 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued November 20, 2017           Decided August 21, 2018

                       No. 15-1219

      UTILITY SOLID WASTE ACTIVITIES GROUP, ET AL.,
                      PETITIONERS

                             v.

          ENVIRONMENTAL PROTECTION AGENCY,
                    RESPONDENT

             WATERKEEPER ALLIANCE, ET AL.,
                    INTERVENORS


  Consolidated with 15-1221, 15-1222, 15-1223, 15-1227,
                    15-1228, 15-1229


     On Petitions for Review of Administrative Action
   of the United States Environmental Protection Agency


     Douglas H. Green and Paul J. Zidlicky argued the causes
for Industry petitioners. With them on the joint briefs were
John F. Cooney, Margaret K. Kuhn, Samuel B. Boxerman, Eric
Murdock, Makram B. Jaber, Joshua R. More, Raghav Murali,
Richard G. Stoll, Lori A. Rubin, and Thomas J. Grever.
Stephen J. Bonebrake, Brian H. Potts, and Aaron J. Wallisch
entered appearances.
                              2
     Thomas Cmar argued the cause for Environmental
petitioners. With him on the briefs were Matthew E. Gerhart,
Mary M. Whittle, and Lisa Evans.

     Perry M. Rosen, Attorney, U.S. Department of Justice,
argued the cause for respondents. With him on the briefs were
Jeffrey H. Wood, Acting Assistant Attorney General, Jonathan
Skinner-Thompson, Attorney, and Laurel Celeste, Attorney,
U.S. Environmental Protection Agency.

   Douglas H. Green, John F. Cooney, and Margaret K.
Kuhn were on the brief for Industry intervenor-respondents.

    Matthew E. Gerhart, Mary M. Whittle, and Lisa Evans
were on the brief for Environmental intervenor-respondents.

    Before: HENDERSON, MILLETT and PILLARD, Circuit
Judges.

    Opinion filed PER CURIAM.

     Opinion concurring in part and concurring in the judgment
in part filed by Circuit Judge HENDERSON.

     PER CURIAM: These consolidated petitions challenge the
Environmental Protection Agency’s 2015 Final Rule
governing the disposal of coal combustion residuals (“Coal
Residuals”) produced by electric utilities and independent
power plants. See Hazardous and Solid Waste Management
System; Disposal of Coal Combustion Residuals from Electric
Utilities (“Final Rule”), 80 Fed. Reg. 21,302 (April 17, 2015).
Coal Residuals make up “one of the largest industrial waste
streams generated in the U.S.” Id. at 21,303. Coal-fired
power plants in the United States burned upwards of 800
million tons of coal in 2012 alone and produced approximately
                               3
110 million tons of solid waste as Coal Residuals. Id. That
waste contains myriad carcinogens and neurotoxins. See
Hazardous and Solid Waste Management System;
Identification and Listing of Special Wastes; Disposal of Coal
Combustion Residuals from Electric Utilities (“Proposed
Rule”), 75 Fed. Reg. 35,128, 35,153, 35,168 (June 21, 2010).
Power plants generally store it on site in aging piles or pools
that are at varying degrees of risk of protracted leakage and
catastrophic structural failure. See 80 Fed. Reg. 21,327–
21,328. The Final Rule sets criteria designed to ensure that
human health and the environment face “no reasonable
probability” of harm from Coal Residuals spilling, leaking, or
seeping from their storage units and harming humans and the
environment. Id. at 21,338–21,339; 42 U.S.C. § 6944(a).

    The statutory framework calling for regulation of solid
waste generation, storage, and disposal has been in place since
1976, when Congress enacted the Resource Conservation and
Recovery Act (“RCRA”), 42 U.S.C. § 6901 et seq., but
regulations implementing RCRA have been long in the
making. The EPA has long studied the Coal Residuals
disposal problem and struggled over how to address its scale,
complexity, and gravity. The agency has been goaded by
public outrage over catastrophic failures at sites storing toxic
Coal Residuals, see 75 Fed. Reg. at 35,132, 35,137, and was
directed by a federal court to devise a schedule to comply with
its obligation to regulate under RCRA, see Appalachian Voices
v. McCarthy, 989 F. Supp. 2d 30, 56 (D.D.C. 2013). Nearly
four decades after Congress enacted RCRA, the EPA finally
promulgated its first Final Rule regulating Coal Residuals in
2015.

  These consolidated petitions—one on behalf of
environmental organizations (“Environmental Petitioners”)
and several others (collectively, “Industry Petition”) for a
                                 4
consortium of power companies and their trade associations
(“Industry Petitioners”)—challenge various provisions of that
Final Rule under the Administrative Procedure Act and RCRA.
RCRA Subtitle D calls on the EPA to promulgate criteria
distinguishing “sanitary landfills,” which are permissible under
the statute, from “open dumps,” which are prohibited. 42
U.S.C. § 6944(a); see id. § 6903(14), (28). The statutory
baseline for the EPA’s criteria for sanitary landfills is that, at a
minimum, they “shall provide that a facility may be classified
as a sanitary landfill and not an open dump only if there is no
reasonable probability of adverse effects on health or the
environment from disposal of solid waste at such facility.” Id.
§ 6944(a). Each claim here relates to what a utility operating
one or more Coal Residuals disposal site(s) must do to qualify
such site as a sanitary landfill that may lawfully operate under
RCRA.

   Shortly before oral argument, the EPA announced its intent
to reconsider the Final Rule, and moved to hold all proceedings
in abeyance. We asked for clarification on the exact
provisions of the Rule that would be subject to reconsideration.
The EPA then filed a separate motion to remand six specific
provisions.

   For the reasons that follow, we deny the EPA’s abeyance
motion, and partially grant its remand motion. We also grant
in part the Environmental Petition and deny the Industry
Petition.

                        I. Background

                                A.

    “Coal Residuals” is a catch-all term for the byproducts of
coal combustion that occurs at power plants. It includes “fly
                               5
ash,” “bottom ash,” “boiler slag,” and “flue gas desulfurization
materials.” See 75 Fed. Reg. at 35,137. These residuals vary
in their size and texture, but all contain “contaminants of * * *
environmental concern.” Id. at 35,138. According to the
EPA, Coal Residuals contain carcinogens and neurotoxins,
including arsenic, boron, cadmium, hexavalent chromium,
lead, lithium, mercury, molybdenum, selenium, and thallium.
80 Fed. Reg. at 21,449. The risks to humans associated with
exposure to the identified contaminants include elevated
probabilities of “cancer in the skin, liver, bladder, and lungs,”
as well as non-cancer risks such as “neurological and
psychiatric effects,” “cardiovascular effects,” “damage to
blood vessels,” and “anemia.” Id. at 21,451. Both cancer
and non-cancer risks to infants “tend[] to be higher than other
childhood cohorts, and also higher than risks to adults.” Id. at
21,466. The risks to plant and animal wildlife include
“elevated selenium levels in migratory birds, wetland
vegetative damage, fish kills, amphibian deformities,
* * * [and] plant toxicity.” 75 Fed. Reg. at 35,172.

     In developing the Final Rule, the EPA collected data on
coal-fired units and their environs, identified hazards for
evaluation, and specified benchmarks of toxicity that it
determined “generally will be considered to pose a substantial
present or potential hazard to human health and the
environment and generally will be regulated.” Final Rule, 80
Fed. Reg. at 21,449, 21,451. The EPA analyzed potential
pathways of contamination to determine those most likely to
pose a reasonable probability of adverse effects on humans or
the environment. Id. at 21,450–21,451. The EPA concluded
that current management practices for Coal Residuals posed
risks to human health and the environment at levels justifying
uniform national guidelines. Id. at 21,303. The main
exposure pathways the EPA found were through waste that
escapes landfills and surface impoundments and then
                                6
contaminates groundwater tapped as drinking water, and
contaminates surface water that comes in direct contact with
fish and other ecological receptors. Id.

     Under most circumstances, the operators of coal-fired
power plants dispose of the waste either by dumping it in dry
landfills or by mixing it with water to channel it to wet surface
impoundments. 80 Fed. Reg. at 21,303. These disposal sites
are massive. On average, landfills span more than 120 acres
and are more than 40 feet deep. Id. Surface impoundments
average more than 50 acres in size with an average depth of 20
feet. Id. As of 2012, there were at least 310 landfills and 735
surface impoundments in the United States currently receiving
coal ash. Id. The EPA identified at least 111 surface
impoundments that are no longer receiving coal ash, but are not
fully closed. See EPA, Regulatory Impact Analysis: EPA’s
2015 RCRA Final Rule Regulating Coal Combustion Residual
(CCR) Landfills and Surface Impoundments at Coal-Fired
Utility Power Plants, 2–3 (2014), Joint App’x (J.A.) 1096.
The record does not specify the number of inactive landfills.
See id. The Rule also addresses circumstances under which
Coal Residuals safely may be “beneficially used”—e.g., to
make cement—thereby reducing the total volume that must be
managed as waste. See 75 Fed. Reg. at 35,212.

     Landfills and surface impoundments both pose threats to
human health and the environment. 80 Fed. Reg. at 21,327–
21,328.     The risks generally stem from the fact that
“thousands, if not millions, of tons [of coal ash are] placed in a
single concentrated location.” Id. These disposal sites are at
risk of structural failure, particularly where they are located in
unstable areas such as wetlands or seismic impact zones. Id.
at 21,304. The sheer volume of Coal Residuals at these sites,
moreover, can force contaminants into the underlying soil and
groundwater, threatening sources of drinking water. Id. at
                                7
21,304–21,305. Surface water bodies—i.e., rivers, lakes, and
streams, see 75 Fed. Reg. at 35,131—are also at risk of
contamination through harmful constituents that migrate
through groundwater, or flow into surface waters as run-off or
wastewater discharge, any of which can lead to environmental
harms such as “wetland vegetative damage, fish kills,
amphibian deformities, * * * [and] plant toxicity.” See id. at
35,172.

     Groundwater contamination is more likely to occur at sites
that are unlined or lack adequate lining between the coal ash
and the soil beneath it. See id.; see also Regulatory Impact
Analysis, 5-22. However, most existing coal ash disposal
sites—70% of landfills and 65% of surface impoundments—
have no liner at all. See Regulatory Impact Analysis, 3-4
nn.104–105, J.A. 1108. And while most new landfills and
surface impoundments are constructed with liners, see 80 Fed.
Reg. at 21,324, not all liners are alike. Composite lining,
which includes a plastic geomembrane and several feet of
compacted soil to act as a buffer, effectively eliminates the risk
of groundwater contamination.           See EPA, Human &
Ecological Risk Assessment of Coal Combustion Residuals
(Risk Assessment), 4-8 to 4-9, J.A. 1110–1111. But many
impoundments are lined only with compacted soil and are
therefore far less protective. See Regulatory Impact Analysis,
5-22, J.A. 1112. The EPA has acknowledged that it “will not
always be possible” to restore groundwater or surface water to
background conditions after a contamination event. See
Response to Comments 50, J.A. 1301.

     Structural failures of surface impoundments pose
additional risks that are more episodic but potentially more
catastrophic than harm from liner leakage. Impoundment dam
ruptures can result in “significant coal slurry releases, causing
fish kills and other ecologic damage, and in some instances
                                8
damage to infrastructure.” 80 Fed. Reg. at 21,457 (footnote
omitted).    The EPA is aware of at least 50 surface
impoundments that are a “high” hazard, see EPA, Coal
Combustion Residuals Impoundment Assessment Reports,
J.A. 446–469, which the Rule defines to mean that “failure or
mis-operation will probably cause loss of human life” in
addition to other harms, 40 C.F.R. § 257.53. The EPA has
tagged another 250 impoundments as posing a “significant”
hazard, see Impoundment Assessment Reports, J.A. 446–469,
where failure or mis-operation is unlikely to kill people, but
would “probably cause economic loss, environmental damage,
or disruption of lifeline facilities, or impact other concerns.”
40 C.F.R. § 257.53. Structural risk is exacerbated at sites
located in geologically unstable areas, such as those with poor
foundation conditions, areas susceptible to earthquakes or
other mass movements, or those with karst terrains. See id.;
80 Fed. Reg. at 21,365–21,367.

     Risks from inactive surface impoundments at inactive
power plants, which the parties refer to as “legacy ponds,” are
also apparent in the record. As with surface impoundments at
active plants, groundwater contamination or catastrophic
structural failure of a legacy pond threatens human health and
the environment. But legacy ponds, which by their nature are
older than most surface impoundments, are “generally unlined”
and unmonitored, and so are shown to be more likely to leak
than units at utilities still in operation. 80 Fed. Reg. at 21,343–
21,344. Without an on-site operator to monitor and maintain
such a unit, consequences of leakage or structural failure may
be amplified. Cf. id. at 21,394 (requiring qualified personnel
to conduct weekly inspections at active surface
impoundments).

   The EPA record reports on the many cases in which
damage has already occurred. “EPA has confirmed a total of
                               9
157 cases * * * in which [Coal Residual] mismanagement has
caused damage to human health and the environment.” 80
Fed. Reg. at 21,325. The EPA recounts that public pressure to
regulate Coal Residuals escalated after an unlined surface
impoundment in Kingston, Tennessee suffered a
“catastrophic” structural failure on December 22, 2008. See
75 Fed. Reg. at 35,132.           The impoundment released
approximately 5.4 million cubic yards of Coal Residual sludge
across 300 acres of land and into the nearby Emory River. See
EPA, Damage Case Compendium:                Technical Support
Document, Volume I: Proven Damage Cases, 143 (2014), J.A.
1192. According to the EPA, the spill was one of the “largest
volume industrial spill[s] in U.S. history.” Id. at 143 n.612,
J.A. 1192. The Coal Residual sludge ruptured a natural gas
line, disrupted power in the area, damaged or destroyed dozens
of homes, and resulted in elevated levels of arsenic and lead in
the Emory River. Id. The resulting river contamination
“completely destroyed” more than 80 acres of aquatic
ecosystems. Id. at 144, J.A. 1193. More than a year after the
spill, the majority of fish collected from the river contained
toxins that rendered them unsafe for human consumption. Id.
The disaster forced the closure of the Emory River for almost
two years. The Tennessee Valley Authority took four years
and spent more than $1.2 billion to remove Coal Residuals and
contaminated sediment from the river and adjoining areas, to
monitor and repair associated damage, and to construct a new
disposal unit. Id. at 148, J.A. 1197.
                               10
                               B.

