist, the current status of these units (e.g.,
capped, dry, closed according to State requirements, still holding
water), and the names, locations, and closure dates of former power
plants that may have these units. Finally, the Agency took comment on
which CCR regulations should apply to legacy CCR surface impoundments
and on suggestions for compliance deadlines.
During the 60-day public comment period, the Agency received over
15,000 comments from environmental groups, four States, one Tribe,
individual utilities, and industry trade associations. The topics
raised in comments included a potential definition of a legacy CCR
surface impoundment, EPA's regulatory authority, the scope and
applicability of the legacy impoundment rule, and regulatory
requirements to propose. Moreover, the comments generally agreed that
EPA must prescribe timeframes for coming into compliance with the
regulations and they recommended timeframes that are shorter than
compliance timeframes in the 2015 CCR Rule.
As noted in the ANPRM, EPA took comment on whether, in light of the
Court's opinion in USWAG, the Agency could reconsider whether it has
the authority to regulate inactive impoundments under RCRA subtitle D.
85 FR 65017-65018 (October 14, 2020). The general consensus from
commenters on the ANPRM was that, because the Court resolved the
question based on the plain meaning of the statute, EPA does not have
the discretion to reinterpret its authority. In addition, no commenter
identified a factual basis for not regulating legacy CCR surface
impoundments that addressed the Court's concern about the risks these
units pose. Id. at 65018. Consequently, EPA is not revisiting the
question of whether it may regulate inactive or legacy CCR surface
impoundments.

E. 2023 Proposed Rule and Comments

On May 18, 2023, EPA proposed revisions to the CCR regulations (88
FR 31982) (``the proposed rule'' or ``2023 proposed rule''). These
revisions included establishing regulations specifying that legacy CCR
surface impoundments are subject to 40 CFR part 257, subpart D and
specifying that owners or operators of legacy CCR surface impoundments
comply with all the appropriate requirements applicable to inactive CCR
surface impoundments at active facilities. In addition, EPA proposed to
establish requirements to address the risks from currently exempt solid
waste management that involves the direct placement of CCR on the land.
EPA proposed to extend a subset of the existing requirements in part
257, subpart D to CCRMU, which was proposed to include CCR surface
impoundments and landfills that closed prior to the effective date of
the 2015 CCR Rule, inactive CCR landfills, and other areas where CCR is
managed directly on the land. This proposal would apply to all active
CCR facilities and all inactive facilities with legacy CCR surface
impoundments. Lastly, EPA proposed to make several technical
corrections to the CCR regulations. These are: (1) To clarify the
definitions of ``feasible'' and ``technically feasible''; (2) To
correct the CFR reference in the definition of wetlands at Sec.
257.61(a); (3) To correct a reference in the groundwater monitoring
scope section; (4) To standardize the references to CCR websites
throughout the CCR regulations; and (5) EPA requested comment on
extending the period for document retention and posting.
The Agency received over 33,500 comments on the proposed rule, with
over 600 unique comments. Commenters included individual electric
utilities and independent power producers, national trade associations,
State agencies, public interest and environmental groups, private
citizens, and entities involved with the beneficial use of CCR. All
public comments submitted in response to the proposal can be found in
the docket for this action. Most commenters focused on the scope of the
proposed rule, definitions, compliance deadlines, and EPA's statutory
authority to regulate CCRMU. Most commenters also requested that EPA
adopt additional requirements to address the risks from CCR units.
EPA's responses to the comments on the proposed rule are addressed
either in this preamble or in a response to comment document available
in the docket to this final rule.
EPA conducted two public hearings on the proposed rule. EPA held an
in-person public hearing in Chicago, Illinois on June 28, 2023. At this
hearing there were 87 speakers and a total of 150 registered attendees.
EPA also held a virtual public hearing on July 12, 2023, using an
internet-based software platform. The platform allowed the public
hearing participants to provide oral testimony using a microphone and
speakers connected to their computers or using a phone. It provided the
ability for any person to listen to the public hearing via their
computer. At the virtual hearing, there were 93 speakers and a total of
353 registered attendees. Testimony at both public hearings focused
generally on EPA's proposed amendments, and on the following topics:
whether to further expand regulation to all CCR, regardless if it was
onsite of a regulated facility; whether to regulate structural fill and
other beneficial uses; enforcement of the CCR regulations; requests for
more engagement with communities; and requests for EPA to amend other
regulations to strengthen corrective action and limit the use of
alternative source demonstrations (ASD). Finally, some commenters
discussed site-specific concerns of facilities near their homes, or
health effects witnessed in communities close to CCR sites, and general
concerns about the health and environmental risks from CCR.

[[Page 38956]]

Transcripts for both public hearings are included in the docket for
this action.

F. 2023 Notice of Data Availability

On November 14, 2023, EPA published a notice of data availability
(NODA), to solicit comments on additional information and statistics
developed in response to comments on the Agency's May 18, 2023 proposed
rule. 88 FR 77941. Some of the information contains data or analysis
obtained directly from comments submitted during the May 18, 2023
proposed rule's comment period, which might aid in the formulation of
the final rule. EPA also solicited comments on a supplemental risk
assessment EPA conducted in response to comments raised on the proposed
rule. This risk assessment builds on the findings of the previous Human
and Ecological Risk Assessment of Coal Combustion Residuals (2014 Risk
Assessment) \4\ and better quantifies the specific risks that may
result from placement of CCR in legacy CCR surface impoundments and
CCRMU. EPA requested comment on all aspects of the assessment including
the validity and propriety of relying on the new information, data, and
analyses contained in the updated risk assessment to inform the final
rule.
---------------------------------------------------------------------------

\4\ U.S. EPA. 2014. ``Human and Ecological Risk Assessment of
Coal Combustion Residuals.'' RIN 2050-AE81. Office of Solid Waste
and Emergency Response. Washington, DC. December.
---------------------------------------------------------------------------

EPA also sought further information on legacy CCR surface
impoundments and CCRMU, including information on the location,
presence, condition, history, and risk associated with any of the
potential legacy CCR surface impoundments or any of the potential CCRMU
within the docket. EPA also requested any information regarding the
presence of water, distance to surface water bodies, proximity to
floodplains, unit size, CCR volume, depth to groundwater, date of CCR
placement, closure status, any corrective action associated with the
unit, and any groundwater monitoring data. EPA also requested comment
on the accuracy of the information that was submitted regarding
potential legacy CCR surface impoundments or potential CCRMU.
Furthermore, EPA sought similar information on any other potential
legacy CCR surface impoundments or potential CCRMU of which EPA may not
be aware or for which we may have incomplete information.
EPA accepted public comment on the NODA until December 11, 2023.
The Agency received over 70 comments on the NODA. Commenters included
individual electric utilities and independent power producers, national
trade associations, State agencies, public interest and environmental
groups, private citizens, and entities involved with the beneficial use
of CCR. All public comments submitted in response to the NODA can be
found in the docket for this action. The majority of commenters focused
on the supplemental risk assessment; some focused on the request for
additional information on the universe of legacy CCR surface
impoundments and CCRMU. EPA's responses to comments received on the
NODA are addressed either in an updated risk assessment (the 2024 Risk
Assessment), this preamble, or in the response to comment document
available in the docket to this final rule.

III. What final action is the Agency taking?

In response to the USWAG decision, EPA is finalizing a provision at
Sec. 257.50(e), specifying that legacy CCR surface impoundments are
subject to 40 CFR part 257, subpart D. EPA is also requiring owners or
operators of legacy CCR surface impoundments to comply with the
following existing requirements in the CCR regulations: installation of
a permanent marker, history of construction, hazard potential
classification, structural stability and factors of safety assessments,
emergency action plan (EAP), air criteria, inspections, groundwater
monitoring and corrective action, closure and post-closure care,
recordkeeping, and notification and CCR website requirements. EPA
further is establishing new compliance deadlines for these newly
applicable regulatory requirements to ensure the owners or operators of
these units have time to come into compliance.
In addition to the revisions EPA proposed to address the USWAG
decision, EPA is establishing requirements to address the risks from
currently exempt solid waste management that involves the direct
placement of CCR on the land. EPA is extending a subset of the existing
requirements in 40 CFR part 257, subpart D to CCRMU, which are CCR
surface impoundments and landfills that closed prior to the effective
date of the 2015 CCR Rule, inactive CCR landfills, and other areas
where CCR is managed directly on the land. These additional
requirements apply to all active CCR facilities, all inactive
facilities with legacy CCR surface impoundments subject to this final
rule, and those active facilities (i.e., facilities producing
electricity for the grid as of October 19, 2015) that ceased placing
CCR in onsite CCR units prior to the effective date of the 2015 CCR
Rule.
EPA is also finalizing alternative closure provisions to allow a
facility to complete the closure by removal in two stages: first, by
completing all removal and decontamination procedures; and second, by
completing all groundwater remediation in a separate post closure care
period.
Lastly, EPA is finalizing several technical corrections to the CCR
regulations. These are: (1) to clarify the definitions of ``feasible''
and ``technically feasible''; (2) to correct the CFR reference in the
definition of wetlands at Sec. 257.61(a); (3) to correct a reference
in the groundwater monitoring scope section; (4) to standardize the
references to CCR websites throughout the CCR regulations; and (5) to
extend the period for document retention and posting.

A. Risks From Legacy CCR Surface Impoundments and CCR Management Units

1. Summary of May 2023 Proposal
The proposal largely relied on the model results from the 2014 Risk
Assessment, as EPA considered the results were equally applicable to
legacy CCR surface impoundments and CCRMU.\5\ This determination was
based on the fact that many of these unregulated units are similarly
constructed, manage the same types of ash, and are frequently located
either at the same or nearby facilities as their regulated
counterparts. In particular, some unregulated units are known to be
located directly adjacent to or beneath currently regulated units.
---------------------------------------------------------------------------

\5\ U.S. EPA. 2014. ``Human and Ecological Risk Assessment of
Coal Combustion Residuals.'' RIN 2050-AE81. Office of Solid Waste
and Emergency Response. Washington, DC. December.
---------------------------------------------------------------------------

The 2014 Risk Assessment concluded that the management practices
that EPA believed were generally in use in 2014 at surface impoundments
and landfills were likely to pose risks to human health through
groundwater exposure within the range that EPA typically considers
warrants regulation. For highly exposed individuals, the cancer risks
from arsenic due to the operation of surface impoundments were as high
as 2 x 10-\4\, while noncancer risks were as high as an HQ
of 5 for arsenic, 2 for lithium, and 2 for molybdenum. Cancer risks
associated with the operation of landfills were estimated to be as high
as 5 x 10-\6\ from the ingestion of arsenic-contaminated
drinking water. In 2015, EPA relied on this risk assessment to support
the regulation of both active CCR units and inactive CCR surface

[[Page 38957]]

impoundments at active facilities. The 2014 Risk Assessment reported
risks for the units that were anticipated to be subject to the 2015 CCR
Rule and therefore drew no conclusions about the older units excluded
from the scope of that rule. Nevertheless, information available in the
record supports the conclusion that these older units can pose risks at
least as high as reported in the 2014 Risk Assessment.
EPA further proposed to find that the risks associated with legacy
impoundments and CCRMU may be even higher than EPA modeled on a
national scale in the 2014 Risk Assessment for active units. First, the
proposal explained that these units have been present onsite for longer
and had more time to leak. In addition, EPA explained that there are
several management practices that have the potential to result in
higher leakage, but that were previously modeled either less frequently
for active units--based on a belief that the practices had declined
over time--or not at all--due to data constraints on a national scale.
These include: (1) The greater prevalence of unlined units; (2) The
greater likelihood of co-management of CCR with coal refuse and other
wastes in surface impoundments, making the overall waste pH far more
acidic and (3) The potential for the units to be constructed below the
water table or to have become inundated with groundwater after
construction. The proposal estimated that the solid waste management
practices associated with legacy impoundments and CCRMU could pose
lifetime cancer risks from arsenic as high as 2 x 10-\5\ to
1 x 10-\3\ (i.e., 2 to 100 cases of cancer for every 100,000
individuals exposed), and non-cancer risks for cobalt as high as an HQ
of 13, mercury up to an HQ of 13, lithium up to an HQ of 3, molybdenum
up to an HQ of 4, and thallium up to an HQ of 2, depending on the
specific management practice. Finally, EPA explained that each of these
practices individually can pose greater risks than those previously
modeled for the currently regulated universe of CCR units, and a
combination of these practices could push risks even higher.
2. 2023 Draft Risk Assessment
A number of commenters claimed that the 2014 Risk Assessment did
not adequately capture various factors associated with legacy CCR
surface impoundments and CCRMU that the commenters believed will result
in significantly different risks than those posed by currently
regulated units. In response to these comments, EPA conducted a
supplemental risk assessment to determine the potential for some of
these factors to affect national risks. This risk assessment built on
the findings of the 2014 Risk Assessment and better quantified the
specific risks that may result from placement of CCR in legacy CCR
surface impoundments and CCRMU.
The 2023 draft supplemental assessment consisted of: (1) Additional
modeling of inactive and closed CCR landfills and surface impoundments
that was actually conducted in 2014 using the same methodology and
data. These results were ultimately not included in the original 2014
Risk Assessment because the units were not regulated under the final
2015 rule, and (2) Some further model runs relying on some updated
data. In addition, EPA modeled the placement of CCR in smaller
quantities than would typically be found in a CCR surface impoundments
or landfill (i.e., smaller CCRMU placements or CCRMU fills) to
determine the potential for these smaller CCRMU placements to
contaminate groundwater. Through this modeling, EPA identified
potential for these fills to contaminate onsite groundwater. Model
results indicated potential for exceedance of groundwater protection
standards (GWPS) at the fill boundary under both high-end and moderate
conditions. These results also showed potential for substantial spread
of the resulting groundwater plume. Under high-end conditions, these
plumes are large and persistent enough to sustain exposures for over a
century or more at average risks of 2 x 10-5 or higher.
Finally, EPA assessed the potential for exposure to radiation from
CCR remaining in the soil (subsurface). EPA found the amount of radon
emitted by CCR is not distinguishable from background soil and so did
not retain this pathway for further consideration. EPA also found
greater potential for risk from gamma radiation as CCR comes to be
located closer to the ground surface due to a reduction in shielding.
An additional sensitivity analysis identified potential for further
risk if CCR becomes mixed with surface soil. Accumulation of CCR can
result in elevated cancer risk from incidental ingestion of arsenic and
radium, in addition to direct exposure to gamma radiation from radium.
For high-end waste concentrations, an eight percent mixture of CCR in
surface soil was found to result in risk on the order of 1 x
10-4.
The 2023 draft risk assessment was made available for public
comment as part of a NODA released on November 14, 2023.
3. Response to Comments on the Proposal and the NODA
The following subsection provides a summary of comments received on
either the proposed rule or NODA that are germane to the risk record
for legacy impoundments and CCRMU. EPA considered these comments as it
worked to finalize the supplemental risk assessment (``2024 Risk
Assessment''). The Agency also received a number of general comments,
which were either editorial in nature or expressed general support or
disapproval for the risk assessment methodology, data, or results.
However, these comments did not provide any specific technical
recommendations or data that could otherwise be used to update the risk
assessment. These general comments did not provide EPA with a basis to
alter or otherwise re-evaluate the risk assessment in response.
a. Comments Related to Applicability of 2014 Risk Assessment
Comment: Several commenters generally affirmed the Agency's risk
basis for regulating historical and inactive disposal units. However,
other commenters argued the Agency's risk record is inadequate to
support regulation of certain legacy impoundments or any CCRMU. Others
contended that because the 2014 Risk Assessment supported regulation of
active landfills and surface impoundments, it is not appropriate to
apply that record to disposal units that previously ceased receipt of
waste. In particular, commenters pointed to the current lack of ponded
water and/or the presence of a cap and vegetative cover that would
reduce infiltration through certain units. Some commenters noted that
State programs may include requirements for unit design, monitoring,
and closure that ensure a cover is present. Commenters stated these
factors must be accounted for through an updated risk assessment.
EPA Response: Claims that the results of the 2014 Risk Assessment
are applicable only to active units represent a fundamental
misunderstanding of scope of the 2014 Risk Assessment. EPA did not only
model units during operation. Instead, the risk assessment modeled the
specific stage of the unit lifecycle anticipated to contribute the most
to long-term risk. For surface impoundments this was during operation,
but for landfills it was after closure. EPA modeled the leakage that
occurred over this one lifecycle stage and tracked the subsequent
migration through groundwater over time. The risks to downgradient
receptors