    Two years after the Kingston disaster, the EPA
promulgated the Proposed Rule announcing its intent to
regulate Coal Residuals under RCRA. See 75 Fed. Reg. at
35,128.1

     A key question for the EPA had long been whether to
regulate Coal Residuals as hazardous waste under the cradle-
to-grave federal hazardous waste management authority
conferred by RCRA Subtitle C, 42 U.S.C. §§ 6921–6939g, or
to treat it as nonhazardous solid waste subject to national
guidelines under Subtitle D, id. §§ 6941–6949a. A waste is
“hazardous” and subject to regulation under Subtitle C only if
it exhibits one of four hazard characteristics: ignitability,
corrosivity, reactivity, or toxicity. See id. § 6921; 40 C.F.R.
§§ 261.11, 261.20–261.24. Under Subtitle C, the EPA
directly regulates all stages of production and disposition of
hazardous wastes, and has administrative enforcement power
as well as authority to initiate or recommend civil and criminal
actions in court. See 42 U.S.C. §§ 6922–6928. Subtitle D, in
contrast, envisions that states are primarily responsible for
regulating disposal of nonhazardous wastes in landfills and
dumps. The EPA’s principal role under Subtitle D is to
announce federal guidelines for state management of
nonhazardous wastes; Subtitle D leaves it up to the states to
“use federal financial and technical assistance to develop solid
waste management plans in accordance with [the] federal
guidelines.” Environmental Def. Fund v. EPA, 852 F.2d
1309, 1310 (D.C. Cir. 1988).

    1
       On several previous occasions, the EPA considered, but
decided against, regulating Coal Residuals under RCRA Subtitle C.
For background on the EPA’s previous determinations on Coal
Residuals, see 75 Fed. Reg. at 35,136–35,137.
                               11

     Substantively, Subtitle D prohibits the disposal of solid
waste in “open dumps,” 42 U.S.C. § 6945(a), and calls on the
EPA to promulgate criteria for determining whether a waste
facility constitutes an open dump—criteria that, if followed,
will ensure “no reasonable probability of adverse effects on
health or the environment from disposal of solid waste at such
facility,” id. § 6944(a). Subtitle D neither grants the EPA
direct enforcement authority nor requires states to adopt or
implement its requirements. See id. § 6941. Enforcement is
left to states’ own policy decisions and to the initiative of
people bringing citizen suits to enforce the federal standards.
See id. §§ 6946–6947, 6972. But see infra Part II.A.
(discussing recent amendments to RCRA).

      The EPA initially published two alternative proposed rules
to govern Coal Residuals, one under each Subtitle, basing the
Subtitle C proposal on the toxicity of Coal Residuals. See 75
Fed. Reg. at 35,146. The proposals drew 450,000 public
comments, the vast bulk of which spoke to the threshold
question of which RCRA Subtitle to use, and the majority of
which supported regulation under Subtitle C. 80 Fed. Reg. at
21,319. Most of the commenters were individuals and
environmental groups pressing for stronger regulation
“because state programs have failed to adequately regulate the
disposal of [Coal Residuals] and because the risks associated
with the management of these wastes are significant.” Id.
Only a handful of states, for example, required any
groundwater monitoring around units holding Coal Residuals,
id. at 21,323–21,324, including only one of the eight states with
the biggest volumes of Coal Residuals, Regulatory Impact
Analysis, G-6, J.A. 1121. On the other hand, the enormous
volume of waste permeated with relatively low concentrations
of toxins posed practical difficulties for any Subtitle C
regulation. See 80 Fed. Reg. at 21,321.
                               12

    Based on many years of analysis, the EPA found “a
compelling need for a uniform system of requirements to
address the[] risks [from Coal Residuals],” and decided to
move forward with a Final Rule. 80 Fed. Reg. at 21,327.
The EPA opted to proceed under the less muscular Subtitle D
even as it continued to study factors potentially supporting
regulating Coal Residuals as hazardous waste under RCRA
Subtitle C. See id. at 21,319–21,327. The EPA thus formally
deferred deciding whether Subtitle C regulation is warranted,
and used its Subtitle D authority to set forth guidelines on
where and how disposal sites for Coal Residuals are to be built,
maintained, and monitored. See 80 Fed. Reg. at 21,302.

     The Final Rule sets minimum criteria for the disposal of
Coal Residuals in landfills and surface impoundments.
Among the provisions of the Final Rule at issue here are
location restrictions on landfills and surface impoundments,
requirements pertaining to lining, structural integrity, and
groundwater monitoring, and criteria for recycling Coal
Residuals for beneficial uses, such as substituting for cement
in road construction, in lieu of keeping it in disposal units. See
40 C.F.R. §§ 257.60–257.74. The Final Rule also sets
compliance deadlines, procedures for closing non-complying
landfills and surface impoundments, and requirements that
operators of these disposal sites make records of their
compliance with the Final Rule publicly available. See id.
§§ 257.100–257.07. We discuss the relevant criteria in more
detail in addressing the merits of the consolidated petitions.
                               13
                               C.

     Two groups of petitioners sought review of the Final Rule.
Environmental Petitioners are an assortment of environmental
groups that includes the Environmental Integrity Project, Sierra
Club, and Hoosier Environmental Council. They generally
claim that EPA did not go far enough to protect the public and
the environment from the harms of Coal Residual disposal.
Specifically, they claim that the Final Rule unlawfully
countenances significant risks of harmful leakage by allowing
unlined impoundments as well as impoundments lined only
with a layer of compacted soil to continue receiving Coal
Residuals. Environmental Petitioners also contend that the
EPA acted arbitrarily and capriciously by exempting from
regulation so-called “legacy ponds”—inactive surface
impoundments at shuttered power plants—given evidence that
legacy ponds are at risk of unmonitored leaks and catastrophic
structural failures. They also make a claim, not raised during
rulemaking, that the EPA violated RCRA’s citizen-suit
provision by failing to require the operators of Coal Residual
disposal sites to timely and publicly disclose records reflecting
their compliance with the Final Rule.

     Industry Petitioners are a collection of industry trade
associations and utilities including the Utility Solid Waste
Activities Group, AES Puerto Rico, LP, the Edison Electric
Institute, the National Rural Electric Cooperative Association,
and the American Public Power Association. They first assert
that the EPA exceeded its statutory authority under RCRA to
set guidelines for facilities where waste “is disposed of,” 42
U.S.C. § 6903(14), by regulating surface impoundments that
no longer actively receive Coal Residuals. They further claim
that the Rule’s restriction on placement of new units and
expansions of existing units near aquifers, 40 C.F.R. § 257.60
(aquifer location restriction), was inadequately noticed, and
                               14
that the Rule’s provision for nonconforming units to continue
in operation if no alternative disposal capacity is available, id.
§ 257.103 (alternative closure provision), arbitrarily and
capriciously excludes cost considerations from its definition of
“available.” Industry Petitioners also challenge the Rule’s
location restrictions and structural integrity criteria governing
units in seismic impact zones. See id. §§ 257.63, 257.73–
257.74.      They contend that the deadline for existing
impoundments’ compliance with those provisions was
arbitrarily shortened from the timeframe in the Proposed Rule,
that the Rule arbitrarily applied the location restrictions to new
but not existing landfills, and that EPA failed to explain the
strict design criteria it adopted for new landfills and
impoundments.

     Environmental Petitioners intervened in Industry’s
petition for review, and vice versa. We consolidated the
petitions. The case has been pending in this court since 2015,
but several procedural matters delayed resolution until now.
In June 2016, we granted the EPA’s unopposed motion to
remand to itself several provisions of the Final Rule not at issue
here that the EPA had decided to vacate. See Per Curiam
Order, Utility Solid Waste Activities Grp. v. EPA, No. 15-1219
(D.C. Cir. June 14, 2016). In doing so, we held all
proceedings in abeyance while the EPA revised portions of the
Rule affected by the vacatur. See id. We then set oral
argument for October 17, 2017.

    Less than a month before oral argument, the EPA
announced that it had granted the petition of several industry
groups to reconsider the Final Rule, and moved us to hold all
proceedings in abeyance. The EPA pointed to Congress’s
recent enactment of the Water Infrastructure Improvements for
the Nation Act (“WIIN Act”), Pub. L. No. 114-322, 130 Stat.
1628 (2016) (codified at 42 U.S.C. § 6945(d)), in December
                              15
2016 that, among other things, amended RCRA Subtitle D to
allow the EPA to approve State permitting programs “to
operate in lieu of [EPA] regulation of coal combustion
residuals units in the State,” provided those programs are at
least as environmentally protective as the existing (or
successor) EPA regulations. 42 U.S.C. § 6945(d)(1)(A).
When we asked EPA to specify which provisions it planned to
reconsider, the EPA filed another motion. That motion sought
to remand provisions of the Rule relating to the beneficial use
of Coal Residuals, alternative compliance provisions, legacy
ponds, and the EPA’s statutory authority to regulate inactive
surface impoundments. We deferred a ruling on both motions
until now.

     On July 30, 2018, the EPA promulgated an amendment to
the Final Rule (i) allowing a state or the EPA, when acting as
a permitting authority, to use alternate groundwater
performance standards, (ii) revising the groundwater
performance standards for certain constituents, and (iii)
extending the timeframe for facilities to cease receiving Coal
Residuals once they are required to close. See Hazardous and
Solid Waste Management System:              Disposal of Coal
Combustion Residuals from Electric Utilities; Amendments to
the National Minimum Criteria, 83 Fed. Reg. 36,435, 36,436
(July 30, 2018).

                 II. Request for Abeyance

                        A. WIIN Act

    At the outset, the EPA requests that this case be held in
abeyance while it considers potential regulatory changes in
response to Congress’s enactment of the WIIN Act, 42 U.S.C.
§ 6945(d). The WIIN Act amended RCRA’s Subtitle D State
permitting scheme. As relevant here, Section 6945(d)
                               16
provides that the Administrator may approve qualified State
“permit program[s] or other system[s] of prior approval and
conditions under State law for regulation by the State of coal
combustion residuals units” to “operate in lieu of [EPA]
regulation of coal combustion residuals units in the
State * * *.” 42 U.S.C. § 6945(d)(1)(A).

     But the Administrator may only approve a state plan if its
standards “are at least as protective as the criteria” set by the
EPA in its corresponding RCRA regulations, specifically
including Coal Residuals regulation, 40 C.F.R. pt. 257. 42
U.S.C. § 6945(d)(1)(C); see id. § 6945(d)(1)(B)(i). The WIIN
Act also provides that a Coal Residuals disposal site can only
qualify as a “sanitary landfill” if it is in full compliance with,
among other things, the EPA’s extant (or successor)
regulations governing Coal Residuals waste sites. 42 U.S.C.
§ 6945(d)(6).

      The EPA argues that the WIIN Act has afforded it new
regulatory options and makes “fundamental changes to RCRA
Subtitle D as applied specifically to [Coal Residuals].” EPA
WIIN Br. 4, 6, 8. On that basis, the EPA asks us to hold the
case in abeyance while it decides whether or not “to alter some
of its regulatory choices[.]” EPA WIIN Br. at 2, 6.

     We decline to exercise our discretion to hold the case in
abeyance. We leave it open for the EPA to address on remand
the relevance of the WIIN Act, the Act’s express incorporation
of the EPA regulations published at 40 C.F.R. Part 257, and its
definition of “sanitary landfill.”
                              17
       III. Environmental Petitioners’ Challenges

           A. Unlined Surface Impoundments

      Environmental Petitioners challenge the Final Rule’s
provision that existing, unlined surface impoundments may
continue to operate until they cause groundwater
contamination. 40 C.F.R. § 257.101(a)(1). They contend
that the EPA failed to show how continued operation of unlined
impoundments meets 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).

     The EPA found that unlined impoundments are
dangerous: It concluded that, among the studied disposal
methods, putting Coal Residuals “in unlined surface
impoundments and landfills presents the greatest risks to
human health and the environment.” 80 Fed. Reg. at 21,451.
The Rule accordingly requires that all new surface
impoundments be constructed with composite lining that
effectively secures against leakage.           See 40 C.F.R.
§ 257.72(a). But it allows existing unlined impoundments to
continue to receive Coal Residuals indefinitely, until their
operators detect that they are leaking. Id. § 257.101(a). Only
once a leak is found must the operator of an unlined
impoundment begin either retrofitting the unit with a
composite liner, or closing it down—a process that the Rule
contemplates may take upwards of fifteen years. Id. §
257.102(f). In view of the record evidence that led the EPA
to conclude that composite liners are needed to ensure that new
impoundments meet RCRA Subtitle D’s “no reasonable
probability” standard, Environmental Petitioners claim that the
Rule’s allowance for continued operation of existing, unlined
                                18
surface impoundments is arbitrary and capricious and contrary
to RCRA.

     The EPA and Industry Intervenors assert that the
composite lining required for new units is not needed for
existing units because most unlined impoundments do not leak,
and an unlined impoundment that is not leaking is not
dangerous. Industry Intervenors emphasize that the record
suggests that “almost two-thirds of unlined impoundments do
not leak,” and they assert that “appropriate controls on
impoundments that do leak” suffice to meet RCRA’s “no
reasonable probability” standard. Industry Intervenor Br. 6–
7. The EPA underscores that it made no finding of any
“reasonable probability that each and every unlined
impoundment will, in fact, result in adverse effects on health
and the environment.” Resp’t Br. 82. It insists that RCRA’s
“no reasonable probability” standard is met by the Rule’s
provisions for “extensive monitoring of groundwater to detect
constituent leaking,” id. at 83, and “immediate action to stop
that leak,” “redress that leak,” and to close the site as soon as a
harmful leak is detected. Oral Arg. Tr. 100:20–100:25.

     The record shows, however, that the vast majority of
existing impoundments are unlined, see Regulatory Impact
Analysis 3-4, J.A. 1108, that unlined impoundments have a
36.2 to 57 per cent chance of leakage at a harmfully
contaminating level during their foreseeable use, see id. at 4-9,
5-22, J.A. 1111–1112, and that the threat of contamination
from unlined units exceeds the EPA’s cancer risk criteria and
thus “generally will be considered to pose a substantial present
or potential hazard to human health and the environment,” 80
Fed. Reg. at 21,449–21,450; see Risk Assessment 5-5, J.A.
1041. It is inadequate under RCRA for the EPA to conclude
that a major category of impoundments that the agency’s own
data show are prone to leak pose “no reasonable probability of
                              19
adverse effects on health or the environment,” 42 U.S.C.
§ 6944(a), simply because they do not already leak.

     The number of unlined impoundments is large. The EPA
identified 735 existing active surface impoundments
throughout the country. Of the 504 sites for which the EPA
was able to collect liner data, approximately 65 per cent were
completely unlined, with most of the rest lined only with
compacted soil or other partial or high-permeability liners.
See Regulatory Impact Analysis 3-4 n.105, J.A. 1108. Only
17 per cent of surface impoundments for which the EPA has
liner data had composite liners—the sole liner type that the
EPA found to be effective in reducing the risk of toxic chemical
leakage to the level that the Agency found acceptable.