[[Page 38958]]

resulting from the modeled leakage were used to represent risk over the
entire unit lifecycle. Consideration of a single lifecycle stage was
necessary because of model constraints and the high computational
burden of tracking shifting configurations for every single unit.
Both landfills and surface impoundments progress through similar
lifecycle stages from construction to closure. Thus, the fact that some
historical and inactive units may no longer contain ponded water or may
have installed a soil cover only places these units in a different
stage of that lifecycle. That does not differentiate the long-term
risks of those units from those previously modeled. In particular,
existing groundwater contamination does not vanish once a unit ceases
operation. As one State commenter noted, ``[g]roundwater contamination
is an important aspect to legacy impoundment closure and should not be
overlooked simply because the impoundment does not contain liquid or
CCR at the date of the final rule.''
By contrast, the 2014 Risk Assessment only modeled landfills after
closure; in other words, EPA assumed that no leakage occurred prior to
closure, while the landfill was operating. EPA only modeled landfills
after closure because based on the assumption that this stage of the
landfill lifecycle would have the greatest contribution to long-term
risk for offsite receptors because the unit would be filled to capacity
and the post closure stage represented the greater period of time over
which leakage can occur. EPA modeled unlined units with a soil cap and
vegetative cover equivalent to the surrounding native soils and found
risks from arsenic as high as 2 x 10-5 for receptors up to a
mile away. Even assuming some landfills have been closed in a manner
more consistent with the existing CCR regulations (i.e., with some kind
of composite cover system), this is unlikely to change the overall
conclusions of the risk assessment. This is because, regardless of the
cover that is ultimately installed, higher leakage can occur throughout
the active life of the unit when the landfill face is open and able to
intercept more precipitation. This conclusion is reinforced by the fact
that facility monitoring reports document that around 20% of currently
active landfills have triggered corrective action. Additionally, EPA
has seen no evidence to suggest that the closure of older units has
been consistently more protective than EPA modeled in 2014. As
discussed in Unit III.B.2.g.iii(a) of the preamble, as part of
developing the 2015 CCR Rule, EPA reviewed State statutes and
regulations, with a more detailed focus on the 16 States responsible
for approximately 74% of the CCR generated in 2009. See 80 FR 21324.
The Agency's review of State programs prior to 2015 found that
oversight of these wastes and the overall protectiveness of particular
programs varied widely. For example, EPA estimated that in 2015,
approximately 20% of the net disposable CCR was entirely exempt from
State regulatory oversight. Similarly, a 2006 joint Department of
Energy (DOE) and EPA study reported that only 19% (three out of 19) of
the surveyed surface impoundment permits included requirements
addressing GWPS (i.e., contaminant concentrations that cannot be
exceeded) or closure/post-closure care. Furthermore, some of the
photographs and descriptions of these older units provided by
commenters indicate extensive growth of trees and other woody
vegetation that can compromise the integrity of any cap present and
increase the rate of infiltration into the unit. For these reasons, the
2014 Risk Assessment is equally representative of the national risks
from historical and inactive landfills.
The 2014 Risk Assessment modeled all surface impoundments during
the active stage of their lifecycle. This was based on the presumption
that the highest rates of leakage would occur while wastewater is
ponded above the ash, because this water creates a large and sustained
hydraulic head that serves to drive leachate into the subsurface.
Although the current configuration of historical and legacy
impoundments may vary, all these units previously held ponded water
during the active stage of their lifecycle. And, in the case of legacy
impoundments, ponded water may still be present. As a result, the
current configuration of the unit is immaterial to the releases that
occurred during operation. For this reason, the modeling approach
relied upon in the 2014 Risk Assessment is equally applicable to
historical and legacy impoundments.
The 2014 Risk Assessment also accurately represents the potential
risks that remains for units that were closed consistent with the 2015
CCR Rule. If the cover system is not adequately maintained after
closure, degradation over time from human or animal activity, natural
settling, freeze-thaw cycles, flooding and other extreme weather
events, and other factors can result in greater leakage from the unit
than designed. In some cases, groundwater monitoring may provide the
only clear evidence the cap is not performing as designed. Thus, the
2014 Risk Assessment accurately describes the risks that can result if
these units are not adequately maintained and monitored in line with
regulatory requirements.
Comment: Multiple commenters argued that historical and inactive
disposal units will generally have a smaller footprint than those
modeled in the 2014 Risk Assessment. For example, some commenters noted
the average sizes of landfills and surface impoundments modeled in the
2014 Risk Assessment were around 120 acres and 50 acres, respectively,
while the estimated average sizes of CCRMU and legacy impoundments in
the proposed rule were both closer to 30 acres. Others cited to the
sizes of individual units that at their facilities to contend that
these units are much smaller than average. These commenters contended
that a smaller footprint would result in a lower mass loading of
groundwater and lower associated risk.
EPA Response: EPA disagrees that the referenced data indicate that
older disposal units are significantly smaller in size than the units
EPA modeled in 2014. The 2014 Risk Assessment relied on data submitted
by facilities in the EPA Surveys to estimate an average active landfill
size of around 120 acres from over 310 landfills and an average active
impoundment size of around 50 acres from over 735 impoundments. The RIA
summary referenced by commenters relies only on data that could be
independently verified by data from posted facility reports and recent
public comments. From the final list of 195 CCRMU and 194 legacy
impoundments, EPA identified data for only one landfill with a size of
90 acres and 47 historical or legacy impoundments with an average size
of 44 acres. Thus, when CCRMU are separately grouped as landfills and
impoundments, the differences in size are not as substantial as
indicated by commenters.
EPA also disagrees that any differences that do exist would result
in substantially lower risks than previously modeled. As part of the
2014 Risk Assessment, EPA modeled 122 landfills and 163 impoundments
that were excluded from the reported risk results because these units
were determined to not be subject to that rule. These excluded units
represent some combination of legacy impoundments, inactive landfills,
and historical disposal units. The average sizes of these previously
excluded units are 77 acres for the landfills and 28 acres for the
impoundments. These sizes are approximately half the size of the units

[[Page 38959]]

identified in the 2014 Risk Assessment or more recent data collection
efforts. However, as discussed in Section 3 of the 2024 Risk
Assessment, the risks associated with these older units are
substantially the same as those for currently regulated units.
Therefore, there is no evidence that these differences in size have a
meaningful impact on national risks, or that the results of the 2014
Risk Assessment are nor equally applicable to legacy impoundments and
CCRMU. While there may be individual disposal units at these sites that
are smaller than average, the model results summarized in the 2024 Risk
Assessment model include landfills as small as 2 acres and impoundments
as small as 0.01 acres. Therefore, there is no indication based on the
data provided that the overall distribution of unit sizes has not been
adequately reflected in the national model.
Finally, EPA notes that individual unit size is not necessarily a
reliable metric to draw conclusions about the overall risk from CCR
disposal at electric utilities. The 2014 Risk Assessment modeled the
risks from each landfill and impoundment separately because it was
difficult to confirm the relative locations and orientations of
different units with data from the EPA Surveys. However, the Agency is
now aware of many sites where multiple units, both landfills and
impoundments, are located immediately adjacent to one another. As a
result, there is potential the 2014 Risk Assessment underestimated site
risk to some degree by not evaluating the combined leakage over the
full contributing area of these adjacent disposal units.
Comment: One commenter stated the 2014 Risk Assessment did not
specifically characterize the risks from impoundments that do not
contain fly ash. This commenter argued that historical and legacy
impoundments are more likely to only contain bottom ash or boiler slag,
as the process of capturing fly ash was not common prior to the 1970s.
Therefore, this commenter concluded that the 2014 Risk Assessment does
not adequately characterize the risks for these older units.
EPA Response: EPA disagrees that the 2014 Risk Assessment does not
address the risks associated with these impoundments. The risk
assessment incorporated porewater data from impoundments that contained
only bottom ash, but EPA did not separately break out risks for this
subset of units because the amount of data available was inconsistent
across the set of modeled constituents. However, available porewater
data show the potential for certain constituents, such as molybdenum,
to leach from bottom ash at levels as high as from fly ash.
Additionally, available monitoring reports for currently regulated
units posted on facility websites document that these units have a
similar potential to contaminate groundwater as units containing other
types of CCR. Of the units designated as managing bottom ash, 32% of
surface impoundments and 38% of landfills have triggered corrective
action. Of the units designated as managing slag, 38% of surface
impoundments have triggered corrective action. No landfills were
identified as dedicated to slag. For comparison, 48% of remaining
surface impoundments and 21% of remaining landfills have triggered
corrective action. Therefore, there is no indication that these types
of units are overall less likely to result in groundwater
contamination.
Comment: One commenter claimed that a nationwide assessment should
not be used to make determinations about the risks at individual sites
or to support national requirements. This commenter stated that, unlike
individual damage cases, the Agency's groundwater model does not
adequately represent the specific conditions at each individual unit.
However, this commenter provided no data to support their broad claims.
One other commenter pointed to data they had identified to contend that
the model does not reflect the specific environmental conditions at
their facility.
EPA Response: The modeling conducted for both the 2014 and 2024
Risk Assessments utilized a probabilistic, site-based approach that
combined site-specific data with more regional and national data
sources. The model incorporated data about the specific location,
dimensions, and liner status of individual disposal units where
available. The aim of this approach is not to assign an exact risk to
each individual unit, but to provide an overall accurate picture of the
potential risks posed by these types of units on a national scale.
Indeed, many of the findings from the 2014 Risk Assessment were
supported by available damage cases. The commenters did not articulate
why they believe the risks associated with individual units fall far
outside the broader distribution of modeled units. But as acknowledged
by the one commenter who did submit data, there is overlap between the
range of conditions modeled and those they identified as present at
their particular facility. EPA does acknowledge that there are some
site conditions that the 2014 and 2024 Risk Assessments were not able
to adequately model, such as waste below the water table. However, this
is why the Agency separately relied on damage cases to identify
additional constituents of potential concern for groundwater
monitoring.
Comment: One commenter stated that EPA should not rely on the
findings of the Environmental Integrity Project's report, ``Poisonous
Coverup: The Widespread Failure of the Power Industry to Clean Up Coal
Ash Dumps'' without independently validating the quality of analyses
conducted for each site.
EPA Response: EPA recognizes that the method used in the cited
report to identify potential exceedances of GWPS is not the same as the
regulatory standard for triggering corrective action. For this and
other reasons, the Agency does not rely on the report as a primary
basis for the current rulemaking or to draw any conclusions about the
monitoring status of any individual unit. Instead, EPA previously
referenced the report as a supplementary source of information that
further supports the findings of the 2014 Risk Assessment.
Specifically, the fact that the constituents identified as risk drivers
in the 2014 Risk Assessment are the same ones detected most frequently
above GWPS indicates that the fate and transport modeling conducted by
EPA was able to correctly identify the constituents most likely to be
released and migrate at environmentally significant concentrations.
While high background concentrations may be present at some of these
sites, many have already triggered corrective action and the Agency
believes that number will increase as more facilities come into
compliance with the rule requirements. Because this report does not
form a basis for the rule, it is not discussed further in the preamble
to the rule or the 2024 Risk Assessment outside of responses to other
comments that cite to the same or similar reports.
b. Comments Related to Draft 2023 Supplemental Risk Assessment
Conceptual Model
Comment: Multiple commenters broadly argued that the draft 2023
risk assessment relied on data and assumptions that represent maximum
values or otherwise reflect worst-case scenarios that could never
occur, and therefore do not represent a ``reasonable probability'' of
adverse impacts and so is not an appropriate basis for regulatory
action.
EPA Response: EPA disagrees that the design of the risk assessment
is inappropriately conservative. Consistent

[[Page 38960]]

with EPA's long-standing practice under RCRA (as well as other agency
programs), an individual with reasonable maximum exposure (RME)
provides the principal basis for evaluating potential human health
risks. An RME scenario is intended to be conservative, while remaining
within the range of possible high-end exposures.\6\ Specifically,
``high end'' has been defined as the part of the exposure distribution
that falls above the 90th percentile, but below the 99.9th
percentile.\7\ Reliance on this type of scenario is intended to protect
sensitive populations. Selection of the data and assumptions
incorporated in the 2024 Risk Assessment is in line with this
objective. Further critiques about the potential for the specific data
and assumptions to overestimate risk are addressed in subsequent
responses.
---------------------------------------------------------------------------

\6\ U.S. EPA. 1989. ``Risk Assessment Guidance for Superfund
Volume I--Part A, Human Health Evaluation Manual.'' EPA/540/1-89/
002. Prepared by the Office of Emergency and Remedial Response,
Washington, DC. December.
\7\ U.S. EPA. 2004. ``An Examination of EPA Risk Assessment
Principles and Practices.'' EPA/100/B-04/00. Prepared by the Office
of the Science Advisor. Washington, DC. March.
---------------------------------------------------------------------------

Comment: Multiple commenters argued that it was inappropriate for
EPA to consider future onsite residential exposures as a basis for
evaluating the potential risks associated with onsite CCR disposal. One
commenter claimed that the estimates of existing populations living
near these facilities used in the 2024 Risk Assessment was both
overestimated and inconsistent with estimates from the Agency's RIA.
One commenter acknowledged that older units tend to be located closer
to population centers. However, others argued that this proximity to
existing populations or water bodies would not make them overall more
likely to become residential in the future. One commenter stated that
EPA should have surveyed the intended land use for facilities or
otherwise directly assessed the likelihood of residential land use.
EPA Response: EPA disagrees that consideration of a population
within a five-mile radius overstates the likelihood of residential
development. Five miles away from a population center is a small
distance for residential development to expand, even in the near
future. Nevertheless, the Agency has updated the population estimates
in the 2024 Risk Assessment to more closely align with reporting in the
RIA and to include both one- and three-mile radii. EPA also disagrees
that consideration of a future residential land use scenario is
inappropriate or unrealistic. The substantial populations already
living near many facilities and the generally higher property value of
land near water bodies are two indicators of the potential for land to
be attractive for future residential land use. Facilities do not
dictate the ultimate use of a property after the land has been sold for
redevelopment. These types of facilities can include considerable
tracts of land beyond that dedicated to waste disposal that may be
considered for a range of different uses. EPA is currently aware of 22
examples in which former electric utilities have been proposed for
residential development, 19 of which are known to have burned coal.\8\
Thus, there is evidence of community interest in residential land use
at these types of facilities.
---------------------------------------------------------------------------

\8\ Memorandum to the Docket: Compilation of News Articles on
Future Land Uses for Electric Utilities.
---------------------------------------------------------------------------

Although future residential use is considered as the RME scenario
in the 2024 Risk Assessment, that does not mean it is the only scenario
EPA considered or on which this final rule is based. Depending on their
location, leakage of Appendix IV constituents from individual CCRMU
fill may migrate off-site at levels of concern. In addition, even if
the constituents from a single CCRMU do not migrate off-site, the
modeling conducted in 2024 confirms that smaller CCRMU fills can
meaningfully contribute to groundwater contamination across a facility.
Concentrations from a single CCRMU can combine with contamination from
other CCRMU, currently regulated CCR units, or legacy CCR surface
impoundments that are also present on the same site. Although EPA did
not model the aggregate or cumulative risk associated with these
potential sources of co-located contamination, at a minimum, EPA
expects that the presence of multiple sources of potential
contamination at the same facility would increase the likelihood of a
contaminant plume that could migrate off-site at levels of concern.
Nor is residential use the only scenario where exposures present
concern. One commenter described donating property to a local
government for recreational uses. Several other commenters described
redeveloping sites as nature preserves. Even under these non-
residential land uses, the is a reasonable potential for exposure (and
consequently risk) to human and ecological receptors if the ash is
subsequently disturbed. For example, as discussed in Section 6 the 2024
Risk Assessment, concentrations of certain contaminants may also pose
risk to wildlife if ash becomes intermingled with surface soil.
Comment: Commenters asserted that consideration of residential land
use is inconsistent with various EPA guidance documents
9 10 11 and Agency cleanup programs. These commenters argued
such guidance instructs EPA to assume that facilities surrounded by
operating industrial facilities will remain industrial unless there is
clear evidence otherwise. These commenters further argued that guidance
instructs EPA to account for institutional controls, such as State or
local zoning laws, that would make residential development or resulting
exposures at individual sites unlikely. Some commenters cited to
specific State requirements they assert would prevent residential land
use or prohibit future use of site groundwater as a source of drinking
water. Others claimed that due diligence reviews would be adequate to
identify and address any remaining sources of contamination before
exposures could occur.
---------------------------------------------------------------------------

\9\ U.S. EPA. 1989. ``Risk Assessment Guidance for Superfund
Volume I Human Health Evaluation Manual (Part A).'' EPA/540/1-89/
002. Prepared by the Office of Emergency and Remedial Response,
Washington, DC. December.
\10\ U.S. EPA. 1991. ``Risk Assessment Guidance for Superfund
Volume I Human Health Evaluation Manual (Part B, Development of
Risk-based Preliminary Remediation Goals).'' Publication 9285.7-01B.
Prepared by the Office of Emergency and Remedial Response,
Washington, DC. December.
\11\ U.S. EPA. 1995. ``Land Use in the CERCLA Remedy Selection
Process.'' OSWER Directive No. 9355.7-04. Prepared by the Office of
Solid Waste and Emergency Response. Washington, DC. May.
---------------------------------------------------------------------------

EPA Response: EPA disagrees that consideration of future
residential land use at these facilities is inconsistent with
applicable guidance and cleanup programs. First, the risk assessment
was conducted to establish minimum national criteria rather than to
clean up an individual site. To determine whether the section 4004(a)
standard will be met at all sites nationwide, as the statute requires,
the Agency needs to evaluate the risks associated with full range of
reasonable scenarios. As discussed in the previous response, there are
numerous examples in the record of instances in which these kinds of
sites have been redeveloped for residential use.
Moreover, as the commenters have acknowledged, the Comprehensive
Environmental Response, Compensation, and Liability Act (CERCLA) and
other cleanup programs only address contamination that has already
occurred. In contrast, national standards for waste management
developed under RCRA section 4004(a) are to prevent environmental
releases

[[Page 38961]]

before they occur. See, USWAG, 901 F.3d at 429-431. As EPA has
previously explained, groundwater contamination is a concern, even if
the aquifer is not currently used as a source of drinking water.
Sources of drinking water are finite, and future users' interests must
also be protected. See, 44 FR 53445-53448.
EPA further disagrees that the risk assessment failed to
appropriately account for existing State and local requirements for
institutional controls that would limit residential exposure. The
purpose of a baseline risk assessment is to provide ``. . . an analysis
of the potential adverse health effects (current or future) caused by
hazardous substance releases from a site in the absence of any actions
to control or mitigate these releases (i.e., under an assumption of no
action).'' \12\ Thus, the intent of the risk assessment is to
characterize the harm that could result if institutional and other
controls are not implemented. This provides a consistent basis to
understand the risks to be controlled and define appropriate national
requirements such as a national requirement for deed restrictions at
all sites at which CCRMU fills will remain in place. The Agency did not
assume that all facilities will ultimately be used for residential
purposes as a consequence of these factors in developing this final
rule.
---------------------------------------------------------------------------

\12\ U.S. EPA. 1989. ``Risk Assessment Guidance for Superfund
Volume I Human Health Evaluation Manual (Part A).'' EPA/540/1-89/
002. Prepared by the Office of Emergency and Remedial Response,
Washington, DC. December.
---------------------------------------------------------------------------

Furthermore, as several commenters have acknowledged, facilities
have not historically been required to identify smaller placements of
ash as a form of disposal and consequently have not maintained reliable
records of where such placements are located. Indeed, most commenters
have acknowledged that they are currently unable to identify all CCRMU
at their existing facilities. These commenters do not explain how due
diligence assessments would reliably identify such placements in the
absence of such records, as such assessments typically rely on
available site records to guide further investigation. Nor do
commenters explain how existing State programs would reliably identify
such placements or otherwise prevent exposures, when the facilities
themselves cannot identify the presence of the ash on-site. This is
reinforced by EPA's review of State programs, which found that the
specific requirements, level of oversight over these wastes, and the
overall protectiveness of individual programs varied widely among
States. See, 80 FR 21324. As a result, EPA currently lacks a record to
conclude that currently unidentified CCRMU fills located across a
facility would be subject to the same institutional controls that are
required for the disposal units the commenters reference. Given the
current absence of national requirements, and that commenters have
generally acknowledged that they have not reliably kept records of the
existence of CCRMU, it is appropriate to evaluate the risks that can
reasonably arise in the absence of institutional controls.
Comment: One commenter argued that EPA had already considered the
practice of disposal below the water table because it had been
discussed in previous risk assessments. Another commenter asserted the
Agency's conceptual model assumed all legacy impoundments were in
contact with the water table. Another commenter stated that EPA cannot
use information about active units to make assumptions about which
historical and inactive units at the same sites are in contact with the
water table, due to differences in unit construction and location. By
contrast, a number of other commenters agreed that because EPA cannot
model the effects of waste below the water table, EPA had previously
underestimated the risks associated with CCR units. Other commenters
argued the conceptual model for surface impoundments did not adequately
distinguish between the types of water that may be present in an
impoundment. These commenters asserted that any residual water
remaining after the unit has been initially drained would not exert the
same hydraulic head within the unit that would drive leachate into the
subsurface during unit operation, and so leakage would more closely
resemble a landfill.
EPA Response: The conceptual models for landfills and surface
impoundments in the 2014 and 2024 Risk Assessments did not evaluate
contact with groundwater. Although these assessments both acknowledged
that this could occur, the scenario could not be incorporated into
groundwater fate and transport modeling as a result of data and model
constraints. Because the 2014 and 2024 Risk Assessments did not
directly model the effects of disposal below the water table, neither
assessment incorporates any assumptions about the prevalence of this
practice in the conceptual model. EPA has acknowledged that its
inability to reliably model the effects of this practice means that its
risk estimates on a national scale underestimate the risks associated
with higher rates of leaching and/or formation of strongly reducing
conditions.
EPA acknowledges the rates of leakage from surface impoundments
will generally decrease after ponded wastewater has been allowed to
drain, reducing the overall hydraulic head across the unit. As such,
discussion in the 2024 Risk Assessment has been updated to clarify the
distinction between water ponded above the ash and porewater within an
impoundment. However, any free liquids that remain within the unit can
still result in higher leakage than would occur if the unit were fully
dewatered. And the amount of ``residual water remaining'' can sometimes
be substantial; in some cases, closed impoundments remain saturated by
20-54 feet of groundwater. See, e.g., 88 FR 31982-319873, 55236.
In addition, regardless of the current configuration of an
impoundment, it is appropriate for the conceptual models in the 2024
Risk Assessment to consider the stage of the unit lifecycle anticipated
to contribute the most to long-term risk. For surface impoundments,
this is when the units are in operation due to the presence of
wastewater ponded above the ash. Subsequent draining of the unit does
nothing to remediate any adverse impacts that occurred during
operation. Furthermore, to the extent that impoundments leak at rates
more similar to landfills after ponded wastewater has been drained, EPA
notes the 2014 Risk Assessment previously modeled the risks from dry
management in landfills and found the potential for unacceptable risk
from these units. Therefore, continued leakage from drained units still
has the potential to sustain releases.
Comment: One commenter affirmed that ``EPA is likely correct in its
observations and assumptions that CCRMU fills `will remain in place
when ownership of the property changes,' and that, `in the absence of
land use restrictions, there is no guarantee [that] engineering
controls will remain in place when the property is redeveloped.' ''
However, multiple commenters argued the conceptual model for CCRMU
fills does not adequately account for the full diversity of CCRMU that
may be present onsite. Various commenters stated that a conceptual
model for fills does not adequately address specific types of
placements, such as use in the construction or closure of CCR disposal
units, storage in waste piles, construction of roadways and railroads,
or spreading on roadways for snow and ice control. Another asserted
that reliance on the similar conceptual models in the 2014 Risk
Assessment to

[[Page 38962]]

evaluate the disposal units and CCRMU fills is inappropriate because
the CCRMU fills will tend to be smaller than the disposal units modeled
in 2014.
EPA Response: EPA disagrees that the conceptual model for CCRMU
fills does not adequately address the configurations of these units.
The majority of the units described by commenters have a concentrated
footprint, such as placement beneath a parking lot. Some of the
specific alternate examples raised by commenters are either already
regulated under the existing regulations (e.g., waste piles) or are
outside the scope of the current rulemaking. For others, there is
little to no information available about the manner or frequency of
such placements that could be used to characterize the units.
Therefore, these types of placements are not considered as part of the
conceptual model for CCRMU fills in the 2024 Risk Assessment.
The commenters do not explain how placement of CCR in a landfill or
impoundment in service of construction or closure of that unit would be
substantially different than the disposal scenarios previously modeled
and found to pose risk. Finally, EPA has proposed and is finalizing the
definition of CCRMU to exclude CCR used in roadbed and associated
embankments.
There is little data that could be used to develop a conceptual
model for diffuse placements, which may occur on a periodic basis. Nor
do commenters provide any data on the manner or frequency of such
placements. As a result, the 2024 Risk Assessment did not model these
types of placements. This represents a source of uncertainty in the
assessment. However, EPA notes that even small placements of CCR can
contribute to broader leakage and have the potential to leak Appendix
III constituents and influence nearby groundwater monitoring.
Therefore, it is still necessary to identify where these types of
onsite placements have occurred.
EPA also disagrees that applying a similar conceptual model for CCR
landfills and CCRMU fills is inappropriate. Specifically, the
conceptual model does not make any upfront assumptions regarding the
sizes of these fills. As described in Section 4 of the 2024 Risk
Assessment, EPA considered a range of potential sizes for these fills
that were smaller than landfills reported in the EPA Surveys.
Comment: One commenter stated that it is inappropriate for the 2024
Risk Assessment to rely on similar data sources as the 2014 Risk
Assessment to characterize environmental parameters, claiming these
data are outdated. Another argued that the conceptual model does not
adequately account for the presence of alternative liners, such as
thick natural clay beneath the units.
EPA Response: First, EPA notes that the 2024 Risk Assessment does
incorporate more recent weather data available from the most recent
version of the Hydrologic Evaluation of Landfill Performance Model,
updated in 2020. As explained in the 2014 Risk Assessment, EPA found
the remaining data sources provide the most recent and representative
data to characterize environmental conditions on a national basis.
Commenters provide no explanation why these data should be considered
outdated. For example, why the soil type present at a site would have
changed substantially since 2014. EPA notes that to the extent that
there is natural clay soil present in the vicinity of a facility, that
would already be reflected through the environmental data.
c. Comments Related to Supplemental Risk Assessment Groundwater Model
Comment: Some commenters asserted that modeled leachate
concentrations are unrealistically high. One commenter specifically
argued that the LEAF data is unable to accurately reflect field
leaching concentrations, citing two EPRI reports comparing LEAF and
field leachate data both collected from the same units.13 14
They separately compared the leachate concentrations modeled in the
risk assessment to field samples collected from around a number of
different landfills. Based on this comparison, the commenter asserted
that the high-end concentrations modeled in the risk assessment were
substantially higher than measured in the field and so unrepresentative
of actual leaching behavior. For these reasons, this commenter
concluded that porewater data provide better representation of leaching
in the field and so EPA should rely on that type of data to model
leakage from CCRMU fills.
---------------------------------------------------------------------------

\13\ EPRI. 2020. ``Leaching, Geotechnical, and Hydrologic
Characterization of Coal Combustion Products from a Closed Coal Ash
Impoundment.'' Palo Alto, CA. June.
\14\ EPRI. 2021. ``Leaching, Geotechnical, and Hydrologic
Characterization of Coal Combustion Products from an Active Coal Ash
Management Unit.'' Palo Alto, CA. February.
---------------------------------------------------------------------------

EPA Reponse: EPA disagrees that leachate concentrations modeled in
the 2024 Risk Assessment are unrealistically high. EPA has previously
demonstrated that the LEAF laboratory leaching tests are ``effective
for estimating the field leaching behavior for a wide range of
materials under both disposal and use conditions.'' \15\ The two
studies cited by commenters do not contradict these findings. Indeed,
one of the cited reports concludes that LEAF Method 1313 measurements
tended to only underestimate porewater concentrations of lithium and
molybdenum and did not consistently overestimate or underestimate
porewater concentrations of arsenic and thallium. These conclusions are
consistent with previous Agency findings that LEAF Method 1313
measurements (1) can underestimate leakage of highly soluble
constituents, such as lithium and molybdenum, if not adjusted to
properly account for the sample liquid to solid ratio and (2) can over
or underestimate leakage of redox sensitive contaminants, such as
arsenic, if not further adjusted with geochemical speciation modeling.
In response to these findings, the Agency has established general
recommendations for how to address these issues.\16\ Modeling of highly
soluble constituents in both the 2014 and 2024 Risk Assessment are
consistent with these recommendations. Sufficient data are not yet
available on the prevalence or magnitude of reducing conditions to
allow EPA to adequately model the effects of these conditions on
leaching behavior at a national scale. However, given that the 2024
Risk Assessment identified potential for extensive groundwater
contamination with overall risks as high as 1 x 10-4 for the
less mobile pentavalent speciation of arsenic, this uncertainty is
unlikely to affect the conclusions of the risk assessment.
---------------------------------------------------------------------------