     Those hundreds of unlined impoundments are at
significant risk of harmful leakage. Of 157 sites where the
EPA confirmed that Coal Residuals have already caused
damage to human health and the environment, the damage
cases “were primarily associated with unlined units.” 80 Fed.
Reg. at 21,452.       The record evidence shows that an
impoundment with composite lining, which the Rule requires
of all new impoundments, has a 0.1 per cent chance of
contaminating groundwater at drinking-water wells a mile
distant from the impoundment perimeter over the course of a
100-year period. Regulatory Impact Analysis 5-22, J.A. 1112.
An unlined impoundment, in contrast, has a 36.2 per cent
chance of contaminating groundwater at such a distance. See
id. And the probability of contamination is higher at distances
closer to the impoundment site, id., J.A. 1112; measured one
meter from the impoundment’s perimeter, the contamination
risk jumps to 57 per cent, id., J.A. 1111. See Risk Assessment
ES-4, J.A. 1083–1084 (“In many of the potential damages
cases, groundwater exceedances were discovered near the
boundary” of the impoundment).              According to the
                                 20
administrative record, then, a significant portion of the 575
identified unlined surface impoundments are likely to
contaminate groundwater.

     Impoundment leakages pose substantial risks to humans
and the environment. The EPA studied a wide range of toxins
present in Coal Residuals, see Risk Assessment ES–4, J.A.
1010, and considered various forms of potential human and
environmental exposures. The EPA uses risk benchmarks in
assessing the propriety of regulatory action. For example, it
treats a cancer risk in excess of 1 x 104, or 1 in 10,000, as one
that “generally will be considered to pose a substantial present
or potential hazard to human health and the environment[.]”
80 Fed. Reg. at 21,449. For non-cancer risks, the EPA
determined that a Hazard Quotient—defined as the “ratio of the
estimated exposure to the exposure at which it is likely that
there would be no adverse health effects,” 75 Fed. Reg. at
35,168—gives rise to such a threat when it is greater than or
equal to 1. See 80 Fed. Reg. at 21,449. Using those
benchmarks and the data it collected from the Risk
Assessment, the EPA found that material human exposures
derive from ingestion of contaminated groundwater or the
consumption of contaminated fish. Id. at 21,450–21,451. 2
The plant and animal exposures the EPA identified as material
derive from contact with contaminated surface water. See id.;
Risk Assessment 5-8, J.A. 1044. The EPA also expressed
concern about the contamination of groundwater that is not
currently used as a source of drinking water because “[s]ources
of drinking water are finite, and future users’ interests must
     2
         The EPA’s Risk Assessment found that unlined
impoundments created an unacceptable human cancer risk as a result
of exposure to two different arsenics, and an unacceptable non-
cancer risk as a result of exposure to one type of arsenic, as well as
lithium, molybdenum, and thallium. See Risk Assessment 5-5, J.A.
1041.
                              21
also be protected.” 80 Fed. Reg. at 21,452. In view of the
record’s limitation of the risk calculus associated with leakage
to the subset of toxins and exposures that the EPA deemed to
present a substantial risk to human health or the environment,
the EPA’s assertion in its brief that, even where it occurs,
leakage “will not necessarily result in contamination of
groundwater, either above allowable regulatory thresholds, or
at all,” is at best a red herring. Resp’t Br. at 85. Every
leakage the EPA record treated as material exceeded regulatory
thresholds. In defending the Rule here, the EPA looks at too
narrow a subset of risk information and applies the wrong legal
test.

     The Final Rule’s approach of relying on leak detection
followed by closure is arbitrary and contrary to RCRA. This
approach does not address the identified health and
environmental harms documented in the record, as RCRA
requires. Moreover, the EPA has not shown that harmful
leaks will be promptly detected; that, once detected, they will
be promptly stopped; or that contamination, once it occurs, can
be remedied.

     On its own terms, the Rule does not contemplate that
contamination will be detected as soon as it appears in
groundwater. The EPA and Industry defend the rule as
RCRA-compliant principally because, they say, it provides for
retrofit with a composite liner or closure of an unlined
impoundment “[o]n the first indication that an unlined unit is
leaking[.]” Industry Intervenor Br. 6. But the required
groundwater sampling need only occur “at least
semiannual[ly],” or perhaps less frequently under certain
geological conditions.   40 C.F.R. § 257.94(b), (d); id.
§ 257.95(c). The Rule thus contemplates that leaks will often
go undetected for many months.
                                22
     By the time groundwater contamination from an unlined
impoundment has been detected, more damage will have been
done than had the impoundment been lined: Leakage from
unlined impoundments is typically quicker, more pervasive,
and at larger volumes than from lined impoundments. See 80
Fed. Reg. at 21,406. Unlike lined impoundments, in which
leaks are “usually caused by some localized or specific defect
in the liner system that can more readily be identified and
corrected,” leakage from unlined impoundments is more
pervasive and less amenable to any quick, localized fix. Id. at
21,371. When an unlined impoundment begins to leak, Coal
Residual sludge “will flow through the unit and into the
environment unrestrained,” such that retrofit or closure of the
unit are typically “the only corrective action strateg[ies] that
[the] EPA can determine will be effective[.]” See id.

     Neither retrofitting nor closure occurs immediately under
the Rule; the timeline contemplates a process that takes from
five to fifteen years. See 40 C.F.R. § 257.102. The EPA
understates the harm its own record evidences by emphasizing
that “leaking unlined impoundments must cease receiving
[Coal Residuals] and initiate closure or retrofit activities within
six months.” Resp’t Br. at 81; see 40 C.F.R. § 257.101(a)(1).
What it neglects to account for is that the Rule gives the
operator a further five years to complete retrofitting or closure
activities. Id. §§ 257.102(f)(1)(ii), 257.102(k)(3). The Rule
also allows the operators of surface impoundments to extend
that window, by up to two years for smaller units and, for units
larger than 40 acres—which most are, see 80 Fed. Reg. at
21,303—for up to ten years, see 40 C.F.R. § 257.102(f)(2)(ii).

     The Rule addresses neither the risks to public health and
to the environment before leakage is detected, nor the harms
from continued leakage during the years before leakage is
ultimately halted by retrofit or closure. See generally 40
                              23
C.F.R. §§ 257.90–257.104.         In defending the Rule as
compliant with RCRA, the EPA did not even consider harms
during the retrofit or closure process. See Resp’t Br. 80–86;
80 Fed. Reg. at 21,403–21,406; cf. Oral Arg. Tr. 102–105 (EPA
counsel unable to identify record evidence regarding how
quickly leaks can be detected or how quickly and thoroughly
responsive action can occur, but referring generally to a
practice of immediate “pump and treat,” which the Rule does
not appear to require). An agency’s failure to consider an
important aspect of the problem is one of the hallmarks of
arbitrary and capricious reasoning. See United States Sugar
Corp. v. EPA, 830 F.3d 579, 606 (D.C. Cir. 2016) (per curiam)
(citing Motor Vehicle Mfrs. Ass’n v. State Farm Mut. Auto. Ins.
Co., 463 U.S. 29, 43 (1983)).

     The EPA’s position suffers additional flaws. The EPA
determined that contaminated surface waters, such as rivers,
streams, and lakes, are the principal pathway of harm to
environmental receptors, but the Final Rule requires only
monitoring of groundwater, and only for levels of
contamination that would harm human health. See 40 C.F.R.
§§ 257.90–257.95 (calling for groundwater monitoring
systems); 75 Fed. Reg. at 35,130 (defining maximum
contaminant level in terms of drinking water safety). Surface
water contamination poses environmental risks from
“[e]levated selenium levels in migratory birds, wetland
vegetative damage, fish kills, amphibian deformities,
* * * [and] plant toxicity,” 75 Fed. Reg. at 35,172, and to
humans through the possible consumption of contaminated
fish, 80 Fed. Reg. at 21,444. These risks exceed the EPA’s
risk criteria for ecological receptors. See Risk Assessment 5–
8, J.A. 1044. And some contamination levels that do not meet
the risk threshold for humans may exceed thresholds for
ecological receptors. See, e.g., id. (noting a risk exceedance
unique to ecological receptors from cadmium). Yet the record
                               24
does not explain how the Rule’s provisions for groundwater
monitoring, followed by corrective action only when human
exposure benchmarks are exceeded, will mitigate these risks.
RCRA requires the EPA to set minimum criteria for sanitary
landfills that prevent harm to either “health or the
environment.” 42 U.S.C. § 6944(a) (emphasis added). The
EPA’s criteria for unlined surface impoundments, limited as
they are to groundwater monitoring for contaminant levels
keyed to human health, only partially address the first half of
the statutory requirement.

     For these reasons, we vacate 40 C.F.R. § 257.101, which
allows for the continued operation of unlined impoundments,
and remand for additional consideration consistent with this
opinion.

                   B. Liner Type Criteria

     Environmental Petitioners next challenge the Final Rule’s
regulation of so-called “clay-lined” surface impoundments. A
clay liner consists of at least two feet of compacted soil to act
as a buffer between the Coal Residual sludge and the local soil.
See Risk Assessment 4-8; J.A. 1024. Even as the Rule
requires all newly constructed surface impoundments to be
built with composite lining, disapproving any new
impoundments lined only with compacted soil, it treats existing
impoundments constructed with the same compacted soil and
no geomembrane as if they were “lined.” See 40 C.F.R.
§§ 257.71(a)(1)(i), 257.96–257.98. The upshot is that such
clay-lined impoundments may stay open and keep accepting
Coal Residuals, subject to groundwater monitoring for leakage,
see 40 C.F.R. § 257.101, but, unlike existing unlined
impoundments—which must begin closure when they leak, id.
§ 257.71(a)(1)—clay-lined impoundments need not begin
closure when they are discovered to be leaking. Rather, their
                              25
operators may attempt to repair them first. Id. §§ 257.96–
257.98. Only if repair is unsuccessful must they then begin
the protracted process to either retrofit with a composite liner
or close.

     The EPA contends that, by requiring the operators of clay-
lined impoundments to fix leaks when they occur, the Rule
comports with RCRA’s mandate to ensure “no reasonable
probability of adverse effects on health or the environment.”
42 U.S.C. § 6944(a); see Resp’t Br. 88–89. For their part,
Environmental Petitioners point to record evidence that clay-
lined units are likely to leak, and contend that the EPA’s
approach “authorizes an endless cycle of spills and clean-ups”
in violation of RCRA. See Envtl. Pet’r Br. 30.

     Clay-lined units are dangerous: “clay-lined units tended
to have lower risks than unlined units” but, the record evidence
showed, they were “still above the criteria” that the EPA set as
the threshold level requiring regulation. 75 Fed. Reg. at
35,144. Clay-lined surface impoundments have a 9.1 per cent
chance of causing groundwater contamination at drinking
water wells at a one-mile distance from the impoundment
perimeter. See Regulatory Impact Analysis 5-22, J.A. 1112.
And, as with unlined impoundments, the EPA acknowledges
that the risk of contamination from leaking clay-lined
impoundments is much higher closer to the impoundment
perimeter. See Risk Assessment 5-39 to 5-40, J.A. 1075–1076
(“[A]rsenic concentrations fall dramatically as the distance
from the [waste management units] increases.”); id. at 5-47 to
5-48, J.A. 1083–1084 (“In many of the potential damage cases,
ground water exceedances were discovered near the boundary
                                26
of the W[aste] M[anagement] U[nit].”). 3 Leaks from clay-
lined units, the EPA found, present cancer and non-cancer risks
that exceed the EPA’s risk criteria. See Risk Assessment 5-5,
5-30, J.A 1041, 1066.

     The EPA’s regulation of clay-lined impoundments suffers
from the same lack of support as its regulation of unlined
impoundments. See supra part III.A. Just as the EPA did not
explain how the Rule’s contemplated detection and response
could assure “no reasonable probability of adverse effects to
health or the environment” at unlined impoundments, it
likewise failed regarding existing impoundments lined with
nothing more than compacted soil. The EPA insists that the
Rule’s criteria ensure that leaks from these clay-lined units will
be “promptly” addressed, thereby satisfying RCRA. Resp’t
Br. 91.

     But here, too, the EPA has failed to show how unstaunched
leakage while a response is pending comports with the “no
reasonable probability” standard.            The problem is
compounded by the Rule’s unsupported supposition that
leaking clay liners, unlike leaking unlined impoundments, can
be repaired. The Rule thus allows an operator of a leaking
clay-lined impoundment time to explore repair even before the
five-to-fifteen year retrofit-or-close clock starts to run. For
starters, the Rule allows operators of lined impoundments up
to five months to complete an assessment of possible corrective
measures, 40 C.F.R. § 257.96(a), and—given the numerous,
complicated steps involved in doing so—allows an additional,
indefinite amount of time to actually select a remedy. See id.
§ 257.97; 80 Fed. Reg. at 21,407–21,408. Once an operator

     3
       The administrative record does not show the exact probability
of contamination from clay-lined units at a one-meter distance. See
Regulatory Impact Analysis, 4-9 to 5-22, J.A. 1111–1112.
                              27
settles on a remedy, it has another three months to initiate
remedial activities. Id. at § 257.98(a). If it turns out that no
effective repair is feasible, or if an attempted repair proves
unsuccessful, only then does the Rule contemplate the
impoundment’s operator will begin the protracted process—
discussed above in connection with the closure of existing,
unlined impoundments—of retrofitting the site with a
composite liner or closing it. There is no evidence in the
record supporting the EPA’s assumption that clay liners are
reasonably susceptible of repair, nor any explanation or
account of how the risks of harm during the lengthy response
periods the Rule allows comport with the “no reasonable
probability” standard.

    Just as with the EPA’s regulation of unlined
impoundments, the Rule’s treatment of clay-lined
impoundments does not capture the full range of health and
environmental harms they pose, as RCRA requires. By
responding only to risks from leakage contaminating
groundwater a mile from the perimeter of the studied
impoundments, and accordingly setting minimum criteria that
focus solely on harms to humans through drinking water
contamination, the EPA has failed to ensure “no reasonable
probability” of adverse effects to the environment, as RCRA
requires. 42 U.S.C. § 6944(a).

     For these reasons, we vacate the Rule insofar as it treats
“clay-lined” units as if they were lined. See 40 C.F.R.
§ 257.71(a)(1)(i).