\15\ U.S. EPA. 2014. ``Leaching Test Relationships, Laboratory-
to-Field Comparisons and Recommendations for Leaching Evaluation
using the Leaching Environmental Assessment Framework.'' EPA 600/R-
14/061. EPA Office of Research and Development. Research Triangle
Park, NC. October.
\16\ U.S. EPA. 2019. ``Leaching Environmental Assessment
Framework (LEAF) How-To Guide: Understanding the LEAF Approach and
How and When to Use It.'' SW-846 Update VII. Prepared by the EPA
Office of Land and Emergency Management. Washington, DC. May.
---------------------------------------------------------------------------

EPA also disagrees that the field data presented by commenters
demonstrates that the modeled concentrations are unrealistic. As a
general matter, these commenters did not make available the underlying
data for the graphs presented or the reports from which the graphs were
drawn. Therefore, it is not possible to fully evaluate these graphs, as
EPA cannot determine how and where these data were collected, how many
individual samples are represented, and how the data were compiled.
Based on

[[Page 38963]]

the limited description provided, it appears that the graphs summarize
data on the average leachate concentrations collected from around
different landfills. Thus, the cited median values would represent a
median of the average measurements from each landfill. This type of
summary does not provide a meaningful understanding of the leaching
potential of CCR. For example, landfills can contain mixtures of
different CCR types and other wastes, which may result in variable
leaching profiles over the footprint of the unit. An average of
measured leachate concentrations can mask regions of higher leaching
potential over many acres. This potential for variable leaching is one
reason why groundwater monitoring wells are required to be spaced along
the entire downgradient boundary of these units. In contrast, CCRMU
fills are smaller in size and more likely to be constructed with a
single source of ash. Additionally, there is no indication of how long
the waste has been present in these landfills prior to sampling. More
soluble constituents can become depleted over time. For example,
Modular Three-Dimension Finite-Difference Ground-Water Flow Model
(MODFLOW) runs conducted for the 2024 Risk Assessment showed that
molybdenum can deplete from the ash anywhere from several years to a
few decades after leaching first began. Thus, these graphs could
understate the full leaching potential of CCR.
Commenters also mischaracterize the results of the probabilistic
analysis. The 90th percentile of all model inputs for leachate
concentration is not the same as the 90th percentile of modeled risks.
There are a number of other model parameters that will influence
contaminant release and subsurface transport. As a result, the model
runs with the highest initial leachate concentrations are not always
the same as those with the highest downgradient concentrations. EPA
reviewed a subset of model runs around the 90th percentile risk result
reported in the 2024 Risk Assessment, representing 1% of all model runs
at 1,000 feet from the waste boundary. This review found the median
leachate concentration representative of these runs was closer to 0.31
mg/L for arsenic and 35 mg/L for molybdenum. There are multiple samples
in the record of porewater or leaching tests with concentrations of the
same order-of magnitude or higher than these concentrations. Therefore,
EPA concludes that the methods used to generate model inputs do not
result in unrealistically high leachate concentrations.
EPA maintains that LEAF leachate provides the most realistic
estimate of long-term leaching potential from CCR placed in fills.
There is little field leachate data for dry-managed CCR available in
the record, as it can be difficult to collect representative samples
from landfills. Additionally, field samples would reflect the specific
waste mixtures and chemistry of these disposal units. Instead, LEAF
provides data on the leaching behavior of individual CCR under a range
of relevant environmental conditions. EPA did consider using
impoundment porewater data to supplement the data on leaching of
lithium because of the lack of LEAF data for this contaminant, and
because lithium is a highly soluble, monovalent ion expected to be less
influenced by specific impoundment chemistry. However, this constituent
was not modeled in the 2024 Risk Assessment due to other data
limitations. The uncertainties associated with exclusion of lithium are
discussed in Section 6 of the 2024 Risk Assessment.
Comment: One commenter asserted that the distribution of leachate
pH values used to represent CCRMU fills is unrepresentative. In
particular, the commenter took issue with the prevalence at which
acidic conditions were modeled within CCR fills. This commenter pointed
to field data collected from CCR landfills to assert that leachate from
fills would rarely be acidic.
EPA Response: EPA disagrees that the modeled leachate pH is
unrepresentative of conditions at smaller CCRMU fills. Modeled leachate
pH is based on the natural pH (or ``own pH'') of the ash sample
measured with LEAF. Thus, these data represent the properties of real
ash samples. Landfills can contain a mixture of different CCR types and
other related waste streams and so it is reasonable that the average pH
of larger landfills may differ from that of individual CCR. At the same
time, regions of individual landfills can be more acidic than average,
which can be masked by consideration of only average values. The
potential for such variations is part of the reason that placement of
monitoring wells is required across the full downgradient boundary of
these landfills. Smaller CCRMU fills are more likely to be constructed
with a single ash type and so it is most appropriate to consider the pH
of individual ash samples, rather than broader landfill conditions. The
uncertainties associated with the modeling of pH are discussed in
Section 6 of the 2024 Risk Assessment.
Comment: One commenter stated that use of a five-mile radius to
draw environmental data for purposes of groundwater modeling is not
adequately justified and inconsistent with both the 2014 Risk
Assessment and Draft 2023 RIA.
EPA Reponse: EPA has reviewed and updated the sampling radius for
environmental and population data. Based on this review, EPA
established the sampling radius for environmental data at two
kilometers (1.2 miles). This is consistent with the methodology applied
in the 2014 Risk Assessment, which the Agency previously found
adequately represented the environmental conditions near units for
which a more precise location at the facility property could not be
determined. EPA established the sampling radii for population data to
be consistent with the rationale outlined in the 2024 RIA.
Comment: Multiple commenters criticized the Agency's use of soil-
water partitioning coefficients (i.e., Kd values) to model contaminant
sorption in the subsurface. These commenters argued that use of
individual Kd values was inappropriate and unable to reflect the
variability of subsurface transport conditions. They also stated that
the Kd values used in the risk assessment for arsenic were biased low
and likely to underestimate retention on soil. These commenters cited
field measurements collected at various locations to assert that actual
values for arsenic are likely to be higher. One commenter cited an
alternative set of Kd values they had calculated to contend that actual
values for arsenic would be orders-of-magnitude different than used in
the risk assessment.
EPA Response: These commenters are incorrect; EPA did not rely only
on individual Kd values for the risk assessment. As part of the 2014
Risk Assessment, EPA previously developed sorption isotherms for each
modeled constituent, which represent the distribution of individual Kd
values calculated and reflect the range of anticipated subsurface
conditions and specific CCR waste characteristics. Each individual
model run in the EPA Composite Model for Leachate Migration with
Transformation Products (EPACMTP) samples from that distribution based
on the key factors for that run (e.g., leachate concentration, pH,
ionic strength). No individual model run will precisely represent
conditions at a particular site. Instead, the model runs collectively
capture the variability of conditions that can occur across sites.
Thus, EPA relies on the model runs in aggregate to draw

[[Page 38964]]

conclusions about the potential for risk nationwide.
EPA also disagrees that the specific Kd values used in MODFLOW are
unrepresentative. The limited number of MODFLOW runs are intended to
further characterize the subset of high-end scenarios modeled in
EPACMTP. Thus, it is entirely reasonable that these model runs are
those more likely to reflect scenarios where pentavalent arsenic is
more mobile in the environment.
The field data shared by commenters for specific CERCLA sites or
agricultural fields are not representative of conditions at CCR
disposal units. As previously noted, the calculated sorption isotherms
reflect the properties of CCR leachate, which can be vastly different
from precipitation infiltrating through soil. In particular, both the
high ionic strength and variable pH of this leachate are expected to
result in different sorption behavior. EPA is also unable to fully
review the Kd values calculated by commenters or compare them with
Agency values because the commenters provided insufficient information
regarding whether and how specific key environmental factors were
considered. Nevertheless, EPA notes that the range of values presented
by commenters falls within the full distribution of Kd values developed
for arsenic in 2014. The full distribution of values is summarized in
Appendix H of the 2014 Risk Assessment, and is the full range of values
EPA sampled from to model groundwater transport in the 2024 Risk
Assessment.
Comment: One commenter stated that any CCR material placed beneath
the soil would become naturally compacted. Another commenter asserted
that the pozzolanic nature of some ash would result in far lower
hydraulic conductivity than EPA modeled.
EPA Response: In the absence of periodic inspections and a well-
maintained cap, there is no guarantee that any ash placed in the ground
will remain undisturbed by human or animal activity, natural settling
or freeze-thaw cycles, flooding and other extreme weather events, or
other unforeseen factors. Given that such disturbances can result in
increased permeability, it was not possible to develop a fixed
probabilistic distribution of conductivities. Instead, EPA modeled
conductivity based on the dominant soil megatexture as described in
Appendix B of the 2014 Risk Assessment. As such, the model assumes the
ash has been subjected to a similar degree of compaction as the
surrounding soil. EPA acknowledges that some fly ash is pozzolanic in
nature. Yet, the commenter provided no information that would indicate
how common it is for this type of ash, which can be marketed for use in
concrete, to be placed in CCRMU fills. EPA is also not aware of any
information that could be used to represent the long-term conductivity
of this ash when left in the field and exposed to the elements.
Comment: One commenter contended that EPA had not adequately
demonstrated that consideration of more recent weather data drawn from
the latest version of the Hydrologic Evaluation of Landfill Performance
model would result in consistently higher infiltration rates than
previously modeled in 2014 for CCR landfills.
EPA Response: The 2023 Draft Risk Assessment proposed that the
higher rates of infiltration modeled for certain soil types with the
new HELP data indicates the potential for higher leaching and risk to
groundwater than previously modeled in 2014. However, because EPA found
that the model results from the 2014 Risk Assessment are sufficient to
support the current rulemaking, the Agency did not conduct the
additional modeling that would be necessary to refine this draft
analysis. As a result, EPA does not rely on this particular analysis to
support the final rule and so it is not included in the 2024 Risk
Assessment.
Comment: Several commenters stated that consideration of a limited
subset of contaminants for groundwater modeling would result in an
underestimation of risk. These commenters further assert that EPA
further underestimated risk by not accounting for the effects of
cumulative exposure to multiple contaminants.
EPA Response: EPA disagrees that the selection of constituents for
groundwater modeling resulted in lower risks than would have otherwise
been identified. The constituents selected for groundwater modeling
were those found to be risk drivers for unlined surface impoundments in
the 2014 Risk Assessment, as these are considered the most likely to
also result in the greatest risks for unlined landfills and comparable
management units. EPA notes that some of the additional constituents
raised by commenters had been previously identified as risk drivers
only for specific CCR types, such as flue gas desulfurization (FGD)
wastes, which are considered far less likely to be used in CCRMU fills.
The commenters presented no new information that could alter the
previous model results and so there is no expectation that inclusion of
these additional constituents would identify risks higher than those
already modeled for the relevant CCR types. Some other additional
constituents raised by commenters lack health benchmarks within the
Office of Land and Emergency Management (OLEM) hierarchy and so could
not be quantitatively evaluated. See, 85 FR 72526. Uncertainties
associated with the selection constituents for modeling is further
discussed in Section 6 of the 2024 Risk Assessment.
Comment: Several commenters argued that a modeling horizon of up to
10,000 years was unrealistic. These commenters stated that such a long
time frame is not consistent with identifying a reasonable probability
of adverse effects because there is no reliable way to predict whether
any receptors will exist that far in the future.
EPA Response: EPA ran the groundwater model until either the
observed groundwater concentration at the receptor point reached a peak
and then fell below a model-specified minimum concentration (1 x
10-\16\ mg/L), or the model had been run for a time period
of 10,000 years. This is the same modeling horizon applied in the 2014
Risk Assessment. The text in the 2024 Risk Assessment has been updated
to make it clear that the selection of a maximum 10,000-year time
horizon does not mean that it typically took that long for
contamination be identified or that all model simulations continue for
the full 10,000 years. EPA also notes that the time to first exceedance
of selected risk criteria is typically considerably less than the time
to the greatest exceedance.
EPA acknowledges that future groundwater use patterns may shift
over time as the number and location of receptors changes, and that it
is unknown whether or how future shifts in receptor locations and other
surface conditions might affect risk. However, EPA notes that all the
contaminants associated with CCR are inorganic and so will remain
present in the environment over the full modeling horizon. As such, a
longer modeling horizon can provide useful information about the
potential duration of groundwater contamination in the absence of
regulation. EPA found that contaminant plumes modeled in MODFLOW did
not fully dissipate for around 2,300 years for arsenic V and 100 years
for molybdenum.
Comment: Multiple commenters argued that EPA was inconsistent with
the 2014 Risk Assessment and overestimated risks for CCRMU fills by not
evaluating the interception of groundwater by surface water.
EPA Response: EPA did not explicitly evaluate interception by
surface water on groundwater fate and transport in the

[[Page 38965]]

2024 Risk Assessment. As acknowledged by commenters elsewhere,
facilities have generally not maintained reliable records about the
location or construction of all CCRMU fills. As a result, it is not
possible for EPA to develop a representative, probabilistic
distribution of the distance from these fills to downgradient water
bodies or offsite receptors. However, given the diversity of reasons
for such placements listed by commenters, there are few limitations as
to where these fills might be located onsite. As a result, there is
greater potential for these fills to be located further away from water
bodies than disposal units, allowing for further contaminant spread
prior to any interception. Therefore, the 2024 Risk Assessment
evaluated the potential magnitude and extent of onsite groundwater
contamination that could occur in the absence of interception. It is
considered unlikely that further quantitative evaluation of
interception would affect the conclusions of the 2024 Risk Assessment.
The reductions in modeled risks attributed interception in the 2014
Risk Assessment were predominantly for median risks. However, the 2014
Risk Assessment still identified high-end risks to offsite receptors,
and it was these risks that formed the basis for the 2015 CCR Rule.
Thus, it is similarly unlikely that quantitative evaluation of surface
water interception would affect the high-end risks reported in the 2024
Risk Assessment, especially because the current assessment considers
onsite groundwater quality prior to discharge to a water body.
Furthermore, as discussed in the 2024 Risk Assessment and in response
to comments elsewhere, the fact that a contaminant plume that has
migrated off-site is intercepted by surface water does not mean that
there is no potential for risk or no need for further action to address
the presence of groundwater contamination onsite.
Comment: Some commenters requested clarification on the prevalence
of different types of liners modeled for the landfills and surface
impoundments previously excluded from the 2014 Risk Assessment. Citing
to data relied upon in the 2014 Risk Assessment, one commenter asserted
that a majority of modeled landfills had some form of liner and that
national regulations should be based on the risks for all units, rather
than those that are unlined.
EPA Response: The handling of liner status for these units was
described in Section 5 of the 2014 Risk Assessment. Of the units
evaluated in the 2014 Risk Assessment, approximately 42% of landfills
and 65% of surface impoundments were modeled as having no engineered
liner system. Of the previously excluded units summarized in the 2024
Risk Assessment, approximately 71% of landfills and 57% of surface
impoundments were modeled as having no engineered liner system. EPA has
updated the discussion of this issue in the 2024 Risk Assessment to
better distinguish the specific liner status modeled for these
different units. Differences in the national risks reported in 2014 and
2024 are largely attributed to the relative prevalence of engineered
liners modeled for each. Modeled risks in both assessments are nearly
the same for the subset of units with no engineered liner.
Far from being an isolated practice, a substantial fraction of the
currently operating landfills across the country have no engineered
liner. Although the 2014 Risk Assessment did model a majority of
landfills as having some form of engineered liner, data that has become
available since then indicates a greater proportion of operating units
lack an engineered liner than EPA previously understood. Furthermore,
the 2014 Risk Assessment modeled the performance of both clay and
composite liners based on the assumption of good construction
practices. However, it has become clear since then that some liner
systems do not perform as modeled. For example, facility reporting
shows that around 10% of composite and alternate-lined units have
already entered into corrective action. Therefore, it is considered
likely that national risks for both landfills and surface impoundments
(including the inactive landfills and legacy impoundments subject to
this final rule) are more similar to those unlined units than
previously modeled.
Nevertheless, the 2014 and 2024 Risk Assessments, which provided
much of the basis for this final rule, modeled the risks associated
with both lined and unlined units. Under RCRA sections 1008(a)(3) and
4004(a), EPA establishes national criteria; because the criteria are
national in scope EPA must evaluate the full range of conditions. In
addition, EPA must establish requirements that will achieve the
statutory standard at all sties subject to the criteria--including
those that pose the greatest risk. Under these provisions, the criteria
may authorize a CCR unit to continue operating ``only if there is no
reasonable probability of adverse effects on health and the environment
from the disposal [or other solid waste management] of solid waste at
such facility.'' 42 U.S.C. 6903(a)(3), 6944(a). Given the requirement
that the standard be met at each facility covered by the regulation, it
is not particularly surprising that the final requirements are driven
by the higher end risks associated with unlined units--especially as
the overwhelming majority of legacy impoundments and CCRMU are expected
to lack the composite liner that would largely mitigate the risks of
CCR units. But that does not mean that the national regulations are not
based on the risks for all units.
Comment: One commenter argued that modeled groundwater
concentrations and associated risk downgradient of smaller CCRMU fills
are unrealistic because they are higher than previously modeled for
landfills and surface impoundments. Other commenters contended that
modeled groundwater concentrations were unrealistic, citing comparisons
to monitoring data for all regulated units in a report by the
Environmental Integrity Project (EIP) \17\ or for some smaller subset
of units. These commenters calculated summary statistics from
concentrations reported for site groundwater monitoring wells to assert
that modeled concentrations were an order of magnitude higher or more
than the concentrations that have occurred in the field.
---------------------------------------------------------------------------