                      C. Legacy Ponds

     The EPA exempted inactive impoundments at inactive
facilities, which are commonly referred to as “legacy ponds,”
from the same preventative regulation applied to all other
                              28
inactive impoundments under the Rule.               40 C.F.R.
§ 257.50(e). The EPA considered it sufficient instead just (i)
to wait to intervene until a substantial environmental or human
harm is “imminent,” 42 U.S.C. § 6973, or (ii) to attempt to
remediate the damage after contamination has occurred. 80
Fed. Reg. at 21,311 n.1; id. at 21,312 n.2. Environmental
Petitioners argue that, because legacy ponds pose at least the
same risks of adverse effects as all other inactive
impoundments, the EPA failed to articulate a rational
explanation for their dissimilar treatment.

     The EPA does not dispute the dangers posed by the
unregulated legacy ponds, but asserts that the difficulties in
identifying the party responsible for legacy ponds justify its
reactive approach. Because the EPA’s own record plainly
contravenes that rationale, and the Rule pays scant attention to
the substantial risk of harm to human health and the
environment posed by legacy ponds, we reject the legacy pond
exemption as arbitrary and capricious.

                               1.

     Legacy ponds are a particular subset of inactive
impoundments. Like all inactive impoundments, they contain
a toxic “slurry” of Coal Residuals mixed with water, but legacy
ponds are not receiving new deposits. 80 Fed. Reg. at 21,457
n.219. What distinguishes legacy ponds from other inactive
impoundments, then, is their location. Legacy ponds are
found at power plants that are no longer engaged in energy
production. In other words, legacy ponds are inactive
impoundments at inactive facilities.

     As a result, legacy ponds present a unique confluence of
risks: They pose the same substantial threats to human health
and the environment as the riskiest Coal Residuals disposal
                               29
methods, compounded by diminished preventative and
remediation oversight due to the absence of an onsite owner
and daily monitoring. See 80 Fed. Reg. at 21,343–21,344
(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. Id. at 21,394; id. at 21,393.

    Nevertheless, the EPA chose to leave legacy ponds on the
regulatory sidelines. 40 C.F.R. § 257.50(e). Unlike all the
other inactive impoundments, EPA adopted a largely hands-off
approach, choosing (i) to respond only after “imminent”
leakage is detected and reported, 42 U.S.C. § 6973(a) (RCRA’s
“imminent and substantial endangerment” provision), or to (ii)
attempt an after-the-spill clean up under the Comprehensive
Environmental Response, Compensation, and Liability Act
(commonly known as the “Superfund” statute), 42 U.S.C.
§ 9601 et seq. See 80 Fed. Reg. at 21,312 n.2 (citing 42 U.S.C.
§ 9608(b)).

     The EPA’s rationale for allowing legacy ponds, in effect,
one free leak was its supposed inability to identify the owners
of legacy ponds. In the absence of an identified owner or
other responsible party, the EPA reasoned, enforcing the Coal
Residuals regime would be difficult with no operator onsite to
generate compliance certifications, conduct inspections, or
otherwise implement the Rule’s substantive requirements.
See 80 Fed. Reg. at 21,344.

    The EPA’s decision was arbitrary and capricious. To
begin with, there is no gainsaying the dangers that unregulated
legacy ponds present. The EPA itself acknowledges the vital
importance of regulating inactive impoundments at active
                              30
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 Fed. Reg. at 35,177; see also 80 Fed. Reg.
at 21,344 n.40.

     The risks posed by legacy ponds are at least as substantial
as inactive impoundments at active facilities. See 80 Fed.
Reg. at 21,342–21,344 (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-health and environmental
disasters in the years leading up to the Rule’s promulgation.
See 75 Fed. Reg. 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 Fed. Reg. at 21,393–21,394. Simply hoping
that somehow there will be last-minute warnings about
imminent dangers at sites that are not monitored, or relying on
cleaning up the spills after great damage is done and the harm
inflicted does not sensibly address those dangers. Certainly it
does not fulfill the EPA’s statutory duty to ensure “no
reasonable probability of adverse effects” to environmental
and human well-being. 42 U.S.C. § 6944(a).

    Confronted by those considerable dangers, the EPA’s
decision to shrug off preventative regulation makes no sense.
The asserted difficulty in locating the owners or operators
responsible for legacy ponds does not hold water. The record
shows that the EPA knows where existing legacy ponds are
                                 31
and, with that and other information, the EPA already is aware
of or can feasibly identify the responsible parties. After all,
the owners and operators of the Dan River, Widows Creek, and
Gambrills, Maryland disasters were all known. See 80 Fed
Reg. at 21,393–21,394; 75 Fed. Reg. at 35,147.

     Also, the EPA has been compiling and maintaining a
database for nearly a decade that identifies legacy ponds and
their owners with specificity. See Regulatory Impact Analysis
for EPA’s Proposed RCRA Regulation of Coal Combustion
Residues, Information Request Responses from Electric
Utilities      (April       30,      2010),     available     at
https://archive.epa.gov/epawaste/nonhaz/industrial/special/fos
sil/web/xlsx/survey_database_041212.xlsx. The Final Rule’s
Regulatory Impact Analysis named more than thirty other
owners and operators of recently, or soon-to-be, retired power
plants where more than 100 legacy ponds are located. This
included a State-by-State list detailing the number of already-
inactive impoundments, and the utility responsible for each
one. See id.; see also J.A. 1104, 1119. The database further
identified 83 power plants that were scheduled to “fully close
all coal-fired” facilities by the time the Rule went into effect,
over 75% of which would house a legacy pond upon closure.
J.A. 1116.

     In sum, the EPA acknowledges that (i) it has the authority
to regulate inactive units, (ii) it is regulating inactive units at
active facilities, (iii) the risks posed by legacy ponds are at least
as severe as the other inactive-impoundment dangers that the
“[R]ule specifically seeks to address, and [(iv)] there is no
logical basis for distinguishing between units that present the
same risks.” 80 Fed. Reg. at 21,343. The EPA also considers
it “quite clear” that older, unlined impoundments, Oral Arg. Tr.
at 94:22—which are primarily legacy ponds—pose “the
greatest risks to human health and the environment,” 80 Fed.
                              32
Reg. at 21,451. Because the administrative record belies the
EPA’s stated reason for its reactive, rather than preventative,
approach—the inability to identify the responsible parties—the
Rule’s legacy ponds exemption is unreasoned, arbitrary, and
capricious.

     D. Inadequate Notice by Owners and Operators

     Because of RCRA’s reliance on citizen enforcement, the
statute requires the EPA to “develop and publish minimum
guidelines for public participation” in the “development,
revision, implementation, and enforcement” of any RCRA
regulation. 42 U.S.C. § 6974(b)(1). The EPA implements
that statutory requirement, as relevant here, by requiring the
owners of Coal Residuals units to “maintain a publicly
accessible Internet site” on which they timely disclose
specified information about their compliance with RCRA
regulations. 40 C.F.R. § 257.107(a).

     The Environmental Petitioners wage several assaults on
the Rule’s Internet notice requirements, arguing that they do
not provide adequate or timely notice to permit the public to
participate in monitoring compliance with the Rule. For
example, the Environmental Petitioners object that the Rule
does not require owners or operators of new Coal Residual
impoundments to post a design certification—an engineer’s
certification that the impoundment’s liner meets the EPA’s
criteria—until sixty days after construction begins. 40 C.F.R.
§ 257.107(f)(1); see 40 C.F.R. § 257.70. That is too late, the
Environmental Petitioners argue, to put the public on effective
notice of any potential design problems. They also object that
the Rule does not require timely public notification about the
design or liner compliance of impoundment expansions, the
structural integrity of facilities, protections against airborne
                              33
coal dust, run-off control, hydraulic capacity requirements, or
the nature of groundwater monitoring efforts.

     The problem for Environmental Petitioners is that,
although they participated in the notice-and-comment
rulemaking process, they never voiced objections to the Rule’s
notice provisions that they now challenge. Having stood
silent during the rulemaking, the Environmental Petitioners
may not now raise their complaints for the first time in their
petition for judicial review. See Military Toxics Project v.
EPA, 146 F.3d 948, 956 (D.C. Cir. 1998); see also City of
Portland v. EPA, 507 F.3d 706, 710 (D.C. Cir. 2007) (“Because
[no] party raised this argument before the [EPA] during the
rulemaking process, however, it is waived, and we will not
consider it.”). The sanction does not exist as a procedural
trap; the notice-and-comment process is in place so that the
agency can consider and—if necessary—revise its proposed
rule in light of public comments. United States v. L.A. Tucker
Truck Lines, Inc., 344 U.S. 33, 37 (1952) (“[O]rderly
procedure and good administration require that objections to
the proceedings of an administrative agency be made while it
has opportunity for correction in order to raise issues
reviewable by the courts.”). The EPA reasonably focuses its
resources on consideration and/or modification of challenged
portions of a proposed rule rather than unchallenged and
apparently uncontroversial portions thereof. See Interstate
Nat. Gas Ass’n of Am. v. FERC, 494 F.3d 1092, 1096 (D.C.
Cir. 2007) (agency must respond to material comments only).
Accordingly, we will not address this claim.

           IV. Industry Petitioners’ Challenges

    Industry Petitioners bring a host of their own challenges to
the Rule. As noted, these claims have dwindled over the
course of this litigation. At the start, Industry Petitioners
                                 34
challenged eighteen provisions of the Final Rule and
questioned the EPA’s statutory authority to regulate inactive
surface impoundments. 4 In response, the EPA filed an
unopposed motion to sever and remand two aspects of the Final
Rule (regarding five regulatory provisions). On June 14,
2016, we granted the motion. Industry Petitioners continued
to challenge the thirteen remaining substantive provisions as
well as to attack the EPA’s statutory authority. In the parties’
proposed oral argument structure, however, Industry
Petitioners moved to dismiss two additional challenges
(regarding three regulatory provisions). We granted that
motion as well.

     Accounting for these interim trims, Industry Petitioners
now assert that the EPA (i) lacks authority to regulate inactive
impoundments; (ii) failed to provide sufficient notice of its
intention to apply the aquifer location criteria to existing
impoundments, to regulate Coal Residual piles of 12,400 or
more tons, and to regulate the temporary storage of Coal

     4
        Industry Petitioners’ initial brief challenged portions of the
following regulations: 40 C.F.R. §§ 257.50(c), 257.100 (inactive
impoundments); 40 C.F.R. § 257.53 (definition of “beneficial use”
and regulation of CCR “pile”); 40 C.F.R. § 257.60 (aquifer location
restrictions); 40 C.F.R. §§ 257.73(e), (f)(1), 257.74(e) (minimum
safety factors); 40 C.F.R. §§ 257.90(d), 257.96(a) (“release”
response); 40 C.F.R. §§ 257.73(a)(4), 257.74(a)(4) (dike
requirements); 40 C.F.R. § 257.103(a)(1)(i), (b)(1)(i) (prohibition on
considering cost and inconvenience); 40 C.F.R. § 257.63(a) (seismic
impact zone landfill requirements); 40 C.F.R. § 257.63(c)(1)
(seismic impact zone deadline); 40 C.F.R. § 257.103 (inclusion of
non-Coal Residuals waste streams in alternative closure provision);
40 C.F.R. §§ 257.95(h)(2), 257.97 (exclusion of risk-based
compliance alternatives).
                                35
Residuals destined for beneficial use; and (iii) acted arbitrarily
in regulating residual piles of 12,400 or more tons, in
regulating on-site Coal Residuals destined for beneficial use,
in eliminating the risk-based compliance alternatives, in
issuing location requirements based on seismic impact zones,
and in imposing temporary closure procedures.5

     The EPA, now supported in part by Industry Petitioners,
requests a remand of several of those issues, namely whether
(i) the EPA has statutory authority to regulate inactive
impoundments, (ii) the EPA arbitrarily regulated Coal
Residuals piles of 12,400 or more tons, (iii) the EPA arbitrarily
regulated on-site Coal Residuals destined for beneficial use,
and (iv) the EPA arbitrarily eliminated risk-based compliance
alternatives.

     We grant the request for voluntary remand of the Coal
Residuals pile-size and beneficial-use issues, and we dismiss
as moot both the claim regarding risk-based compliance
alternatives and the accompanying notice challenges. As to
all remaining issues, we deny remand, and we deny the
Industry Petitioners’ petition for review.

            A. EPA’s Motion for Voluntary Remand

    We have broad discretion to grant or deny an agency’s
motion to remand. See Limnia, Inc. v. Department of Energy,
857 F.3d 379, 381, 386 (D.C. Cir. 2017). We generally grant
an agency’s motion to remand so long as “the agency intends


     5
       These challenges encompass the following regulations (or
portions thereof): 40 C.F.R. §§ 257.50(c), 257.100 (inactive
impoundments); 40 C.F.R. § 257.103(a)(1)(i), (b)(1)(i) (alternative
closure requirements); 40 C.F.R. § 257.63(a), (c)(1) (seismic impact
zone requirements).
                              36
to take further action with respect to the original agency
decision on review.” Id. (emphasis omitted). Remand has
the benefit of allowing “agencies to cure their own mistakes
rather than wasting the courts’ and the parties’ resources
reviewing a record that both sides acknowledge to be incorrect
or incomplete.” Ethyl Corp. v. Browner, 989 F.2d 522, 524
(D.C. Cir. 1993). Remand may also be appropriate if the
agency’s motion is made in response to “intervening events
outside of the agency’s control, for example, a new legal
decision or the passage of new legislation.” SKF USA Inc. v.
United States, 254 F.3d 1022, 1028 (Fed. Cir. 2001)
(discussing National Fuel Gas Supply Corp. v. FERC, 899 F.2d
1244, 1249 (D.C. Cir. 1990) (per curiam)). Alternatively,
“even if there are no intervening events, the agency may
request a remand (without confessing error) in order to
reconsider its previous position.” Id. at 1029.

    In deciding a motion to remand, we consider whether
remand would unduly prejudice the non-moving party.         See
FBME Bank Ltd. v. Lew, 142 F. Supp. 3d 70, 73 (D.D.C. 2015).
Additionally, if the agency’s request appears to be frivolous or
made in bad faith, it is appropriate to deny remand. See SKF
USA, 254 F.3d at 1029; see also Lutheran Church-Missouri
Synod v. FCC, 141 F.3d 344, 349 (D.C. Cir. 1998) (denying
FCC’s “novel, last second motion to remand” because it was
based on agency’s non-binding prospective policy statement).

     To start, we decline the EPA’s request to remand the
challenge to the agency’s authority to regulate inactive
impoundments so that it can reconsider its interpretation of the
statute, for two reasons. First, the EPA’s statutory authority
over inactive sites necessarily implicates the Environmental
Petitioners’ claim regarding legacy ponds. So, even if
Industry Petitioners are willing to go along with a remand,
Environmental Petitioners are not and remand would prejudice
                               37
the vindication of their own claim. Second, this claim
involves a question—the scope of the EPA’s statutory
authority—that is intertwined with any exercise of agency
discretion going forward. Given that, the EPA has not met its
burden of justifying its last-minute request for a remand in this
case, and we proceed to the merits on this issue.