\17\ EIP. 2022. ``Poisonous Coverup: The Widespread Failure of
the Power Industry to Clean Up Coal Ash Dumps.''
---------------------------------------------------------------------------

EPA Response: The 2014 Risk Assessment modeled risks from landfills
and surface impoundments to receptors located up to a mile away from
these units. The 2024 Risk Assessment modeled the magnitude and extent
of contamination extending from smaller CCRMU fills, including the
likelihood of exceedance of GWPS at the waste boundary of the unit. It
is entirely reasonable that concentrations and risk closer to the waste
boundary are higher than EPA modeled in 2015 up to a mile away from a
unit.
EPA disagrees that the modeled groundwater concentrations are
contradicted by available monitoring data. First and foremost, EPA
modeled the long-term potential for groundwater contamination that may
occur in the absence of regulatory action. Thus, monitoring data from
units of variable age and operational status do not represent a one-to-
one comparison. Second, field monitoring data can diverge from model
results as a result of improper well installation. As just one example,
EPA is aware of multiple instances where monitoring wells are located
far from the waste boundary, in some cases, hundreds of feet away. See,
for example, 88 FR 55239. Third, EPA used EPACMTP to model

[[Page 38966]]

concentrations along the centerline of the plume and to provide a best
estimate of contaminant transport potential to inform further modeling
with MODFLOW. Even if all wells in a network were properly installed
and spaced, there is no guarantee that any individual well will
intersect with the exact point of highest concentration; some wells may
not intersect with the plume at all. Finally, the 90th percentile
concentration modeled is not intended to correspond precisely to a 90th
percentile of well concentrations. Instead, it reflects an RME scenario
that is conservative, while remaining within the range of possible
high-end exposures. The EIP dataset cited by commenters do show
multiple instances of well concentrations at individual landfills of
the same order of magnitude as modeled in the 2024 Risk Assessment or
even higher. Further, in the case of arsenic, modeled GWPS exceedances
between 26 and 19 for arsenic III and V are of a similar magnitude as
the exceedance of 16 estimated by one commenter based on the EIP
report. Therefore, EPA maintains that the magnitude of modeled
groundwater concentrations is realistic.
Comment: Some commenters claimed that EPA had not justified
modeling groundwater concentrations at fixed distances along the
centerline of the plume or within the upper five feet of the aquifer
and had not demonstrated how this approach compares with the 2014 Risk
Assessment, which modeled concentrations within the top 30 feet of the
aquifer.
EPA Response: The goal of modeling with EPACMTP was to identify the
potential magnitude of GWPS exceedances at the waste boundary and
potential for contaminant spread to support further modeling with
MODFLOW. For both goals, a sampling along the centerline of the plume
and to a depth of five feet was determined to be most relevant portion
of the aquifer for consideration for the reasons documented in the 2024
Risk Assessment. Because different scenarios were modeled in the two
risk assessments, a comparison with the results of 2014 Risk Assessment
is not relevant here.
Comment: EPA received several comments regarding a graph from the
2023 Draft Risk Assessment, which summarized modeled risks from the
2014 Risk Assessment for unlined landfills as a function of unit size.
Commenters stated that it demonstrated that risks consistently decline
below a certain acreage and that smaller units do not warrant
regulation because they pose less risk. One commenter stated that the
underlying model runs for the 2014 Risk Assessment were not made
available alongside the graph and so its validity could not be
confirmed.
EPA Response: One purpose of the referenced graph was to
demonstrate that risks remain above levels of concern over a broad
range of unit sizes modeled in the 2014 Risk Assessment. However, upon
further review, EPA has determined that the graph incorrectly
summarized model results for receptors of all age cohorts into one
figure. This has the potential to bias the plotted risks low. However,
filtering the model runs for only (1) unlined landfills, (2) where
drinking wells are located closer than surface water bodies, and (3)
where an adult was exposed results in a relatively small number of
model runs. EPA is concerned that this number of runs is not sufficient
to reflect national variability or support broader conclusions about
risk. As such, EPA does not rely on this line of evidence to support
the final rule and so it is not included in the 2024 Risk Assessment.
EPA cautions the data presented in the graph was for landfills and
so use of this graph to draw conclusions about the risks from surface
impoundments is not appropriate. EPA further cautions that it is not
appropriate to use the referenced graph to identify a specific unit
size below which landfill risks are not possible. The graph summarized
the results of the 2014 Risk Assessment, which modeled risks to offsite
receptors up to a mile away from the waste boundary. The risks
identified based on these receptors provided a robust basis for the
2015 CCR Rule. Yet, this does not mean these are the only relevant
risks. EPA's longstanding and consistent policy (across numerous
regulatory programs) has been that groundwater contamination is a
significant concern that merits regulatory action in its own right,
whether or not the aquifer is currently used as a source of drinking
water. The 2024 Risk Assessment identifies the potential for CCRMU
fills to contaminate groundwater above levels of concern. Where CCR
landfills and surface impoundments are located at the same sites even
more extensive contamination can occur as a result of their larger
size. As such, these disposal units warrant regulation to protect
groundwater resources, regardless of their size.
Comment: One commenter questioned why MODFLOW--Unstructured Grid
(USG) was used to model groundwater transport, stating that MODFLOW 6
is more commonly used. This commenter also inquired why the model was
not run in steady-state mode. They further argued that insufficient
information had been provided to allow for evaluation of the design of
MODFLOW model runs. Finally, the commenter identified a potential
discrepancy in the reported model inputs for EPACMTP and MODFLOW.
EPA Response: MODFLOW-USG was selected for its ability to: (1)
Simulate flow and transport in both the unsaturated and saturated zones
without the need for additional modeling packages and (2) Simulate
groundwater flow and transport sequentially without the need for
reading cell by cell flow and transport. Steady state simulations were
not used because they do not provide a time series representation of
plume evolution. EPA has reviewed the model documentation to ensure
that this and other relevant information raised by commenters was made
clear in the 2024 Risk Assessment. However, EPA notes that this and
much of the other specific information raised by commenters was
previously described in the 2023 Draft Risk Assessment. EPA did not
incorporate the full output files for all MODFLOW model runs because
the file size would become prohibitively large to manage. The level of
documentation of model inputs and outputs is consistent with that
provided for EPACMTP. The identified discrepancy between EPACMTP and
MODFLOW inputs were the result of a typo, which has been corrected.
Comment: One commenter stated that EPA had not provided sufficient
evidence to support its conclusion that the location of legacy
facilities that were not modeled in 2014 could result in somewhat
higher risks for this subset of units compared to those previously
modeled units.
EPA Response: EPA previously found that the locations of legacy
facilities were clustered in the eastern half of the country. As a
result, the rates of precipitation at these facilities will tend to be
higher than modeled for the nation as a whole. Higher precipitation can
result in greater vertical infiltration and subsequent leakage down to
groundwater. The Agency has not conducted further sensitivity analyses
to support this contention, as this argument is not central to the
findings of either the risk assessment or the rulemaking. Instead,
discussion in the 2024 Risk Assessment has been updated to clarify that
the primary finding is that there is no indication based on geography
that these additional units would be exposed to substantially different
environmental conditions than EPA modeled in 2014.
Comment: Multiple industry commenters argued that modeled

[[Page 38967]]

arsenic risks do not warrant regulation because the associated
concentrations often fall below the current maximum contaminant limit
(MCL). One commenter noted that 70 percent of runs identified peak
arsenic concentrations below the MCL at the unit boundary. In contrast,
environmental advocacy groups stated that cancer risks within the OLEM
risk range can occur at even lower levels. Another asserted it was
inappropriate for EPA to identify risk based on modeled concentrations
above GWPS because corrective action requires ``a statistically
significant level exceeding the groundwater protection standard.''
EPA Response: First, EPA notes that arsenic is only one of the
contaminants modeled. Molybdenum was found to be above the associated
GWPS on a more frequent basis. Indeed, EPA identified exceedances for
this contaminant at both the 90th and 50th percentile results. EPA
disagrees that risks identified below MCLs do not pose a concern. MCLs
are not purely risk-based and can incorporate other considerations,
such as the technical feasibility of reliably achieving even lower
levels. As environmental commenters have pointed out, the arsenic MCL
in particular represents a concentration that can fall outside the OLEM
risk range. As such, these standards should be understood as values
that corrective action must achieve and not levels that never warrant
concern. Indeed, EPA established GWPS at the unit boundary with the
intent to limit downgradient transport of contamination above this
level and prevent the same magnitude of risk identified in the risk
assessment.
EPA also disagrees that a statistically significant increase above
GWPS is an appropriate standard for risk modeling. It is not clear, nor
do commenters articulate, how such a statistical analysis would be
conducted as part of the model. Thus, EPA believes this comment
represents a general misunderstanding of both groundwater monitoring
programs and probabilistic analysis. Statistical analysis is used in
groundwater monitoring programs because factors, such as natural
fluctuations in groundwater and uncertainty from sampling or laboratory
analysis procedures, can introduce variability into the broader
dataset. In this context, statistical analysis allows evaluation of the
broader data and identification of an exceedance of GWPS with a
specified level of certainty. However, numerical models are not subject
to the same constraints. A model tracks the fate and transport of all
contaminant mass from the point of release to the point of exposure.
Therefore, no additional steps required to confirm that an identified
exceedance of GWPS resulted from leakage from the modeled unit.
Comment: Several commenters stated that the toxicity value used for
arsenic underestimated risks from groundwater, citing draft values they
assert would increase modeled arsenic risks by an order of magnitude or
more.
EPA Response: The Agency's current risk estimates are based on the
same cancer slope factor of 1.5 mg/kg/d-\1\ for arsenic in
EPA's Integrated Risk Information System (IRIS). EPA is currently in
the process of reviewing this slope factor and has released a draft
toxicological review, which, if finalized without revision, would
increase the individual risk estimates for arsenic by a factor of
approximately 35. See, 88 FR 71360. However, the Agency has not yet
finalized this updated IRIS reassessment, and EPA cannot base a final
decision on a draft IRIS value that is subject to revision. Nor did EPA
receive any other information during the development of this final rule
that would help to resolve this uncertainty. The current IRIS values
thus represent the best data available to the Agency until the IRIS
reassessment is complete.
d. Comments Related to Supplemental Risk Assessment Soil Model
Comment: One commenter contended that radionuclides and non-
radionuclides have different health endpoints and so it is not
appropriate to treat the resulting risks as additive.
EPA Response: EPA disagrees that it is inappropriate to consider
the cumulative risk from chemical and radiological contaminants. EPA
policy is to treat the risk resulting from exposure to multiple
carcinogens as additive.\18\ Agency policy is also to evaluate the
risks from exposure to radionuclides in the same manner as chemical
contaminants.\19\ Therefore, it is appropriate to evaluate the
cumulative cancer risk from chemical and radiation contaminants.
However, EPA notes that considering chemical and radiological risks
separately would not alter the overall conclusions of the analysis, as
each have demonstrated potential to individually result in risk
exceeding EPA's levels of concern. Uncertainties associated with
umulative risk is further discussed in Section 6 of the 2024 Risk
Assessment.
---------------------------------------------------------------------------

\18\ U.S. EPA. 1989. ``Risk Assessment Guidance for Superfund
Volume I Human Health Evaluation Manual (Part A).'' EPA/540/1-89/
002. Prepared by the Office of Emergency and Remedial Response,
Washington, DC. December.
\19\ U.S. EPA. 2014. ``Radiation Risk Assessment at CERCLA
Sites: Q&A.'' OSWER 9285.6-20. Prepared by the Office of Land and
Emergency Response. Washington, DC. June.
---------------------------------------------------------------------------

Comment: EPA received comments that argued the U.S. Geological
Survey coal quality (COALQUAL) database does not adequately account for
several factors that may affect bulk content of the resulting ash, such
as: CCR type, regional variability, coal rank, mining practices, coal
preparation prior to combustion, and the presence of unburnt carbon
remaining after combustion. Another commenter stated that because the
risk assessment addresses historical disposal of CCR, sampling of the
COALQUAL database should be updated to reflect production over time,
rather than current production. Finally, one commenter argued that
differences identified between activity calculated from COALQUAL data
and measured elsewhere in the literature demonstrates that handling of
COALQUAL data is likely to overestimate concentrations in the ash.
EPA Response: The Agency acknowledges that the bulk contaminant
content of specific CCR samples can be influenced by a range of
factors, such as the manner in which a coal sample is prepared and
combusted. As detailed in Section 6 of the 2024 Risk Assessment, EPA
considered the information provided by commenters on the potential for
mining practices, residual unburnt carbon, and coal washing to affect
estimated ash concentrations and concluded these factors are likely to
have a minimal or inconsistent effect on overall distribution of
concentrations. EPA did determine that concentrations of some
contaminants are sensitive to differences in region and coal rank and
so reviewed the Energy Information Administration (EIA) coal production
reports referenced by commenters when updating the weighting of
available samples.
For purposes of modeling groundwater exposure, EPA did not use the
COALQUAL database to estimate the leachable content of CCR in the 2024
Risk Assessment. Previous reviews of EPACMTP summarized in the 2014
Risk Assessment did not identify leachable content as among the
sensitive model parameters. Even at lower bulk concentrations, there is
often sufficient soluble mass present to support sustained leaching.
Instead, EPA represented leachable content using available LEAF data in
a manner

[[Page 38968]]

consistent with the 2014 Risk Assessment and Agency guidance.\20\
---------------------------------------------------------------------------

\20\ U.S. EPA. 2019. ``Leaching Environmental Assessment
Framework (LEAF) How-To Guide: Understanding the LEAF Approach and
How and When to Use It.'' SW-846 Update VII. Prepared by the EPA
Office of Land and Emergency Management. Washington, DC. May.
---------------------------------------------------------------------------

For purposes of modeling soil exposure, EPA retained use of the
COALQUAL database in the 2024 Risk Assessment to calculate the bulk
content of thorium and uranium of CCR. In this instance, use of
COALQUAL provides information about the relative levels of each
contaminant, which allowed for a more refined estimate of cumulative
exposure that provides a more direct comparison with relevant
benchmarks. As discussed in Section 6 of the 2024 Risk Assessment, EPA
also considered available EIA data when updating the calculation of
bulk content for these two contaminants and found that concentrations
of both are less sensitive than other contaminants to regional
geography. Therefore, further efforts to refine these calculations are
considered unlikely to result in changes that would affect the overall
conclusions of the evaluation.
The bulk contaminant content calculated from COALQUAL represents a
mixture of fly ash and either bottom ash or boiler slag, collectively
referred to in the 2024 Risk Assessment as the ``whole ash.'' Because
fly ash is generated in the greatest volumes during coal combustion,
the calculated bulk content primarily reflects this type of CCR.
However, other available data sources indicate that the activity of fly
ash and bottom ash are not substantially different. EPA has seen no
indication that the activity of boiler slag would differ markedly from
that of bottom ash. The whole ash does not include any CCR generated by
scrubber systems and similar pollution control technologies. However,
these CCR types are not considered relevant to the evaluation of CCRMU
fills. EPA further discusses the uncertainties associated with these
different types of CCR in Section 6 of the 2024 Risk Assessment.
Based on the comments received, EPA reviewed the available data on
radioactivity drawn from the literature. This review led to the removal
of several samples that were determined to be duplicative and removed
all the data for one study because it was determined to not be
representative of the broader ash generated at the facility.
Altogether, the data removed represent a small fraction of the overall
dataset. This review also identified some inaccuracies in how samples
were described and averaged to avoid biasing the overall dataset toward
individual facilities that reported a greater number of samples. This
had resulted in more samples being averaged together than was intended.
The database presented as part of the 2024 Risk Assessment has been
updated along with a summary of these updates. Following these
corrections, the updated summary statistics for thorium align more
closely with those calculated with COALQUAL. Therefore, there is
general agreement between these two datasets. It is inevitable there
will be some differences between datasets developed through different
methodologies. In particular, any individual study may not reflect the
full variability of coal produced over time. However, the magnitude of
differences between activities drawn from COALQUAL and the broader
literature are small on an absolute basis and consequently would not
affect the overall conclusions of the risk assessment. Therefore, EPA
concludes that COALQUAL can provide a reasonable estimate of both
median and high-end ash activity.
Comment: One commenter critiqued multiple individual model inputs
used in RESRAD as likely to overestimate potential for radon exposure.
They also stated that the risk assessment should consider an additional
scenario with RESRAD of CCR disposed at the ground surface to provide a
consistent frame of reference to compare risk results obtained from
RESRAD and the preliminary remediation goal (PRG) calculator. Other
commenters separately commented that the assumed presence of some soil
cover is inappropriate, referencing one CCRMU purported to have been
placed with the intent to level out the ground surface and without any
additional soil cover.
EPA Response: EPA has not established default parameters for
modeling of radon fate and transport. Nor is there currently enough
information available on a national scale to develop distributions that
could be sampled probabilistically. Instead, EPA previously conducted a
deterministic analysis for radon exposure by specifying high, moderate,
and/or low values for model inputs to capture the range of potential
exposure. EPA first modeled risk with all inputs set to moderate values
to identify a baseline risk more representative of the central
tendency. From this baseline, EPA adjusted each individual input to
lower or higher values to better understand which inputs exert the
greatest influence on modeled risks and support development of an RME
scenario. However, EPA ultimately concluded that the rate of radon
emanation from CCR is not distinguishable from background soil and so
the Agency did not develop this RME scenario or draw final conclusions
about risk from radon exposure. For this same reason, EPA did not
retain the quantitative evaluation of radon in the 2024 Risk
Assessment.
Some CCRMU fills may currently be uncovered, but EPA was not able
to confirm the status of the specific unit identified by the commenter
based on the information provided. Nevertheless, EPA maintains it is
unlikely that future residential construction would occur in the
absence of some initial soil cover. It is generally anticipated
residential construction sites will cover any exposed land with topsoil
or turf to support uniform lawn growth. However, this does not
guarantee this soil cover will be adequately maintained by residents
into the future. As such, EPA agrees it is appropriate to evaluate a
scenario of CCR without any soil cover to provide a bounding estimate
of potential risk and a more direct link between the primary and
sensitivity analyses. This updated scenario is discussed in Section 6
of the 2024 Risk Assessment.
Comment: Some commenters raised concerns about the sensitivity
analysis conducted with the PRG calculator. One commenter asserted that
the PRG calculator is intended for use with contaminated soils and is
inappropriate for comparison against undiluted CCR. This commenter
further argued that the sensitivity analysis conducted with the PRG
calculator is overly generic and did not incorporate scenario-specific
inputs, such as the potential for greater soil cover, shorter exposure
duration, and ability of radon to emanate from CCR. Finally, this
commenter stated that the degree of mixing of soil with CCR would not
result in activities higher than either background or applicable or
relevant and appropriate requirements (ARARs), concluding that the
evaluation of radiation risk should consider contributions from
background soils when presenting risk results. Another commenter stated
that the ARAR was only exceeded around the 90th percentile
concentrations and that regulation based on 90th percentile
concentrations is not appropriate.
EPA Response: EPA disagrees that the PRG calculator is not
applicable to the modeled scenario of CCR intermixed with soil. The
commenters provide no rationale for this assertion beyond the fact that
the PRG calculator nominally identifies soil as an environmental media
of interest. This is reasonable as it would quickly become overwhelming
to identify a comprehensive list of sludges, sediments, and other soil-
like

[[Page 38969]]

materials that might be encountered at cleanup sites. EPA notes that
the exposure assumptions incorporated into the PRG calculator are
equally relevant for CCR intermixed with soil. CCR consist of small
particulates that can be readily intermixed with the soil and result in
exposures through the exact same routes, specifically incidental
ingestion and direct exposure to gamma radiation.
EPA also disagrees that the analysis of exposure to CCR mixed with
soil is overly simplistic. First, the presence of additional cover soil
is already considered in the main analysis and is not relevant to the
types of exposures explicitly considered in the sensitivity analysis.
Second, because EPA concluded the rate of radon emanation from CCR and
soil were not distinguishable, the sensitivity analysis explicitly does
not incorporate risk from inhalation of radon gas. Only a relatively
small fraction of the radon generated from fly and bottom ash is
expected to escape into the ambient air and these losses can be
counteracted by upward migration from deeper ash. Therefore, it is
unlikely that further consideration of radon emanation would have
substantial impacts on exposures through incidental ingestion or direct
gamma exposure. Third, the model parameters used to characterize
exposure to gamma radiation in the PRG calculation are generally the
same as in RESRAD and other available models. Finally, exposure factors
selected for use in the PRG calculator are consistent with Agency
policy for characterizing an RME scenario and many of the remaining
parameters are based on extensive modeling.21 22 23
---------------------------------------------------------------------------

\21\ Oak Ridge National Laboratory. 2014. ``Area Correction
Factors for Contaminated Soil for Use in Risk And Dose Assessment
Models.'' ORNL/TM-2013/00. Oak Ridge, TN. September.
\22\ Oak Ridge National Laboratory. 2014. ``Gamma Shielding
Factors for Soil Covered Contamination for Use in Risk and Dose
Assessment Models.'' ORNL/TM-2013/00. Oak Ridge, TN. September.
\23\ Oak Ridge National Laboratory. 2020. ``Bateman Equation
Adaptation for Solving and Integrating Peak Activity into EPA ELCR
and Dose Models.'' ORNL/TM-2020/1780. Oak Ridge, TN. September.
---------------------------------------------------------------------------

EPA generally only considers contributions from disposed wastes to
risk when conducting national risk assessments under RCRA. Background
concentrations may contribute to risk when present and can sometimes be
higher than concentrations modeled in a risk assessment. Although
constituent concentrations in undisturbed environmental media can be
highly variable, they are often relatively low in concentration. As a
result, consideration of these concentrations would generally have no
impact on the overall conclusions of a national risk assessment.
Therefore, consideration of background concentrations is more
appropriate on a site-specific basis when risk managers are determining
the need for and scope of corrective action. EPA recognizes that a
focus on background is more common for discussion of radioactivity,
particularly when providing context for the associated risks to the
broader public. However, as one point of reference, EPA has found that
the median activities of fly and bottom ashes already fall close to the
standard of 5 pCi/g radium-226+228 above background soil, which has
been adopted as an ARAR for some cleanups under Superfund and State
programs (i.e., around 4.3 pCi/g higher).\24\ Additionally, EPA has
found that high-end radium-226+228 activity in CCR has the potential to
be nearly 10 pCi/g higher than typical background soil. Thus, there is
real potential for mixing of CCR with soil to further increase any risk
already associated with background.
---------------------------------------------------------------------------

\24\ U.S. EPA. 1998. ``Use of Soil Cleanup Criteria in 40 CFR
part 192 as Remediation Goals for CERCLA Sites.'' OSWER Directive
9200.4-25. Office of Emergency and Remedial Response and Office of
Radiation and Indoor Air. Washington, DC. February.
---------------------------------------------------------------------------

Commenters are correct that mixing small quantities of CCR with
soil may not result in a surface soil activity above the ARAR. For
high-end CCR activity, this would require a roughly equal mixture of
soil and ash. However, risks are still possible at activities below the
ARAR. The PRG calculator estimates that an increase of only 1.13 pCi/g
of the thorium-232 decay chain or 1.45 pCi/g of the uranium-238 decay
chain in surface soils could increase cancer risk for residential
receptors by 1 x 10-\4\. Such risks can result from
relatively low mixtures of CCR and soil, which are possible if ash
beneath the soil surface is disturbed. As a result, EPA has identified
ARAR of 5 pCi/g above background as equally applicable to subsurface
contamination that may be disturbed in the future and concluded ``it
would not generally be appropriate to allow backfilling with material
with concentration higher than 5 pCi/g.'' Uncertainties associated with
background concentrations are further discussed in Section 6 of the
2024 Risk Assessment.
Comment: One industry commenter presented an analysis they had
conducted comparing the concentrations of certain inorganic
constituents in CCR to soil screening levels. The commenter contended
this analysis demonstrated that ``even daily direct contact to trace
elements in coal ash would not pose a significant risk to human
health.''
EPA Response: EPA did not evaluate the potential soil risks for
human health associated with many of the constituents considered in the
cited analysis. The Agency believes that any risk from additional
constituents would be mitigated by the rule requirements that address
the risks identified for radionuclides. However, EPA notes that the
cited analysis is not sufficient to demonstrate a lack of risk for
these additional constituents on a national scale. The ash
concentrations reported for some constituents are already near or above
the health benchmarks, indicating some potential for risk. Further, the
reported ash concentrations are based on samples from a limited number
of geographically constrained facilities. As a result, the reported
concentrations may not reflect the broader variability of potential
concentrations from across the region or country. In particular, EPA
notes there is evidence in the regulatory record of arsenic
concentrations approaching an order of magnitude higher than considered
in this analysis.
Comment: Several commenters argued that EPA underestimated risk by
not considering other potential exposure pathways, specifically
inhalation of loose CCR.
EPA Response: EPA selected direct exposure gamma radiation and
incidental ingestion of soil as the pathways for evaluation because
these represent the most direct routes of exposure to contamination in
the soil. EPA agrees that inhalation is another pathway through which
future receptors could be exposed if CCR becomes intermixed with
surface soil. Quantitative evaluation of this pathway would require
additional model inputs that could further increase the uncertainty of
results on a national scale, such as the degree of vegetative cover and
mean wind speed. However, EPA notes the default PRGs for inhalation of
the uranium-238 decay chain in secular equilibrium is nearly three
orders of magnitude higher than for external exposure to gamma
radiation and two orders of magnitude higher than for incidental
ingestion of soil. As a result, it is unlikely consideration of this
pathway would substantially increase calculated risk. Therefore, this
pathway does not represent a major source of uncertainty in the
evaluation. EPA acknowledges that there may be other exposure pathways
that could occur if CCR is mixed with surface soil. These are further
discussed in Section 6 of the 2024 Risk Assessment.

[[Page 38970]]