     The EPA also initially requested a remand of its decision
to exclude certain risk-based compliance measures. On July
30, 2018, however, the EPA promulgated amendments to the
Final Rule. See Hazardous and Solid Waste Management
System: Disposal of Coal Combustion Residuals From
Electric Utilities; Amendments to the National Minimum
Criteria (Phase One, Part One) (“Final Rule Amendments”), 83
Fed. Reg. 36,435 (July 30, 2018).             The Final Rule
Amendments provide certain risk-based compliance measures
and site-specific engineering certifications. Accordingly, we
dismiss as moot Industry Petitioners’ challenge to 40 C.F.R.
§§ 257.95(h) and 257.97. See National Min. Ass’n v.
Department of Interior, 251 F.3d 1007, 1011 (D.C. Cir. 2001)
(dismissing challenges as moot due to “substantial changes” in
regulations and declaring “[a]ny opinion regarding the former
rules would be merely advisory”).

     For the remaining requests—(i) the regulation of Coal
Residuals piles; (ii) the Proposed Rule’s notice of the Coal
Residuals pile regulation; and (iii) the 12,400-ton threshold for
beneficial use (and notice thereof)—we grant the EPA’s
motion to remand.6 First and foremost, the EPA has explained
that it plans to reconsider these provisions and has submitted a

    6
       Specifically, we remand without vacating 40 C.F.R § 257.53
(definition of “beneficial use” and regulation of Coal Residuals
“pile”).
                               38
proposed timeline to the court, thereby satisfying the
requirement for remand that it “take further action with respect
to the original agency decision on review.” Limnia, 857 F.3d
at 386 (emphasis omitted). Second, although the WIIN Act
does not affect the validity of the Rule itself, it does provide
the EPA with new tools to pursue its regulatory goals. See 42
U.S.C. § 6945(d)(4) (incorporating enforcement provisions of
Sections 6927 and 6928).7

     The EPA has explained that the Final Rule was
promulgated with the understanding that there would be no
regulatory “overseer,” and therefore the Final Rule itself
should “account for and be protective of all sites, including
those that are highly vulnerable.” 80 Fed. Reg. at 21,311; id.
(explaining how “the requirement to establish national criteria
and the absence of any requirement for direct regulatory
oversight” influenced the Final Rule). Although a one-size-
fits-all national standard might have been necessary for the
self-implementing Final Rule, more precise risk-based
standards are both feasible and enforceable under the
individualized permitting programs and direct monitoring
provisions authorized by the WIIN Act. See Oral Arg. Tr.
37:12–37:14 (counsel for EPA explaining that certain
provisions of the Final Rule “cry out for site specific
enforcement”). Thus, the regulatory tools authorized by the
WIIN Act support the EPA’s request to reconsider certain
provisions of the Rule. See SKF USA, 254 F.3d at 1028.


    7
        On March 23, 2018, the Consolidated Appropriations Act of
2018 was signed into law. Pub. L. No. 115-141, 132 Stat. 348. It
allocates funds to the EPA to “implement[] a coal combustion
residual permit program under” the WIIN Act. Id. at Division G,
Title II. Accordingly, with its recently acquired funding, the EPA
is to “implement a permit program” in non-participating states. 42
U.S.C. § 6945(d)(2)(B).
                              39
     Moreover, the provisions we now remand stand
unchallenged on their merits; accordingly, no party will suffer
prejudice from remand without vacatur. See FBME Bank, 142
F. Supp. 3d at 73. Indeed, at this stage in the litigation, all
parties agree that the “beneficial use” and “Coal Residuals
pile” provisions should stay in effect—at least until a new rule
is promulgated. See EPA Remand Mot. 2 (“EPA seeks
remand of these provisions without vacatur, and thus they
remain in place and fully applicable[.]”). Moreover, the only
parties that object to remand—Environmental Petitioners—did
not challenge any of the relevant provisions in their petition;
rather they defended the provisions as Intervenors. See
generally Envtl. Intervenor-Resp’t Br. 14–22. Accordingly,
any opinion we issue regarding these provisions would be
wholly advisory; it would resolve no active case or controversy
and would award no relief. See Chafin v. Chafin, 568 U.S.
165, 172 (2013) (case is non-justiciable if court is unable to
grant concrete relief to any party).

     We conclude that there is no reason to opine on the
“beneficial use” and “Coal Residuals pile” provisions that the
EPA wants remanded. At oral argument, the court pressed
Industry counsel as to why Industry Petitioners did not simply
dismiss their petition rather than acquiescing in the EPA’s
motion. Oral Arg. Tr. 49–52. Industry counsel did not
provide a clear answer. But he did make two concessions:
First, he declared that Industry does not oppose remand. Id. at
50:16–50:23. Second, he acknowledged, “on a remand * * *
the petition * * * is dismissed as a practical matter.” Id. at
51:6–51:10 (emphasis added). Counsel is correct in one
respect.    When combined with the statutory provision
requiring any challenge to be brought within 90 days of the
Rule’s promulgation, the legal effect of remand without
vacatur is simple: The Rule remains in force and Industry
Petitioners cannot bring another challenge until and unless the
                               40
EPA takes additional regulatory action.            42 U.S.C.
§ 6976(a)(1) (petition for review “shall be filed within ninety
days from the date of * * * promulgation”). In effect, Industry
Petitioners have withdrawn their petition with respect to the
provisions for which it does not oppose remand.

     Accordingly, we deny the EPA’s motion to remand to
itself Industry Petitioners’ challenge to the EPA’s regulation of
inactive impoundments and Environmental Petitioner’s
challenge to the non-regulation of legacy ponds. We
otherwise grant the motion to remand without vacatur.

                 B.   Substantive Challenges

     After deciding the issue of remand, we are left with
Industry Petitioners’ statutory argument and its three APA
challenges to the Final Rule.

      1. Authority to Regulate Inactive Impoundments

    Industry Petitioners first challenge the EPA’s regulatory
authority to set any standards at all for inactive impoundments.
That claim is without merit. Because those inactive sites
house waste in “open dumps,” 42 U.S.C. § 6944, RCRA’s
plain text unambiguously confers regulatory authority on the
EPA.

     By its terms, RCRA empowers the EPA generally to
define “which facilities shall be classified as sanitary landfills
and which shall be classified as open dumps[.]” 42 U.S.C.
§ 6944. Section 6943 of RCRA, in turn, incorporates those
classification standards into minimum criteria for State
regulatory plans. Id. § 6943. Those statutory minimums
both require States to “provide for the closing or upgrading of
all existing open dumps” and prohibit “the establishment of
                                 41
new open dumps[.]” Id. § 6943(a)(2), (3). The statute also
provides that, “[a]t a minimum,” the EPA must define sanitary
landfills to include only facilities where “there is no reasonable
probability of adverse effects on health or the environment
from disposal of solid waste[.]” 42 U.S.C. § 6944(a). 8 In
this way, the statute creates a binary world: A facility is a
permissible sanitary landfill, or it is an impermissible open
dump. The EPA regulates both.

     While the statute allows the EPA to establish criteria for
distinguishing between “open dumps” and “sanitary landfills,”
it also offers some definitions of its own. RCRA defines
“open dump” as “any facility or site where solid waste is
disposed of which is not a sanitary landfill” or a site regulated
under RCRA Subtitle C’s more rigorous hazardous waste
provisions. 42 U.S.C. § 6903(14). The statute likewise
defines “sanitary landfill” as “a facility for the disposal of solid
waste [that] meets the criteria published under section 6944,”
id. § 6903(26), and that operates in accordance with the
“applicable criteria for coal combustion residuals units under”
40 C.F.R. Part 257 or its successor regulations, id.
§ 6945(d)(6).

     Finally, RCRA defines “disposal” as “the discharge,
deposit, injection, dumping, spilling, leaking, or placing of any
solid waste or hazardous waste into or on any land or water” in
a manner by which “such solid waste or hazardous waste or
any constituent thereof may enter the environment[.]” 42
U.S.C. § 6903(3).


     8
       As noted earlier, supra, Part II, we leave open on remand the
definitional and substantive impact on the EPA’s discretion of the
WIIN Act’s express incorporation of the extant or successor EPA
regulations under 40 C.F.R. Part 257 into the statutory definition of
“sanitary landfill.”
                               42

     Notwithstanding that broad assignment of regulatory
authority, see 42 U.S.C. § 6912, Industry Petitioners argue that
“inactive” impoundments—sites that contain, but no longer
receive new, solid waste—cannot be “open dumps” within the
EPA’s regulatory ambit. Seizing on the phrase “is disposed
of” in the definition of an “open dump,” id. § 6903(14), they
contend that the site must actively receive new waste to come
within the statutory definition of a regulable waste disposal
dump. Industry Petitioners also argue that the words used to
define “disposal”—“discharge, deposit, injection, dumping,
spilling, leaking, or placing,” id. § 6903(3)—all require present
and ongoing activity.

     RCRA’s reach, however, is not so narrow as Industry
Petitioners suppose. Rather, a straightforward reading of the
statute’s language allows for the regulation of inactive sites.

     We start by recognizing that, in RCRA, Congress
delegated to the EPA “very broad” regulatory authority over
waste disposal. In re Consolidated Land Disposal Regulation
Litig., 938 F.2d 1386, 1388 (D.C. Cir. 1991). We therefore
review the Industry Petitioners’ challenge under the two-step
Chevron framework.          Under this rubric, if RCRA is
unambiguous, its text controls. See Chevron, U.S.A., Inc. v.
Natural Res. Def. Council, Inc., 467 U.S. 837, 842–843 (1984);
see also City of Arlington v. FCC, 569 U.S. 290, 297 (2013)
(holding that an agency’s interpretation of the “jurisdictional”
reach of its governing statute merits Chevron deference). If,
on the other hand, the statute is silent or equivocal, we ask only
whether the agency has offered a reasonable interpretation of
the statute. Chevron, 467 U.S. at 843.

     Resolution of this issue begins and ends with RCRA’s
plain text. The definition of “open dump,” which is the key
                                  43
term at issue, does not use the word “disposal.” It uses the
phrase “is disposed of”: An “open dump” is “any facility or
site where solid waste is disposed of[.]” 42 U.S.C. § 6903(14)
(emphasis added). To divine its proper meaning, we must
interpret the operative phrase “is disposed of” as a whole.
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. See Florida Dep’t of
Revenue v. Piccadilly Cafeterias, Inc., 554 U.S. 33, 39 (2008)
(describing a past participle as a “verb form” that reaches “past
or completed action”) (quoting AMERICAN HERITAGE
DICTIONARY 1287 (4th ed. 2000)); Sherley v. Sebelius, 644
F.3d 388, 403 n.4 (D.C. Cir. 2011) (Henderson, J., dissenting)
(noting that the statute at issue “combine[d] the present tense
‘are’ with the past participle ‘destroyed’” to “signify conduct
that ha[d] already occurred”) (citations omitted).9

     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

     9
        The concurring opinion notes that Piccadilly Cafeterias was
ultimately resolved as a Chevron step two case. Concur Op. 4 n.1.
True enough. But before the Court got to the Chevron step two
stage of its analysis, it first endorsed, as the “more natural reading”
of the relevant text, Florida’s construction of the past participle as
“unambiguously limit[ing]” certain tax exemptions in bankruptcy
proceedings. 554 U.S. at 39, 41. Only then did the Court, for
argument’s sake, “assum[e]” that the relevant text were
“ambiguous,” and hold that any ambiguity would fall in Florida’s
favor. Id. at 41. The Court, in short, never found any ambiguity in
the past participle’s coverage of “past or completed action[s],” and
in fact embraced that more natural meaning. Id. at 39. We too give
Congress’s adjectival past participle “is disposed of” its natural
meaning.
                               44
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” in at a
site that is still operating.

     Tellingly, not even Industry Petitioners embrace the full
import of their interpretation. They agree that previously
deposited waste “is disposed of” at an impoundment site, so
long as the site is actively accepting new waste. But if EPA’s
authority reaches only active disposal, it stands to reason that
its authority over the site extends only to that newly deposited
(or actively leaking) waste. But Industry Petitioners do not
push this point—probably because, as a practical reality, waste
is no less “disposed of” at a site the day after operations cease
than it was the day before. That is, the waste previously
dumped is still currently “placed” or “deposited” there. 42
U.S.C. § 6903(3), (14). In other words, the pile of Coal
Residuals retains its regulated status whether or not anyone
adds to the pile.

     Think of it this way: If a kindergarten teacher tells her
students that they must clean up any drink that “is spilled” in
the room, that would most logically be understood to mean that
a student must clean up her spilled drink even if the spill is
already completed and nothing more is leaking out of the
carton. A student who refused to clean up that completed spill
(as Industry Petitioners would have it) might well find himself
on time out.

    What’s more, the Industry Petitioners’ reading butts up
against the binary world created by the statute. RCRA creates
two categories for Subtitle D waste: open dumps and sanitary
landfills. Industry Petitioners offer no explanation of where
                                45
“inactive” sites fit into their understanding of that landscape.
Nor do they explain why, once the last person turns off the
lights, Congress’s concern for the substantial health and
environmental dangers posed by that pile of toxic waste would
completely evaporate. As our concurring colleague aptly
notes, “the disposal of [Coal Residuals] in an impoundment is
not a discrete act. If it were, the EPA would regulate only the
transfer of [Coal Residuals] from a power facility into an
impoundment, at which point the ‘disposal’ would end.”
Concur Op. at 8.

     The concurring opinion spies ambiguity only by splitting
the operative verb “is disposed” into two distinctly analyzed
parts: “is” and “disposed.” Concur Op. 2–4. But just as
courts must not “construe statutory phrases in isolation,” we
surely must read a single verb “as a whole” and not in pieces.
United States v. Morton, 467 U.S. 822, 828 (1984). Even
more so, we must give effect to the whole adjectival phrase “is
disposed of.” A site where garbage “is disposed of” is the
place where garbage is dumped and left. The status of that site
does not depend on whether or not more garbage is later piled
on top. A garbage dump is a garbage dump until the deposited
garbage is gone.

     In short, as facilities “where solid waste is disposed of,”
42 U.S.C. § 6903(14), inactive impoundments are “open
dumps,” unless they fall into one of two statutory exceptions—
neither of which the Industry Petitioners claim applies to their
inactive impoundments. 10 And no one denies that the EPA
has authority to regulate (and to prohibit) “open dumps.”


    10
       The two exceptions, which Industry Petitioners do not
contend apply here, are for “sanitary landfills,” as defined by the
                                   46

     Instead, the Industry Petitioners point to cases interpreting
the term “disposal” in the Superfund statute, 42 U.S.C. § 9601
et seq., to apply only to ongoing disposals. True enough. But
those cases turned on the Superfund statute’s different
language, which is “at the time of disposal,” not the RCRA
phrase “is disposed of.” See id. § 9607(a) (responsible
persons subject to recovery costs under the Superfund statute
include “any person who at the time of disposal of any
hazardous substance owned or operated any facility at which
such hazardous substances were disposed of”). The specific
signification of that language lies at the heart of those court
rulings. See Carson Harbor Vill., Ltd. v. Unocal Corp., 270
F.3d 863, 871 (9th Cir. 2001) (“We must decide in this case
whether the Partnership Defendants * * * owned the
contaminated property ‘at the time of disposal of any hazardous
substance.’”) (citing 42 U.S.C. § 9607(a)(2)).11

    The Superfund statute also contains an “innocent
landowner” defense by which a person can avoid liability if
“the disposal or placement of the hazardous substance”
occurred prior to that party’s acquisition of the property. 42
U.S.C. § 9601(35)(A). That strengthens the notion that “at the
time of disposal,” as used in the Superfund statute, is time-

EPA, 42 U.S.C. § 6944, and sites housing “hazardous” waste
regulated separately under RCRA Subtitle C, id. § 6921 et seq.
     11
        See also ABB Indus. Sys., Inc. v. Prime Tech., Inc., 120 F.3d
351, 356 (2d Cir. 1997) (“Under [the Superfund statute], a prior
owner or operator is a responsible party if it controlled the site ‘at the
time of disposal’ of a hazardous substance.”); United States v.
CDMG Realty Co., 96 F.3d 706, 712–713 (3d Cir. 1996) (“HMAT
contends that Dowel is liable as a person who owned or operated the
facility ‘at the time of disposal’ of a hazardous substance.”); Joslyn
Mfg. Co. v. Koppers Co., 40 F.3d 750, 760 (5th Cir. 1994) (similar).
                               47
dependent and refers to the act of placing the waste in the
holding site. See Carson Harbor Vill., 270 F.3d at 882.
RCRA’s distinct language comes with no such limiting textual
indicia.

     In short, the fundamental flaw in the Industry Petitioners’
effort to limit EPA regulation to active impoundments is that
they focus on the wrong text. For all their efforts to explain
the meaning of the single word “disposal,” they fail to grapple
with the full phrase “is disposed of.” RCRA is explicit that
inactive sites may qualify as open dumps if they are facilities
where waste “is disposed of,” regardless of whether they are
also facilities where more “disposal” continues to occur. As
is often true in statutory interpretation, the words make all the
difference.

     Even if the text were ambiguous, the EPA’s interpretation
is eminently reasonable under Chevron step two. First, the
same reasons supporting our interpretation of the plain
statutory text demonstrate with even greater force the
reasonableness of the EPA’s interpretation.

     Second, the EPA’s interpretation directly advances
RCRA’s stated regulatory purpose. RCRA directs the EPA to
develop standards that limit permissible waste sites “[a]t a
minimum” to those with “no reasonable probability of adverse
effects on health or the environment from disposal of solid
waste[.]” 42 U.S.C. § 6944(a). No one denies that inactive
impoundments can have significant adverse environmental and
health effects. In fact, the EPA persuasively explains that
inactive sites often pose even greater health risks given their
age and accompanying deterioration. 80 Fed. Reg. at 21,343
(indicating that “the risks are primarily driven by the older
existing units”); see also id. (noting that leaks into the Dan
                              48
River from an inactive impoundment occasioned publication of
this very Rule).

     The EPA’s construction of the text is thus consistent with
a straightforward reading of statutory text and RCRA’s central
purpose. See In re Consolidated Consol. Land Disposal
Regulation Litig., 938 F.2d at 1389 (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 under
Chevron step two).

     For all of those reasons, the Industry Petitioners’ attempt
to confine the EPA’s authority to only active impoundments
fails.

      2. Notice Challenge to Aquifer Requirements

     Under 5 U.S.C. § 553, an agency is required to give notice
of a proposed rule and allow interested parties to comment on
the rule before it is promulgated. Although the final rule need
not be identical to the proposed rule, it must be the “logical
outgrowth” thereof. Shell Oil Co. v. EPA, 950 F.2d 741, 747
(D.C. Cir. 1991) (per curiam). “A rule is deemed a logical
outgrowth if interested parties ‘should have anticipated’ that
the change was possible, and thus reasonably should have filed
their comments on the subject during the notice-and-comment
period.” Northeast Md. Waste Disposal Auth. v. EPA, 358
F.3d 936, 952 (D.C. Cir. 2004) (per curiam) (citing City of
Waukesha v. EPA, 320 F.3d 228, 245 (D.C. Cir. 2003)).

     The Final Rule requires that all surface impoundments be
located no fewer than five feet above the uppermost aquifer or,
alternatively, that the owner or operator of the impoundment
demonstrate that the impoundment will not be subject to a
hydraulic connection with the groundwater supply as
                                49
groundwater levels fluctuate over the course of the year.12 40
C.F.R. § 257.60(a); see 80 Fed. Reg. at 21,361. Industry
Petitioners argue that the EPA did not give adequate notice that
this provision would apply to existing surface impoundments
because the proposed regulation applied only to “[n]ew [Coal
Residuals] landfills and new [Coal Residuals] surface
impoundments[.]” 75 Fed. Reg. at 35,241.13

     The Industry Petitioners’ argument ignores the plain
language of the preamble to the Proposed Rule, which declares:
“[b]y contrast [to landfills] * * * the proposed regulations
would apply all of the location restrictions to existing surface
impoundments.” 75 Fed. Reg. at 35,198 (emphasis added).
This is exactly what the Final Rule prescribes. See 40 C.F.R.
§ 257.60. Indeed, the Rule is not only the “logical outgrowth”
of the Proposed Rule; it faithfully tracks the goals set forth in
the preamble. See Shell Oil Co., 950 F.2d at 747. The
preamble—and the Proposed Rule as a whole—advised the

     12
        A “hydraulic connection” means a connection between the
[Coal Residuals] unit and the underground water table. 80 Fed.
Reg. at 21,362. The EPA received comments explaining that
“fluctuations in groundwater levels in many geological settings can
exceed ten feet over the course of the year.” Id. at 21,361. To
account for this change in aquifer levels, the EPA revised its
definition of “uppermost aquifer” to “specify that the measurement
of the upper limit of the aquifer must be made at a point nearest to
the natural ground surface to which the aquifer rises during the wet
season.” Id. at 21,362.
     13
         In the preamble to the Final Rule, the EPA acknowledged
that, “[i]n the proposed rule, the regulatory language should have
included ‘all surface impoundments’ as opposed to only ‘new
surface impoundments.’” 80 Fed. Reg. at 21,360.
                                  50
public that the EPA was at least considering applying the
aquifer restrictions to existing impoundments, thereby inviting
Industry’s comments on the topic. Id.14

               3. Seismic Impact Zone Criteria

     The Final Rule contains two seismic impact requirements.
First, the Rule imposes safety assessment criteria on surface
impoundments over a specific size. 40 C.F.R. § 257.73(e).
These criteria had an implementation deadline of October 17,
2016. Id. § 257.73(f). Because the compliance deadline
lapsed before oral argument, Industry Petitioners voluntarily
dismissed this challenge. See Sept. 27, 2017 Per Curiam
Order Granting Motion to Dismiss.

    Second, every new Coal Residual landfill and landfill
expansion, as well as any new and existing surface
     14
         Although the EPA may not “bootstrap notice from a
comment,” the sheer volume of Industry Petitioners’ comments on
this very provision confirms that notice was adequate. Fertilizer
Inst. v. EPA, 935 F.2d 1303, 1312 (D.C. Cir. 1991) (internal
quotation marks omitted). The EPA explains: “Overwhelmingly,
the issue receiving the most comment was EPA’s intention to subject
existing [Coal Residuals] surface impoundments to all of the new
location criteria.” 80 Fed. Reg. at 21,360. Industry Petitioners’
comments confronted the aquifer location restrictions, including
their applicability to existing surface impoundments, head-on. See,
e.g., Comments of the Utility Solid Waste Activities Group on
Proposal, Nov. 19, 2010, J.A. 775 (“EPA states in the preamble to
the proposal that it intends to subject existing surface impoundments
to all of these new location restrictions * * * .”) (emphasis omitted).
When combined with the clarity of the preamble, Industry
Petitioners’ comments illustrate that it was both aware of, and
troubled by, the aquifer restrictions.
                                 51
impoundment, is subject to location restrictions that prohibit
operation in a “seismic impact zone” 15 unless the facility
demonstrates that it has the appropriate structural components,
including liners, leachate collection and removal systems and
surface water control systems. 40 C.F.R. § 257.63(a). For
existing surface impoundments, the deadline for demonstrating
compliance with the Rule is October 17, 2018—four and one-
half years after the Rule was promulgated. Id. § 257.63(c)(1).

     Industry Petitioners attack the seismic impact zone
requirements on three fronts; they argue that the EPA was
arbitrary and capricious in: (i) shortening the operating life
for existing impoundments from five years to four years; (ii)
applying the seismic impact zone location restriction to new
Coal Residual landfills and landfill expansions; and (iii)
regulating the structure of Coal Residual landfills based on a
2,500-year seismic event. The parties brief these three issues
separately, and we likewise address—but reject—each of
Industry Petitioners’ challenges in turn.

                     a. Operating Expiration

    Industry Petitioners argue that, although the Proposed
Rule had a five-year operating expiration for impoundments,
the Final Rule arbitrarily reduced that window to four years.
Industry Pet’rs’ Br. 45. As a corollary, Industry Petitioners
argue that four years is not enough time for impoundment
owners and operators to switch from wet to dry Coal Residuals
disposal. Industry Pet’rs’ Reply Br. 21–22.



     15
       “Seismic impact zone means an area having a 2% or greater
probability that the maximum expected horizontal acceleration,
expressed as a percentage of the earth’s gravitational pull (g), will
exceed 0.10 g in 50 years.” 40 C.F.R. § 257.53.
                                52
     Industry Petitioners’ arguments misconstrue both the
Proposed Rule and the Final Rule. The section of the
Proposed Rule that Industry Petitioners cite for the five-year
deadline (proposed 40 C.F.R. § 257.65(a)) does not apply to
the seismic impact zones; instead, it applies to “unstable areas.”
See 75 Fed. Reg. at 35,242–35,243. Indeed, the Proposed
Rule does not prescribe an explicit operating deadline for
seismic impact zones at all.

     Moreover, even assuming the proposed five-year deadline
for “unstable areas” applies to seismic impact zones, the
Proposed Rule reads: “Existing [Coal Residuals] landfills and
surface     impoundments        that    cannot      make       the
demonstration * * * must close by [date five years after the
effective date of the final rule].” 75 Fed. Reg. at 35,242
(brackets in original). The “must close by” language in the
Proposed Rule is different from the language of the Final Rule,
which demands only that the regulated facility “complete the
demonstration [that the site has met the relevant structural
requirements] no later than October 17, 2018.” 40 C.F.R.
§ 257.63(c)(1).        Contrary to Industry Petitioners’
representation, then, the Final Rule gives the disposal sites four
years before they must demonstrate compliance. See id.
Only if they fail in that demonstration must they begin the
closure process. Id. And once the closure process begins,
they have at least five years to complete it. See id.
§ 257.102(f)(1)(ii).16

     16
        Manifesting additional flexibility, the Final Rule’s closure
timeframe may be extended up to ten years (in consecutive two-year
periods) “if the owner or operator can demonstrate that it was not
feasible to complete closure of the [Coal Residuals] unit within the
required timeframes due to factors beyond the facility’s control.”
Id. § 257.102(f)(2)(i)–(2)(ii)(B).        Accordingly, in some
circumstances the impoundment need not complete the closure
process until 19 years after the Rule’s enactment date.
                                53

     Once the Rule’s timeline is correctly understood, there is
nothing in the record to suggest the Rule’s operating deadline
is arbitrary and capricious. Indeed, Industry’s comments
confirm that the Rule’s timeline will provide a sufficient period
for a non-compliant facility to close (within nine years, and
more if it meets the extension requirements). See, e.g.,
Comments of American Elec. Power Co. on Proposal at 5, J.A.
581 (“[A]t some locations, it will take at least four years from
the time the new [Coal Residuals] rule becomes effective to
accomplish the wet-to-dry conversion and to accomplish the
switch to dry.”); Comments of SCANA Corp. on Proposal at 7,
J.A. 579 (“The time frame required to site, design, permit, and
construct a landfill in today’s regulatory environment is at least
5 to 10 years.”). The EPA’s conclusions are consistent with
Industry Petitioners’ comments. See 75 Fed. Reg. at 35,202
(“[Under Subtitle C,] EPA believes that five years will, in most
cases, be adequate time to complete proper and effective
facility closure and to arrange for alternative waste
management * * * . EPA is aware of no reason that the time
frames would need to differ under subtitle D * * * .”). In sum,
we conclude that the EPA’s operating timeline is not arbitrary
and capricious.

        b.    Seismic Restrictions for New Landfills

     The seismic location restrictions apply to impoundments
as well as new landfills and landfill expansions, but they do not
apply to existing landfills. 40 C.F.R. § 257.63(a). This
distinction reflects, inter alia, the EPA’s determination that
“the risks associated with [Coal Residuals] surface
impoundments are substantially higher than the risks
associated with [Coal Residuals] landfills, by approximately an
order of magnitude.” 80 Fed. Reg. at 21,360. Industry
Petitioners argue that, if landfills are universally less dangerous
                               54
than surface impoundments, they should not be subject to the
same seismic standard as surface impoundments. In other
words, the argument goes, if it is acceptable to exempt existing
landfills from the seismic location restrictions, it is acceptable
to exempt new landfills as well. Because Industry Petitioners
failed to make this argument before the EPA, however, we
reject it.17

     “Under ordinary principles of administrative law a
reviewing court will not consider arguments that a party failed
to raise in timely fashion before an administrative agency.”
Sims v. Apfel, 530 U.S. 103, 114 (2000) (Breyer, J., dissenting);
accord Natural Resource Def. Council, Inc. v. EPA, 25 F.3d
1063, 1073 (D.C. Cir. 1994) (“We do not reach the merits of
this challenge because petitioners failed to raise this question
of statutory and regulatory construction before the agency
during the notice and comment period. They have therefore
waived their opportunity to press this argument in court.”); see
discussion, supra, at 33.