e. Comments Related to Site Monitoring Data
Comment: Some commenters stated that, as part of any further risk
assessment efforts, EPA should incorporate data that have been
collected as part of the monitoring programs required by either the
2015 CCR Rule or prior State programs. Such data might include site
hydrogeology from borings around the units and groundwater quality
sampled from monitoring wells. These commenters claimed these data are
more recent and more relevant to characterizing the actual nature and
extent of contaminant release at individual sites.
EPA Response: There are multiple reasons why it is neither
practical nor prudent to incorporate site-specific monitoring data into
national fate and transport modeling. First, there are documented
concerns about the quality and reliability of these data. For example,
EPA has identified significant deficiencies in the monitoring networks
at each facility for which the Agency has completed reviews under the
Part A (85 FR 53516, August 28, 2020) and Part B (85 FR 72506, November
12, 2020) Rules. It is unlikely such deficiencies are isolated to this
specific subset of facilities. Monitoring wells that are located too
far apart, installed in the wrong aquifer, or otherwise inadequately
installed would result in data that are incomplete or unrepresentative
of relevant site conditions. Thus, use of these data would require
thorough review prior to use. Much of the site characterization data
are not required to be posted on facility websites and so would take
substantial time to compile and review for the over 1,000 individual
landfills and surface impoundments. Further, it is highly unlikely that
any identified deficiencies could be remedied within a reasonable
timeframe.
Second, the hydrogeologic data that have been collected in support
of well installation can provide an incomplete or erroneous picture of
site conditions for the purpose of fate and transport modeling. For
example, at sites with lower conductivity soils, EPA has previously
raised concerns that collection of hydrogeologic data with a focus on
characterizing the predominant soil type can underestimate the
prevalence of more localized deposits of higher conductivity soil and
other discontinuities that can serve as preferential flow pathways to
groundwater. See, 85 FR 72519. Therefore, the current approach to
probabilistic characterization of soil and aquifer characteristics
using more local data sources is believed to provide the most reliable
means to capture the potential variability of conditions across
different facilities and represent contaminant fate and transport on a
national scale. Furthermore, EPA notes that consideration of more site-
specific data would not be expected to change the fact many units are
known to be constructed on relatively permeable soils. As a result,
further refinements on the hydrogeology modeled at each individual site
is unlikely to alter overall model results, which show contaminants can
escape from these units and spread considerable distances through
groundwater.
Third, groundwater monitoring only provides a snapshot in time of
groundwater concentrations at each well location. It is not obvious,
nor do commenters articulate, how these data would be applied to model
long-term unit leakage. Factors such as natural fluctuations in
background groundwater concentrations make it difficult to apportion
measured concentrations from individual sampling events into the
specific contributions from background and unit leakage. That is why
groundwater monitoring programs rely on statistical analysis of data
across numerous sampling events to make a binary determination whether
or not contaminant concentrations downgradient of a unit have increased
above background and GWPS. Even if it were practical to utilize these
monitoring data, groundwater samples do not provide broader information
about the progression of leakage over time. Specifically, groundwater
samples do not provide information on the magnitude of source leachate
concentrations, how long the unit has been leaking, or any indication
of the potential magnitude and extent of contamination in the future.
EPA modeling previously showed that the magnitude and extent of a plume
may not peak until decades or centuries after the unit first beings to
leak. As a result, incorporation of groundwater monitoring samples into
a model would require a number of additional assumptions about the site
characteristics and conditions that could substantially increase the
overall uncertainty of model results.
Finally, EPA is not aware of similar site-specific data available
for the subset of smaller CCRMU intended for purposes other than
disposal. As several commenters have acknowledged, facilities have not
typically maintained reliable records of the locations of all these
smaller units. Thus, any modeling of these units must, by necessity,
draw on other datasets to characterize the potential for environmental
release and subsequent contaminant fate and transport.
f. Comments Related to Additional Risk Drivers
Comment: Multiple commenters asserted that risks higher than those
modeled in the 2014 Risk Assessment are unlikely for landfills. One
commenter stated that the previous risks modeled for unlined landfills
are ``only slightly above'' the point of departure at 2 x
10-\5\ and so, even if most CCRMU landfills are unlined, it
would not result in risks higher than this value.
EPA Response: The national risks reported in the 2014 Risk
Assessment were based on the understanding of relative liner prevalence
at the time of that assessment. However, it has since become clear that
an even greater proportion of regulated unit have no engineered liner
and there is no evidence that CCRMU landfills are lined to any greater
degree. Additionally, EPA notes that the 2014 Risk Assessment modeled
both clay-lined and composite-lined units under the assumption of good
construction practices that achieved the regulatory performance
standard. However, it has become clear since then that some liner
systems do not achieve this standard. For example, facility reporting
shows that around 10% of regulated units with composite or alternate
liners have already entered into corrective action. Therefore, even for
those units that do have some form of engineered liner, there is
potential for national risks to be higher than previously modeled. For
all these reasons, national risks for both currently regulated and
CCRMU landfills are only expected to be more similar to those
previously modeled for unlined landfills. Furthermore, EPA has
identified additional factors that have the potential to result in even
higher risks than modeled, but that could not be fully quantified as
part of either the 2014 or 2024 Risk Assessment. These include co-
disposal with coal refuse and disposal in contact with the water table.
The greater prevalence of unlined units makes it even more likely these
additional factors will occur at unlined units. The combination of
these factors has the potential to result in national risks even higher
than previously modeled.
Comment: One commenter acknowledged that the 2014 Risk Assessment
had demonstrated the potential for co-disposal with coal refuse to
increase risk from surface impoundments. However, multiple others
argued that the same assessment shows that neither co-disposal with
coal

[[Page 38971]]

refuse nor extreme pH conditions increase risks for landfills.
Specifically, commenters pointed to one sensitivity analysis summarized
in Table 5-6 of the 2014 Risk Assessment that concluded modeled risks
did not exceed the point of departure for any subset of the modeled pH
conditions. One commenter argued the Agency's conclusions are not based
on actual observations of CCR porewater and groundwater quality at
sites where coal refuse is managed. This commenter stated that not all
units that accepted coal refuse will contain enough to affect the
broader chemistry of the unit and not all coal refuse will contain
enough pyrite to influence pH. This commenter further argued that,
where acidic conditions and higher leachate concentrations do occur, it
will not necessarily result in higher downgradient groundwater
concentrations due to other site-specific factors. To support this
argument, the commenter summarized findings from multiple EPRI reports
that analyzed field samples from around several landfills and surface
impoundments believed to have accepted coal refuse.
Response: These commenters misrepresent the findings of the
referenced sensitivity analysis. This analysis represents a parsing of
groundwater model runs conducted in 2014 as a function of leachate pH.
This analysis incorporates model results for a substantial number of
lined units, which can mask the effects of leachate pH due to the low
overall leakage rates from these units. As such, this sensitivity
analysis does not support any conclusions about the impacts of pH on
risks from unlined units. Further, very few model runs were conducted
at highly acidic pH; the sensitivity analysis did not summarize any
results for a pH lower than around 4. Thus, this analysis also does not
support any conclusions about the risks associated with highly acidic
conditions.
Available LEAF leachate data used to model landfills show that many
constituents, including arsenic, can leach at highest concentrations
near one or both extremes of the pH scale. The effects of these higher
concentrations are reflected in the sensitivity analysis, with higher
risks observed around a highly basic pH of 13. Therefore, this
sensitivity analysis is consistent with the broader risk record and
shows that extreme pH conditions can result in higher risk.
The commenters are also incorrect that the risk record is not based
on observations of CCR porewater. EPA relied on empirical measurements
of porewater to support modeling of surface impoundments in 2014, which
included samples co-disposed with coal refuse. As acknowledged by some
commenters, these data supported identification of higher risks from
these co-disposed wastes in impoundments. Corresponding pH data are not
available for every porewater sample, but available data do show the
potential for highly acidic pH around 1, roughly equivalent to stomach
acid. The cited EPRI reports do not contradict the finding that co-
disposal can affect CCR leaching behavior. As summarized by the
commenter, these reports found that a third of units had impacts to
unit pH and porewater chemistry. Individual units had potential or
confirmed impacts on groundwater quality, causing at least one to
trigger remedial measures by the facility. EPA further notes that these
reports provide only a snapshot in time of the environmental impacts
associated with disposal in this subset of disposal units. As a result,
there remains potential for future releases beyond the waste boundary
if these conditions persist.
Comment: Multiple commenters asserted that waste disposed below the
water table would not result in higher risks from surface impoundments
than previously modeled in the 2014 Risk Assessment. These commenters
generally argued the hydraulic head present in an operating impoundment
from ponded wastewater will result in greater leakage than groundwater
flowing through a unit. One commenter presented a hypothetical
comparison of the relative hydraulic flux from a unit due to ponded
water, infiltrating precipitation, and contact with groundwater to
argue that the presence of a ponded water would result in higher
leakage. Others pointed to analyses from the 2014 Risk Assessment,
which compared leakage from surface impoundments before and after
dewatering, to argue that risks from impoundments remaining in
groundwater would be lower. Others further argued that the lower
hydraulic conductivity of some ash would limit flow through the
impoundment and cause groundwater to preferentially flow around the
unit.
Several commenters presented data from groundwater monitoring
conducted at individual units to assert that risks are more likely to
result from the hydraulic head in active impoundments than the
intersection of waste with the water table. The presented data depict
concentrations of boron, a highly soluble constituent that one
commenter noted was selected for its ``insensitivity to redox
conditions.'' These plots generally show concentrations of boron to
decrease over time after the impoundments were taken out of service,
though that pattern was not universal. Some commenters went further,
concluding that eliminating the hydraulic head in the unit would allow
any prior groundwater contamination to naturally attenuate. Conversely,
other commenters pointed to a documented case study where groundwater
concentrations increased after ponded water was drained to contend that
contact with the water table can result in higher releases.\25\
---------------------------------------------------------------------------

\25\ EPRI. 2001. ``Evaluation and Modeling of Cap Alternative at
Three Unlined Coal Ash Impoundments.''
---------------------------------------------------------------------------

EPA Response: A number of the commenters misconstrue the findings
of the 2014 Risk Assessment, which did not include any assessment of
the effects of CCR disposal within the water table. EPA was unable to
quantitatively model the risks associated with this management practice
because there was little data on how common the practice was or the
extent to which it would affect groundwater chemistry. Instead, these
commenters are referring to a comparison of the risks resulting from
surface impoundments during operation and post-closure (i.e., after
free liquids had been eliminated consistent with Sec.
257.102(d)(2)(i)) that was undertaken to understand if only modeling
these units only during operation might underestimate peak risks. EPA
only concluded that continued leakage after elimination of free liquids
and closure would rarely result in higher peak risks. Thus, this
assessment did not consider the effects of disposal below the water
table or draw any conclusions about the risks associated with this
practice.
When waste is managed above the water table, any leakage out of the
unit must first infiltrate down through unsaturated subsurface soils
and then mix with groundwater before it can flow beyond the waste
boundary. As a result, downgradient groundwater concentrations can end
up substantially lower than the original leachate concentration. In
contrast, when waste is disposed below the water table, the entire
volume of groundwater in contact with the CCR and all water
infiltrating from above would become undiluted leachate. As the
thickness of CCR below the water table increases, the volume of
leachate generated can increase substantially based on the sheer size
of these disposal units. There is no evidence the properties of CCR
would reliably limit transport of this leachate away from the unit.
Rather, the hydraulic gradient of the aquifer will continue to drive
continued flow

[[Page 38972]]

through the unit. The hydraulic conductivity of different CCR overlaps
with that of common aquifer materials. Even in instances where the
average conductivity within a unit is lower than the surrounding
aquifer, these units often contain different ash types and other
wastes. This can lead to stratification within the unit that creates
regions of higher conductivity and allows for greater flow. For all
these reasons, there is potential for sustained leakage from units when
waste is disposed below the water table. Whether or not the magnitude
of this continued leakage is greater than from water ponded in an
impoundment does not address the potential for such leakage to cause a
release or sustain one that began when water was still ponded in the
unit. Such comparisons also ignore that the waste would also be in
contact with groundwater while the unit operates, greatly increasing
the likelihood of groundwater mounding around the impoundment and
increased contaminant transport in all directions.
It is not feasible to draw conclusions based on the small and
curated sample of units presented by commenters. Various factors can
complicate any interpretation of the presented graphs. First, boron is
a highly soluble constituent that can washout at high concentrations
into small amounts of water. Thus, the extent to which decreases in
concentration over timeframes of a long as a decade or more simply
represent the depletion of this highly soluble constituent from the ash
is unclear. Second, unit geometry may not be uniform and consistently
intersect with the groundwater table, resulting in more spatially
isolated releases that cause higher concentrations in some wells and
not others. Third, at sites with intermittent contact with groundwater,
predefined sampling dates may not align with periods when contact with
groundwater occurs. Therefore, it is not possible to draw meaningful
conclusions, either at these sites or more broadly, based on the data
provided. As pointed out by other commenters, there are also examples
available where sustained contract with groundwater after a unit is
drained resulted in increased groundwater concentrations of other
Appendix III constituents.
The fact that downgradient concentrations have decreased at some
impoundments after the unit was drained despite ongoing contact with
groundwater does not prove such reductions will be sustained or further
groundwater releases will not occur. As one EPRI report concluded,
``the existence of saturated ash will greatly reduce the effectiveness
of any cap design when the facility is underlain by geologic materials
with high hydraulic conductivity, because groundwater will continue to
leach ash constituents.'' \26\ Thus, removal of ash from groundwater
may be the only reliable means of source control for these units.
---------------------------------------------------------------------------

\26\ EPRI. 2001. ``Evaluation and Modeling of Cap Alternative at
Three Unlined Coal Ash Impoundments.''
---------------------------------------------------------------------------

Comment: Several commenters agreed that use of porewater to
represent leakage from impoundments is appropriate. However, these
commenters also raised concerns that available porewater data collected
during the active life of an impoundment may underestimate the risks
associated with legacy impoundments because it may not accurately
reflect leachate concentrations after the unit has ceased operation. As
one example, they cited potential for reducing conditions to form
through prolonged contact between waste and groundwater.
By contrast, one commenter asserted that elevated arsenic
concentrations identified in the two journal articles EPA referenced in
the proposal are only representative of that one site and that the
majority of available impoundment porewater data have lower
concentrations than reported in those articles.27 28 The
commenter also noted the data presented in the journal articles were
collected in support of an EPRI report, which found these
concentrations had not translated to exceedances of GWPS in
downgradient wells.\29\ Based on this finding, the commenter concluded
leachate concentrations alone are not a reliable indicator of which
units will cause groundwater contamination due to variable site
geochemistry and hydrogeology.
---------------------------------------------------------------------------

\27\ Wang, X., A.C. Garrabrants, Z. Chen, H.A. van der Sloot,
K.G. Brown, Q. Qiu, R.C. Delapp, B. Hensel, and D.S. Kosson. 2022.
``The Influence of Redox Conditions on Aqueous-Solid Partitioning of
Arsenic and Selenium in a Closed Coal Ash Impoundment.'' Journal of
Hazardous Materials. 428:128255.
\28\ Wang, X., H.A. van der Sloot, K.G. Brown, A.C. Garrabrants,
Z. Chen, B. Hensel, and D.S. Kosson. 2022. ``Application and
Uncertainty of a Geochemical Speciation Model for Predicting
Oxyanion Leaching from Coal Fly Ash under Different Controlling
Mechanisms.'' Journal of Hazardous Materials. 438:129518.
\29\ EPRI. 2020. ``Leaching, Geotechnical, and Hydrologic
Characterization of Coal Combustion Products from a Closed Coal Ash
Impoundment.''
---------------------------------------------------------------------------

EPA Response: EPA agrees that porewater samples remain the best
available data to represent leakage from operating surface
impoundments. These field samples provide empirical data on leakage
from various mixtures of CCR and other wastes managed under
consistently saturated conditions. EPA also acknowledges there can be
uncertainties associated with field data submitted to the Agency, which
might lead to an underestimation of concentrations in the field. One
example is the potential for stronger reducing conditions to form after
a unit has been closed as a result of less oxygenated water
infiltrating through the unit. As acknowledged by commenters, however,
there is not sufficient data to characterize the magnitude or extent of
such conditions on a national basis. Therefore, the impact of this
uncertainty is not known.
EPA disagrees that the arsenic concentrations identified in the
referenced studies should be considered an isolated occurrence. These
studies clearly demonstrate that: (1) Sustained contact with
groundwater can result in stronger reducing conditions than dry
management, (2) Reducing conditions can cause higher leaching of
arsenic, and (3) LEAF methods can underestimate actual leaching from
CCR under reducing conditions by as much as an order of magnitude.
Given that disposal beneath the water table is a more common practice
than previously understood, there exists the real potential for higher
leachate concentrations in the field than previously modeled,
particularly at landfills modeled with LEAF data.
EPA does agree that initial leachate concentrations are not the
sole determining factor for contaminant fate and transport. As
discussed in response to previous comments, this fact is reflected in
Agency modeling. Individual model runs with the highest leachate
concentrations are not always those with the highest risk. However,
factors that will tend to push the overall distribution of leachate
concentrations higher will also tend to push modeled nationwide risks
higher because of the greater likelihood that higher leachate
concentrations will occur at sites where these concentrations can more
readily spread. Thus, the greater prevalence of units in contact with
groundwater has the potential to result in higher risks on a national
scale than previously modeled.
Finally, EPA notes that groundwater monitoring only represents a
snapshot in time and does not necessarily provide any indication of the
potential for future contamination. In the case of the studied unit, it
is not known whether reducing conditions formed during or after
operation. As such, there remains potential for future releases if the
unit remains in contact with groundwater