     This fundamental principle of administrative law applies
squarely to Industry Petitioners’ challenge. Natural Resource
Def. Council, 25 F.3d at 1073. In the Proposed Rule, the EPA
explained that, because many Coal Residuals disposal sites are
within seismic impact zones, it was “concerned that such
facilities would be unable to meet the requirements, because
retrofitting would be prohibitively expensive and technically
very difficult in most cases, and [they] would therefore be
forced to close.” 75 Fed. Reg. at 35,198. Accordingly, the
EPA sought comments on “the number of existing [Coal
Residuals] landfills located in these sensitive areas” and the

    17
       The EPA makes it failure-to-exhaust argument in its opening
brief. Rep’t Br. 71–72. Industry’s reply brief offers no rebuttal.
See generally Industry Pet’r’s Reply Br.
                                 55
corresponding effect their closure would have on the national
disposal capacity. 80 Fed. Reg. at 21,360. In spite of the
invitation to comment, Industry Petitioners cannot point to any
record evidence that they questioned the application of the Rule
to new Coal Residuals landfills.18

     Put differently, the EPA did not address the argument that
new Coal Residuals landfills or landfill expansions should be
exempted because the public comments gave no reason to
question the position it announced in the Proposed Rule.
“Indeed, the notion that a yet-to-be built landfill need not
comply with basic seismic location restrictions that are
designed to avoid the potentially catastrophic events identified
in the record, borders on irrational.” Resp’t Br. 73. In light
of Industry Petitioners’ failure to alert the EPA to the issue
while the latter was promulgating the Final Rule, we decline
reach it.

                 c.   The 2,500-Year Standard

     Both the seismic location restrictions and the seismic
safety assessment criteria incorporate a 2,500-year standard.
80 Fed. Reg. at 21,384. This means a disposal site in a seismic
impact area must be designed to withstand the maximum
expected impact of a 2,500-year earthquake. Id. In
establishing the 2,500-year standard, the EPA considered
multiple engineering sources, including (i) Federal Guidelines
for Dam Safety: Earthquake Analyses and Design of Dams,

     18
        Instead, comments focused on the non-regulation of existing
landfills, responding to the Proposed Rule’s conclusion that applying
the seismic location restrictions to existing Coal Residuals landfills
could cause “disposal capacity shortfalls * * * [that] raise greater
environmental and public health concerns than the potential failure
of the [Coal Residuals] landfills in these locales.” 80 Fed. Reg. at
21,360.
                               56
issued by the Federal Emergency Management Agency
(FEMA), and (ii) Minimum Design Loads for Buildings and
Other Structures, International Building Code, a publication of
the American Society of Civil Engineers (ASCE). 80 Fed.
Reg. at 21,384; id. at 21,384–21,385 nn.98–99. The EPA also
consulted geological sources, including the criteria of the
National Earthquake Hazards Reduction Program (NEHRP) of
the U.S. Geological Survey. 75 Fed. Reg. at 35,201. Further,
the Final Rule’s 2,500-year standard precisely mirrors the
EPA’s regulations governing municipal solid waste
management. 75 Fed. Reg. at 35,193 (referencing 40 C.F.R.
§ 258.18).

     In light of the engineering, geological and regulatory
sources informing and supporting the 2,500-year standard,
Industry Petitioners face an uphill battle. They nonetheless
challenge the application of the seismic location restrictions to
landfills—as opposed to impoundments—because landfills
pose comparatively fewer risks than impoundments. Thus,
although FEMA’s dam safety guidelines are applicable to dam-
like impoundments structures, ASCE’s International Building
Code is applicable to buildings, and EPA’s municipal landfill
regulations are applicable to urban landfills, Industry
Petitioners argue that Coal Residuals landfills are different and
should be subject to a less demanding standard. In short, it
asserts that the rule is overprotective and therefore arbitrary
and capricious. We disagree.

    Industry Petitioners’ argument rests on the assumption that
the EPA adopted the 2,500-year standard “without
explanation.” Industry Pet’rs’ Br. 48. To the contrary, the
EPA first examined the structures of municipal landfills and
concluded that they were “very similar to those found at [Coal
Residuals] disposal facilities, and the regulations applicable to
such units would be expected to address the risks presented by
                                 57
the constituents in [Coal Residuals] wastes.” 75 Fed. Reg. at
35,193 (referencing 40 C.F.R. § 258.18). It then cross-
referenced the 2,500-year standard with the criteria adopted by
the U.S. Geological Survey and other engineering experts
before adopting the Final Rule. Id. at 35,201. Indeed, some
Industry members conceded that “the NEHRP/USGS
2%PE/50y [2,500-year] standard provides a sufficient margin
of safety.” Comments of the Southern Company at 34, J.A.
481. Industry Petitioners may disagree, but the EPA’s
reasoning was fully explained and is supported by the record.

    Conversely, Industry Petitioners have not cited any record
evidence that either challenges or provides an alternative to the
2,500-year standard. The best they can do is highlight
comments stating generally that the rule is “overly
protective.”19 Industry Pet’rs’ Br. 47–48. This broad stroke


    19
       Industry Petitioners claim that one commenter suggested a
250-year standard. See Comments of FirstEnergy Corp. at 11, J.A.
598.      Again, Industry Petitioners misread the record.
FirstEnergy’s comment declares:

              EPA intends to incorporate seismic
         performance in section 257.63 of the proposed rule.
         One alternative suggested by EPA is the use of
         seismic impact zones.        A second alternative
         suggests adopting criteria of the National
         Earthquake Hazards Reduction Program (NEHRP)
         of the U.S. Geological Survey, which was used to
         develop national seismic hazard maps. It appears
         the horizontal acceleration expressed as 0.01g in
         250 years in the agency’s first approach closely
         matches the 2% ground motion probability in 50
         years that the seismic maps are based upon.
                                 58
does not carry their argument very far. Once the EPA selected
the Subtitle D rather than the Subtitle C regulatory path, it was
charged with developing uniform national standards rather
than implementing a site-specific permit program. See
generally 42 U.S.C. § 6944(a) (requiring EPA to develop
minimum criteria for all disposal sites). Consistent with that
mandate, the EPA developed criteria for all climates and
conditions within seismic impact zones. Accordingly, it is of
no moment that the criteria might be “overprotective” for a
western landfill located miles from any water source. See
Comments of Electric Power Research Institute on Proposal at
89, J.A. 596 (explaining that “cap and liners” may not be
necessary in “western areas where * * * the total rainfall is less
than 10 inches per year”). Congress demanded national
minimum standards that ensure “no reasonable probability of
adverse effects on health or the environment.” 42 U.S.C.
§ 6944(a). The 2,500-year standard does just that.

             4. The Alternative Closure Option

     RCRA states in plain terms that the “open dumping of
solid waste * * * is prohibited.” 42 U.S.C. § 6945(a). Thus,
if a disposal site is classified as an open dump, it must either
retrofit or close. See id. The Final Rule stays true to the
statutory mandate. Under the Final Rule, certain events—
such as groundwater sampling that reveals an excess of Coal
Residuals constituents in the water table—establish the
disposal site as an “open dump,” which triggers the Rule’s
closure requirements. 40 C.F.R. § 257.101. If the closure

Id. Thus, the “250 years” corresponds to the horizontal acceleration
rate rather than a “ground motion probability” calculation such as the
one upon which the 2,500-year model is based (2% in 50 years =
100% in 2,500 years). It is not a free-standing 250-year standard.
That is, FirstEnergy does not appear to offer an alternative standard.
                                 59
requirements are triggered, the surface impoundment or
landfill ordinarily has six months to either retrofit its facility or
to stop receiving Coal Residuals and to begin the closure
process. Id. § 257.101(a)(2), (4). In other words, the
statutory (and regulatory) presumption is that a non-compliant
disposal site—one that is polluting the groundwater—will
close. Id.

     Notwithstanding this presumption, the Rule includes an
“alternative closure” exemption that allows a non-compliant
Coal Residuals disposal site (an “open dump”) to receive Coal
Residuals for an additional five years before it ceases
operations. 40 C.F.R. § 257.103. In order to qualify for the
alternative closure exception, the owner or operator must
certify that, inter alia: “No alternative disposal capacity is
available on-site or off-site.” Id. § 257.103(a)(1)(i). In
making the certification, “[a]n increase in costs or the
inconvenience of existing capacity is not sufficient to support
qualification under this section.” Id.

     Describing the rationale for its alternative closure
exemption, the EPA explained that it did not want to force
facilities to close and create power shortages “because there is
no place in which to dispose of the resulting waste.” 80 Fed.
Reg. at 21,423.         The preamble includes an example:
“[W]hile it is possible to transport dry ash off-site to [an]
alternate disposal facility[,] that simply is not feasible for wet-
generated [Coal Residuals]. Nor can facilities immediately
convert to dry handling systems. As noted previously, the law
cannot compel actions that are physically impossible.” Id.

     Industry Petitioners argue that ignoring costs and
inconvenience in the alternative disposal criteria is arbitrary
and capricious because it effectively renders the exemption a
nullity: “If costs or inconvenience cannot be evaluated, off-
                               60
site disposal capacity—no matter where it is located or how
much it will cost to send [Coal Residuals] there—will always
be ‘available’ somewhere.” Industry Pet’rs’ Br. 38–39. At
oral argument, Industry Petitioners lamented that they might be
required to hire a fleet of 1,000 vacuum trucks in order to
transfer wet Coal Residuals to an off-site disposal facility.
Oral Arg. Tr. 23:22–23:23. This result, it argues, would make
nonsense of the alternative closure requirements.

     Industry Petitioners’ hyperbole faces a roadblock. As the
United States Supreme Court has explained, if the Congress
directs the EPA to “regulate on the basis of a factor that on its
face does not include cost, the Act normally should not be read
as implicitly allowing the agency to consider cost anyway.”
Michigan v. EPA, 135 S. Ct. 2699, 2709 (2015) (citing
Whitman v. American Trucking Ass’ns, 531 U.S. 457, 469–472
(2001)). Applying this rule, the Court held that the EPA is
prohibited from considering costs when developing its primary
ambient air quality standards under the Clean Air Act because
the statute does not mention costs but instead demands
standards “requisite * * * to protect the public health with an
adequate margin of safety.” American Trucking, 531 U.S. at
475–476 (quoting 42 U.S.C. § 7409(b)(1)). Thus, “public
health” provided the statutory measuring stick in that instance,
notwithstanding flexible words such as “requisite” and
“adequate” that the trucking industry suggested might allow
the agency to consider costs. Id. at 468.

    Simply put, “to prevail in their present challenge,
[Industry] must show a textual commitment of authority to the
EPA to consider costs.” American Trucking, 531 U.S. at 468.
Under any reasonable reading of RCRA, there is no textual
commitment of authority to the EPA to consider costs in the
                                61
open-dump standards.20 RCRA’s statutory language instructs
the EPA to classify a disposal site as a sanitary landfill and not
an open dump only “if there is no reasonable probability of
adverse effects on health or the environment from disposal of
solid waste at such facility.” 42 U.S.C. § 6944(a) (emphasis
added). There is no explicit mention of costs in section 6944;
nor is there any flexible language such as “appropriate and
necessary” that might allow the EPA to consider costs in its
rulemaking. See Michigan v. EPA, 135 S. Ct. at 2709. This
stands in stark contrast with other sections of Title 42—such as
the Bevill Amendment—where the Congress expressly
required the EPA to consider, inter alia, “the costs of * * *
alternatives” in determining whether Coal Residuals should be
classified as hazardous waste. See 42 U.S.C. § 6982(n)(6).

     With Michigan v. EPA and American Trucking, then, it is
far from clear that the EPA could consider costs even if it
wanted to. See Michigan v. EPA, 135 S. Ct. at 2707
(explaining that “appropriate and necessary” language could
require consideration of costs in some contexts but not others).
In any case, there is no statutory support for the assertion that
EPA was required to consider costs in developing its
alternative closure plan. Excluding consideration of costs and
convenience may narrow the alternative closure exemption but
including cost and convenience would appear to violate
RCRA’s statutory mandate and run afoul of Supreme Court
precedent. The EPA was neither arbitrary nor capricious in its
decision to avoid testing that legal limit.




     20
        At oral argument, neither Industry Petitioners nor the EPA
could identify a statutory provision that allows the EPA to consider
costs. Oral Arg. Tr. 83:15–83:23; 116:02–116:10.
                               62
                       V. Conclusion

     In sum, we deny the EPA’s motion for us to hold these
petitions in abeyance. We grant in part the EPA’s motion for
a voluntary remand, remanding to the EPA the provisions in
the Final Rule pertaining to (i) the definition of “Coal
Residuals Piles,” see 40 C.F.R. § 257.53; (ii) the 12,400-ton
beneficial use threshold, see id.; and (iii) the alternative
groundwater protection standards, see id. § 257.95(h)(2). We
deny the EPA’s motion to remand the provisions in the Final
Rule pertaining to inactive surface impoundments and landfills
at active power plants, see id. §§ 257.50(c), 257.100, and
inactive surface impoundments at inactive power plants, see id.
§ 257.50(e).

     On the claims raised by Environmental Petitioners, we
hold that the EPA acted arbitrarily and capriciously and
contrary to RCRA in failing to require the closure of unlined
surface impoundments, in classifying so-called “clay-lined”
impoundments as lined, and in exempting inactive surface
impoundments at inactive power plants from regulation. We
therefore vacate and remand the provisions of the Final Rule
that permit unlined impoundments to continue receiving coal
ash unless they leak, see id. § 257.101(a), classify “clay-lined”
impoundments as lined, see 40 C.F.R. § 257.71(a)(1)(i), and
exempt from regulation inactive impoundments at inactive
facilities, see 40 C.F.R. § 257.50(e). We reject as forfeited
Environmental Petitioners’ challenges to the Final Rule’s
public notice provisions.

     Regarding the Industry Petitioners’ claims, we hold that (i)
the EPA has statutory authority to regulate inactive
impoundments; (ii) the EPA provided sufficient notice of its
intention to apply the aquifer location criteria to existing
impoundments; (iii) the EPA did not arbitrarily issue location
                              63
requirements based on seismic impact zones; and finally (iv)
the EPA did not arbitrarily impose temporary closure
procedures. As to the regulation of Coal Residuals piles of
12,400 tons or more and the regulation of Coal Residuals
destined for beneficial use, we remand to the agency as
requested. We dismiss as moot the two accompanying notice
challenges and the issue of risk-based compliance alternatives.
                                                  So ordered.
                               1
     KAREN LECRAFT HENDERSON, Circuit Judge, concurring
in part and concurring in the judgment in part: A central
question before us is whether the EPA exceeded its statutory
authority under the Resource Conservation and Recovery Act
(RCRA), 42 U.S.C. §§ 6901 et seq., by applying its Final Rule,
80 Fed. Reg. 21,302 (Apr. 17, 2015), to an impoundment that
no longer receives coal combustion residuals (CCR) after the
effective date of the Rule and thus becomes “inactive.” The
answer to this question turns on our interpretation of the
statutory phrase “is disposed of.” My colleagues conclude
that the verb “to be,” when conjugated in the present tense
(“is”), unambiguously applies to disposal that occurred entirely
in the past. I disagree and accordingly concur in the judgment
with respect to Section IV.B.1 of the opinion. I join all other
sections of the per curiam opinion in full.