[[Page 38973]]

and continues to leak such elevated arsenic concentrations.
g. Comments Related to Complete Exposure Pathways
Comment: Multiple commenters asserted that EPA must demonstrate the
existence of a complete exposure pathway to justify regulatory action,
which some defined as exposures that have already occurred.
Specifically, commenters stated that ``the presence of groundwater
contamination alone does not constitute a risk'' and ``in many cases no
one is drinking the water or contacting the CCR materials.'' One
commenter presented a summary of analyses that had been conducted
across 27 sites, which concluded that groundwater risks do not exist at
most sites because no drinking water wells are currently present.
Another commenter asserted that the high-end risks identified in the
2014 Risk Assessment assumed that receptors were exposed immediately
downgradient of the disposal units. This commenter went on to state
that complete exposures would not occur at the many sites adjacent to
water bodies because groundwater contamination would be intercepted by
surface water first and that the 2014 Risk Assessment found no risks
warranting regulation for surface water. Several other commenters also
claimed that groundwater quality should be measured at the facility
boundary because that would be more representative of a complete
exposure pathway.
EPA Response: Section 4004(a) of RCRA requires EPA to establish
requirements that will ensure no reasonable probability of adverse
effects both to human health and the environment. See, 42 U.S.C.
6944(a). EPA therefore disagrees that only the presence of receptors
within the impact sphere of a contaminating facility merits
consideration. EPA's longstanding and consistent policy (across
numerous regulatory programs) has been that groundwater contamination
is a significant concern that merits regulatory action in its own
right, whether or not the aquifer is not currently used as a source of
drinking water.
Once a potentially harmful constituent has leached from a disposal
unit into groundwater, whether the constituent ultimately causes
further damage by migrating into drinking water wells does not diminish
the significance of the environmental damage caused to the groundwater
under the site, even where it is only a potential future source of
drinking water. As EPA explained in the preamble to the original 1979
subtitle D criteria, EPA is concerned with groundwater contamination
even if the aquifer is not currently used as a source of drinking
water. Sources of drinking water are finite, and future users'
interests must also be protected. See, 44 FR 53445-53448. (``The Act
and its legislative history clearly reflect Congressional intent that
protection of groundwater is to be a prime concern of the criterion. .
. . EPA believes that solid waste activities should not be allowed to
contaminate underground drinking water sources to exceed established
drinking water standards. Future users of the aquifer will not be
protected unless such an approach is taken.''). See also, 80 FR 21453.
The commenters' approach is also inconsistent with Agency guidance,
which states that a ``. . . pathway is complete if there is (1) a
source or chemical release from a source, (2) an exposure point where
contact can occur, and (3) an exposure route by which contact can
occur.'' \30\ The guidance goes on to state that ``. . . exposure
assessments are concerned with current and future exposures.'' Thus, a
key consideration in evaluating risk is the potential for future
exposure. If it were necessary to wait for exposures to occur as a
prerequisite for action, an untold number of receptors could be subject
to potential harm. Further, implementation of corrective action is not
instantaneous and so this harm could persist for some time after
receptor exposures are first identified. Commenters do not explain how
such delayed action could be considered protective of human health and
the environment, and so meet RCRA's standard. See, USWAG, 901 F3d at
429-431.
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\30\ U.S. EPA. 1989. ``Risk Assessment Guidance for Superfund
Volume I: Human Health Evaluation Manual (Part A).'' EPA/540/1-89/
002. Prepared by the Office of Emergency and Remedial Response.
Washington, DC. December.
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Commenters also misrepresent the findings of the 2014 Risk
Assessment regarding surface water interception. EPA modeled a
distribution of distances for both groundwater wells and surface water
bodies, accounting for interception whenever a water body was located
closer than a well. Thus, reported high-end risks do not include any
assumptions about the proximity of receptors to the units. Even if
direct exposure to groundwater from use as a drinking water source is
considered unlikely due to the potential for interception by nearby
surface water, that does not justify no further action. EPA did
identify the potential risks from individual disposal units to
ecological receptors present in these water bodies and human receptors
who fish from those water bodies, as well as associated damage cases,
which is why constituents, such as cadmium and mercury, were added to
the Appendix IV list of constituents. Additionally, surface water
bodies are large and highly interconnected systems that are likely to
have multiple electric utilities, as well any number of other
industrial sources, located along their banks. If all these facilities
were allowed to freely discharge to a water body solely because no
individual release posed risk, the cumulative impacts can result in
risk to surface water resources and nearby receptors. The 2015 CCR Rule
addresses the potential for such risk by specifying corrective action
must ``remove from the environment as much of the contaminated material
that was released from the CCR unit as is feasible.'' 40 CFR
257.97(b)(3). Thus, dilution of a groundwater plume into surface water
could not be considered a presumptive remedy. This requirement is
consistent with guidance for OLEM programs that specify the need to
prevent groundwater contamination above GWPS from contaminating other
aquifers or environmental media.\31\
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\31\ U.S. EPA. 2009. ``Summary of Key Existing EPA CERCLA
Policies for Groundwater Restoration.'' OSWER Directive 9283.1-33.
Prepared by the Office of Solid Waste and Emergency Response.
Washington, DC. June.
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EPA also disagrees that a point of compliance at the facility
boundary would provide a better estimate of actual risk than the waste
boundary. Again, the commenter disregards that the contamination of the
aquifer is an adverse effect on the environment, not simply a potential
risk to subsequent receptors. Consequently, the regulations require
facilities to address the contamination at the first available point,
that is, when it first leaves the unit. There are several additional
reasons that the waste boundary is the appropriate point of compliance.
First, a point of compliance at the facility boundary would result in
greater potential for current residences or water bodies immediately
adjacent to the facility boundary to be exposed before the presence of
contamination can be confirmed. Second, the facility boundary may be a
significant distance away from the waste boundary, which would allow
contamination to increase and spread for some time before triggering
corrective action. The further contamination is allowed to increase and
spread, the more difficult it may become to clean it up due to factors
such as complex contaminant chemistry and site hydrogeology. This may
render

[[Page 38974]]

large volumes of groundwater unusable for drinking water or other
purposes. Finally, EPA has previously documented numerous instances
where, once the contaminant plume has migrated off-site and impacted
private water wells, a utility has purchased these properties, thereby
rendering the off-site contamination, ``on-site,'' further delaying
corrective action. See, 80 FR 21456. For all these reasons, EPA
considers the waste boundary to provide the most consistent and
protective basis on which to establish evidence of a release.
4. 2024 Final Risk Assessment
EPA identified risks to groundwater from active CCR landfills and
surface impoundments, as well as to inactive CCR surface impoundments
at active utilities in the 2014 Risk Assessment, which are now
regulated under the 2015 CCR Rule. The results of EPA's further
analyses in the final 2024 Supplemental Risk Analysis confirm that the
findings on the risk from active units from the 2014 Risk Assessment
are equally applicable to units that ceased receipt of waste prior to
2015 and either closed or became inactive. This final rule therefore
relies upon the 2014 Risk Assessment, the additional data and analysis
presented in the March 2023 proposal indicating that the legacy CCR
surface impoundments and CCRMU would be expected to have risks even
higher than previously modeled, and the 2024 Supplemental Risk
Assessment. Each of these is discussed in turn below.
a. Summary of 2014 Risk Record
In the 2014 Risk Assessment EPA conducted a national-scale,
probabilistic analysis that characterized potential risks to human and
ecological receptors associated with leakage from CCR surface
impoundments and landfills in operation at that time. A combination of
models was used to predict fate and transport of contaminants through
the environment, receptor exposures, and the resulting risks to human
and ecological receptors. The specific exposure routes evaluated were:
(1) Human inhalation of particulate matter blown from open management
units, (2) Human ingestion of crops and livestock raised on nearby
fields, (3) Human ingestion of groundwater used as a source of drinking
water, (4) Human ingestion of fish caught from freshwater streams, and
(5) Ecological contact with and ingestion of surface water and
sediment. Site-specific data were used where available, supplemented by
regional and national data to fill data gaps, to capture the
variability of waste management practices, environmental conditions,
and receptor behavior. EPA reported risks for both highly exposed
individuals and more moderately exposed individuals. Risks to highly
exposed individuals represent a reasonable maximum estimate that
members of the general population might be exposed to, which were
calculated as the 90th percentiles of all probabilistic model results.
Risks to moderately exposed individuals represent a more typical
estimate that members of the general population might be exposed to,
which were calculated as the 50th percentiles of all probabilistic
model results.
Under RCRA, EPA typically relies on a risk range to determine the
point at which regulation is appropriate. This policy was first
developed in the context of determining whether to regulate (or
``list'') wastes as hazardous under subtitle C of RCRA. See 80 FR
21449; 59 FR 66075-66077, December 22, 1994. However, over the years
EPA has relied on this risk range more broadly to determine whether
regulation is warranted under both subtitles C and D of RCRA. See 75 FR
35193 (``Although the statutory standards under subsections C and D
differ, EPA has historically interpreted both statutory provisions to
establish a comparable level of protection, corresponding to an
acceptable risk level ranging between 1 x 10-\4\ and 1 x
10-\6\.'').
Thus, to determine whether there is a reasonable probability of
adverse effects on health or the environment from the disposal or other
solid waste management of solid waste, EPA typically uses as an initial
cancer risk ``level of concern'' a calculated risk level of 1 x
10-\5\ (one in one hundred thousand) or an HQ above 1.0 for
any noncarcinogenic risks. See, 80 FR 21,449. For example, wastestreams
or activities for which the calculated high end individual cancer-risk
level is 1 x 10-\5\ or higher generally are considered
candidates for regulation. Wastestreams or activities with risks
calculated to be 1 x 10-\4\ (one in ten thousand) or higher
generally will be considered to pose a reasonable probability of
adverse effects on health or the environment and generally will be
regulated. Wastestreams or activities for which these risks are
calculated to be 1 x 10-\6\ (one in one million) or lower,
and lower than 1.0 HQ or environmental risk quotients for any
noncarcinogens, generally will be considered not to pose a reasonable
probability of adverse effects on health or the environment, and
generally will not be regulated. Id.
EPA first evaluated national-scale risks in the 2014 Risk
Assessment, which provides a snapshot in time of potential risks across
the country. This was accomplished by weighting risks from individual
management practices in proportion to the anticipated prevalence of
those practices. National-scale risks provide important context as to
whether risks are a systemic issue that warrant national regulations or
are limited in scope and better addressed through more targeted
actions. The Agency's evaluation found that the management practices
that EPA believed were generally in use in 2014 at surface impoundments
and landfills were likely to pose risks to human health through
groundwater exposure within the range that EPA typically considers
warrants regulation. For highly exposed individuals, the cancer risks
from arsenic due to the operation of surface impoundments were as high
as 2 x 10-\4\, while noncancer risks were as high as an HQ
of 5 for arsenic, 2 for lithium, and 2 for molybdenum. Cancer risks
associated with the operation of landfills were estimated to be as high
as 5 x 10-\6\ from the ingestion of arsenic-contaminated
drinking water. In contrast, all risks for moderately exposed
individuals fell below EPA's risk range. This was largely attributed to
the fact that many facilities are located next to major water bodies
and so contaminant plumes were frequently intercepted by these water
bodies before they could reach private wells.
EPA next evaluated the risks associated with individual management
practices at surface impoundments and landfills. This was accomplished
by filtering the national-scale model runs to focus only on those that
included the practice of interest and using the filtered set of runs to
calculate risks associated with that specific practice. These
individual risks provide important context about the range of
contaminants and practices that could pose risk at individual sites.
The Agency's evaluation identified two specific management practices
that could lead to risks higher than those identified in the national
risk estimates.
The first practice EPA evaluated was the disposal of CCR in unlined
and clay-lined units. Management in unlined surface impoundments
resulted in cancer risks for arsenic up to 3 x 10-\4\, as
well as noncancer risks for lithium up to an HQ of 3, molybdenum up to
an HQ of 4, and thallium up to an HQ of 2. Management in unlined
landfills resulted in cancer risks for arsenic up to 2 x
10-\5\. The larger increase in arsenic risks identified for
unlined landfills above those for national-scale landfills (2 x
10-\5\ vs. 5 x 10-\6\) compared to unlined and
national-scale

[[Page 38975]]

impoundments (3 x 10-\4\ vs. 2 x 10-\4\) is
because a larger proportion of landfills nationwide were initially
modeled as having a liner. Since promulgation of the 2015 CCR Rule, it
has become clear that more units are unlined than originally estimated.
Thus, it is anticipated that national-scale risks for landfills would
actually be closer to those for unlined landfills (2 x
10-\5\), rather than the lower nation-wide estimates
reported in the 2014 Risk Assessment.
Although clay-lined units tended to have lower risks than unlined
units, they still had potential to result in risks within the range
that EPA considers for regulation under RCRA. Management in clay-lined
impoundments with a liner thickness of three feet resulted in cancer
risks for arsenic of up to 7 x 10-\6\ and noncancer risks
for lithium up to an HQ of 2, while management in similarly unlined
landfills resulted in cancer risks for arsenic up to the 1 x
10-\5\. The larger increase in arsenic risks for unlined
impoundments above those for clay-lined impoundments (1 x
10-\5\ vs. 7 x 10-\6\) compared to unlined and
clay-lined landfills (2 x 10-\5\ vs. 1 x 10-\5\)
is because the layer of low conductivity clay counteracts the hydraulic
head in impoundments that would otherwise freely drive greater volumes
of leachate into the subsurface.\32\ In contrast, leachate generation
in both types of landfills is limited far more by the rate of
precipitation. As a result, EPA further considered how reducing the
modeled clay liner thickness of impoundments to the minimum allowable
standard of two feet would affect arsenic risk and found it would
increase to as high as 2 x 10-\5\.
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\32\ The somewhat higher risks identified for clay-lined
landfills compared to similarly lined impoundments are likely
related to site-specific conditions, such as where in the country
these units are located.
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The second practice evaluated was the management of wastes with an
extreme pH. In particular, empirical porewater data revealed that co-
disposal of CCR with other wastes in surface impoundments had the
potential to result in a highly acidic pH, cancer risks for arsenic up
to 1 x 10-\3\, and noncancer risks for cobalt and mercury up
to an HQ of 13 and 5, respectively. Laboratory leaching test data also
indicated that highly acidic and basic CCR wastes have the potential to
leach similarly high arsenic concentrations, up to an order of
magnitude higher than under more neutral conditions. Only a small
number of previous landfill model runs considered acidic conditions
based on the information available about conditions in active units;
identified risks for these units were driven by more basic conditions.
Thus, to the extent that conditions at either extreme of the pH scale
are more prevalent than previously estimated, it is likely that overall
risks from disposal in both surface impoundments and landfills would be
even higher than modeled.
EPA acknowledged in the 2014 Risk Assessment that there were some
additional management practices that could result in higher risk at
individual sites, but that could not be quantitatively modeled with the
data available at the time. One specific example provided was of CCR
disposal below the water table. EPA was unable to quantitatively model
the associated risks as there was little data on how common this
practice was or the extent to which it could affect groundwater
chemistry. Because EPA could not quantitatively model these management
practices (and because the Agency had no information to indicate that
it was a current, widespread management practice), EPA noted only that,
based on its review of damage cases, the damage from the placement of
CCR in sand and gravel pits was almost always associated with CCR being
placed in contact with water, which indicated that the placement of CCR
in contact with water can lead to higher risks than from dry disposal.
80 FR 21352. EPA further explained that ``in this situation, the
sorption that occurs in the unsaturated zone of the risk assessment
model does not occur in the field. This and other site-specific risk
factors could lead to additional contamination beyond what was modeled
nationwide.'' 2014 Risk Assessment at pages 5-48. As a consequence, EPA
specifically included sand and gravel pit

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