                               I.

     I believe there are three tiers to the statutory question.
First, RCRA directs the EPA to promulgate regulations that
draw a dividing line between “sanitary landfills” and “open
dumps.” 42 U.S.C. §§ 6944-45. Generally speaking, a
sanitary landfill is a disposal site that complies with the EPA’s
regulations and presents “no reasonable probability of adverse
effects on health or the environment.” Id. § 6944(a). By
contrast, “any solid waste management practice or disposal of
solid waste . . . which constitutes the open dumping of solid
waste or hazardous waste is prohibited.” Id. § 6945(a).
Second, RCRA defines an “open dump” as “any facility or site
where solid waste is disposed of which is not a sanitary landfill
which meets the criteria promulgated under [§ 6944].” Id.
§ 6903(14) (emphasis added).              Third, RCRA defines
“disposal” as

       the discharge, deposit, injection, dumping,
       spilling, leaking, or placing of any solid waste
                              2
       or hazardous waste into or on any land or water
       so that such solid waste or hazardous waste or
       any constituent thereof may enter the
       environment or be emitted into the air or
       discharged into any waters, including ground
       waters.

Id. § 6903(3).

     To interpret RCRA’s text, we turn to the familiar two-step
framework of Chevron, U.S.A., Inc. v. Natural Resources
Defense Council, Inc., 467 U.S. 837 (1984). Thus, we begin
with the statutory language and ask whether the Congress “has
directly spoken to the precise question at issue.” Id. at 842.
If the language is plain, our inquiry ends, as we must “give
effect to the unambiguously expressed intent of Congress.”
Id. at 843. If “the statute is silent or ambiguous with respect
to the specific issue,” however, we defer to the EPA’s
interpretation so long as it is “based on a permissible
construction of the statute.” Id.

     We do not alter our analytical framework when the case
presents a question of an agency’s “jurisdiction” or core
statutory authority. City of Arlington v. FCC, 569 U.S. 290,
297 (2013) (“[T]he distinction between ‘jurisdictional’ and
‘nonjurisdictional’ interpretations is a mirage.”). If “the
reality is that [the statute] is ambiguous,” it is our duty to
declare it so and proceed to the second step of the Chevron
analysis. AT&T Corp. v. Iowa Utils. Bd., 525 U.S. 366, 395
(1999).

                              II.

     I believe the text—and more precisely, the grammatical
structure—of RCRA’s definition of “open dump” is temporally
ambiguous. See United States v. Wilson, 503 U.S. 329, 333
                                3
(1992) (“Congress’ use of a verb tense is significant in
construing statutes.”). Under RCRA, an “open dump” is a site
where solid waste “is disposed of.” 42 U.S.C. § 6903(14).
The operative verb is the present tense of the infinitive “to be”
(“is”). The Dictionary Act tells us that “unless the context
indicates otherwise . . . words used in the present tense include
the future as well as the present.” 1 U.S.C. § 1. By
implication, therefore, the Dictionary Act “instructs that the
present tense generally does not include the past.” Carr v.
United States, 560 U.S. 438, 448 (2010). It is plain, therefore,
that “is” does not mean “was.”

     The verb’s present tense formation takes on additional
meaning because the “Congress could have phrased its
requirement in language that looked to the past . . . but it did
not choose this readily available option.” Gwaltney of
Smithfield, Ltd. v. Chesapeake Bay Found., Inc., 484 U.S. 49,
57 (1987). It could have conjugated the infinitive “to be” in
any number of ways to unambiguously include past disposal:
“is or was disposed of”; “had been disposed of”; or “has been
disposed of.” See CHICAGO MANUAL OF STYLE ONLINE
§§ 5.118-35       (17th     ed.    2017),      available      at
www.chicagomanualofstyle.org//home.html             (explaining
tenses generally). The Congress could also have included
unambiguous temporal phrases such as: “ever”; “at any time”;
“past or present”; or “beginning on a date certain.” It did not
do so. The present tense of section 6903(14) therefore
suggests that an “open dump” does not include any
impoundment where solid waste “was disposed of.”

     Significantly, the Congress used temporally unambiguous
language in other RCRA provisions. For example, RCRA’s
“substantial endangerment” provision plainly applies to past
actions; it allows a state or individual to bring suit against “any
person . . . who has contributed or who is contributing to the
                                  4
past or present . . . disposal of any solid or hazardous waste
which may present an imminent and substantial endangerment
to health or the environment.” 42 U.S.C. § 6972(a)(1)(B)
(emphases added). RCRA Subtitle C provides that the EPA
must conduct “corrective action for all releases of hazardous
waste or constituents from any solid waste management
unit . . . regardless of the time at which waste was placed in
such unit.” Id. § 6924(u) (emphasis added). I believe there
can be no reasonable dispute that these provisions apply to past
as well as present and future actions. By itself, therefore, “is”
at least suggests that the EPA is precluded from including past
acts of disposal in the definition of an “open dump.”

      The ambiguity comes from the second part of the phrase:
“disposed of.” A past participle like “disposed” is not singular
in its purpose; it is defined as “[a] verb form indicating past or
completed action or time that is used as a verbal adjective in
phrases such as baked beans and finished work.” Fla. Dep’t
of Revenue v. Piccadilly Cafeterias, Inc., 554 U.S. 33, 39
(2008) (quoting AMERICAN HERITAGE DICTIONARY 1287 (4th
ed. 2000) (emphasis removed)). In other words, a past
participle can serve either as a verb (i.e., the pecans were
covered in chocolate) or as an adjective (i.e., the chocolate-
covered pecans). Moreover, in verb form, a past participle can
indicate past (i.e., the pecans were covered in chocolate),
present (i.e., the pecans are covered in chocolate) or future
action (i.e., the pecans will be covered in chocolate). In short,
there is nothing unambiguous about a past participle, at least
when construed without context.21


     21
        My colleagues cite two authorities for their conclusion that a
statutory past participle unambiguously signifies retroactive effect.
Neither authority decides the issue. First, in Florida Department of
Revenue v. Piccadilly Cafeterias, Inc., 554 U.S. 33, 41 (2008), the
                                 5
     I believe “disposed of” must be read in conjunction with
RCRA’s definition of “disposal,” which includes the
“discharge, deposit, injection, dumping, spilling, leaking, or
placing” of solid waste into certain areas. 42 U.S.C. §
6903(3). Circuit courts disagree about whether “disposal”
includes the “passive migration” of contaminants, such as a
slow leak from an inactive CCR impoundment. Compare
Carson Harbor Vill., Ltd. v. Unocal Corp., 270 F.3d 863, 867
(9th Cir. 2001) (en banc) (concluding that “the migration of
contaminants on the property does not fall within the statutory
definition of ‘disposal’”), with Nurad, Inc. v. William E.
Hooper & Sons Co., 966 F.2d 837, 846 (4th Cir. 1992) (holding
past owners liable for “disposal” of hazardous wastes that

Supreme Court assumed the statute at issue was temporally
ambiguous and resolved the interpretive question at Chevron’s
second step. Moreover, in Sherley v. Sebelius, the majority found
ambiguity in a statute that prohibited funding for “research in which
a human embryo or embryos are destroyed.” 644 F.3d 388, 390
(D.C. Cir. 2011) (emphasis added) (internal quotation marks
omitted). It did so in spite of applicable regulations defining
research as “a systematic investigation, including research
development, testing and evaluation, designed to develop or
contribute to generalizable knowledge.” Id. at 394 n.* (quoting 45
C.F.R. § 46.102(d)).       Notwithstanding this temporally broad
definition, the majority declared that the “definition of research is
flexible enough to describe either a discrete project or an extended
process.” Id. at 394. I dissented, challenging the majority’s
interpretive fallacy that “research” can be dissected into “free-
standing pieces” rather than read as a “systematic [and ongoing]
investigation.” Id. at 402-04 (Henderson, J., dissenting). Thus, I
did not find the phrase “are destroyed” unambiguous standing alone;
in my view, the explicit connection to research funding—and the
correct definition of “research”—clarified the temporal scope of the
statute to include past conduct. Id.
                               6
leaked from underground storage tank notwithstanding they
were not owners “at the time of disposal”). Because these
cases arise in a different statutory context, 22 they are not
precisely on point regarding the question of the EPA’s
authority to regulate inactive impoundments. Nonetheless,
they illustrate the ambiguity in the statutory definition of the
word “disposal”; if courts disagree about the meaning of
“disposal,” that disagreement strongly suggests there is
ambiguity in the words “disposed of.” See Final Rule, 80 Fed.
Reg. at 21,346 (surveying caselaw interpreting “disposal”).

     Although there is some temporal tension between the
present tense “is” and the past participle “disposed,” it can be
explained by statutory context. See Brown v. Gardner, 513
U.S. 115, 118 (1994) (“Ambiguity is a creature not of
definitional possibilities but of statutory context.”).
Industry’s entire argument hinges on three words—“is
disposed of”—in the definition of “open dump.” 42 U.S.C. §
6903(14). But “open dump” is also defined by what it is not:
a “sanitary landfill.” Id. The statutory categorization is
binary: a disposal site is either a sanitary landfill or an open
dump and the EPA is directed to promulgate regulations that
distinguish between the two. Id. § 6944. Thus, as the EPA
promulgates new regulations that may shift the contours of
what constitutes a “sanitary landfill,” see 42 U.S.C. § 6912(b)
(RCRA regulations “shall be reviewed and, where necessary,
revised not less frequently than every three years”), the
definition of “open dump” will morph as well, see Appalachian
Voices v. McCarthy, 989 F. Supp. 2d 30, 56 (D.D.C. 2013)
(“requir[ing] the EPA to submit a proposed scheduling order
setting forth a proposed deadline by which it will comply with
    22
       The cited cases interpret the Comprehensive Environmental
Response, Compensation, and Liability Act of 1980 (CERCLA),
which incorporates RCRA’s definition of “disposal.” 42 U.S.C.
§ 9601(29) (incorporating 42 U.S.C. § 6903(14)).
                                 7
its statutory obligations” under RCRA). Although not every
interpretation of “open dump” may be reasonable, see
Michigan v. EPA, 135 S. Ct. 2699, 2708 (2015) (“Chevron
allows agencies to choose among competing reasonable
interpretations of a statute; it does not license interpretive
gerrymanders[.]”), RCRA’s mandated flexibility contemplates
that the regulatory meaning of “open dump” can change over
time and thus fits the definition of “ambiguity.” See
Ambiguity, WEBSTER’S THIRD NEW INTERNATIONAL
DICTIONARY 66 (3d ed. 1993) (“admitting of two or more
meanings”).

                                III.

     Although I believe the statute is temporally ambiguous, I
nonetheless agree that the EPA reasonably concluded that it has
the authority to regulate inactive impoundments.             See
Chevron, 467 U.S. at 843 (deference to agency’s interpretation
required so long as it is “based on a permissible construction of
the statute”). In reviewing the reasonableness of an agency’s
interpretation, we look to the statute’s structure and purpose as
well as to precedent, Nat’l Ass’n of Home Builders v. Defs. of
Wildlife, 551 U.S. 644, 666 (2007), keeping in mind that
Chevron “does not require the best interpretation [of the
statute], only a reasonable one,” Van Hollen, Jr. v. FEC, 811
F.3d 486, 492 (D.C. Cir. 2016) (internal quotation marks
omitted).

     First, regarding the definition of “disposal,” we have
rejected a similar “linguistic point that ‘[d]isposal . . . is not a
continuing activity but occurs anew each time waste is placed
into or on land.’” In re Consol. Land Disposal Regulation
Litig., 938 F.2d 1386, 1389 (D.C. Cir. 1991). In doing so, we
noted that RCRA’s “equation of ‘disposal’ with ‘leaking,’
which is a continuous phenomenon rather than a discrete event,
                              8
is enough to blunt the sting of the petitioners’ point.” Id. In
that case, we concluded that the petitioners’ suggested
interpretation was, “at most an alternative reading of the
statute, not an argument as to why the EPA’s reading of the
statute is unreasonable.” Id. Thus, we upheld as reasonable
the EPA’s interpretation of “disposal” to include “continuous”
leaking; we can apply a similar reading today. Indeed, the
record “demonstrates that unlined surface impoundments
typically operate for 20 years before they begin to leak.” See
80 Fed. Reg. at 21326-27; see also 40 C.F.R. §§ 257.70-72
(imposing liner requirements to prevent leaking).            As
discussed in Section IV.B.1 of the per curiam opinion, the risk
of leaking does not decrease in an inactive impoundment—
indeed, it can increase.         Because “disposal” includes
“leaking”—and because “leaking” does not necessarily cease
upon an impoundment’s closure—the EPA reasonably
concluded that CCR continues to be “disposed of” even after
an impoundment stops receiving CCR. See 75 Fed. Reg.
35,128, 35,159 (June 21, 2010) (“historical or legacy sites”
pose leaking risk).

     Second, an impoundment where CCR “is disposed of” is
different from an impoundment that is actively receiving
additional CCR. 42 U.S.C. § 6903(14). As the EPA
suggests, if an individual were to stand on an impoundment
dam looking out over thousands of tons of wet CCR and ask
“is this an impoundment where ‘solid waste is disposed of,’”
the answer would be “yes.” EPA Br. 22. Put differently, the
disposal of CCR in an impoundment is not a discrete act. If it
were, the EPA would regulate only the transfer of CCR from a
power facility into an impoundment, at which point the
“disposal” would end. Of course, the reality is that CCR
disposal and its resulting health hazards occur over long
periods of time. See 80 Fed. Reg. at 21,309 (“estimated time
to peak potential exposures of CCR through groundwater
                               9
migration to drinking water wells is 75 years” and estimated
CCR unit lifespan is 40 to 80 years). CCR is not like a bag of
trash that a homeowner places on the curb to be picked up.
The homeowner releases control of the bag once he deposits it
and the garbage truck makes its rounds. In contrast—and by
definition—an impoundment owner or utility operator does not
relinquish control of the CCR once it is impounded. See 40
C.F.R. § 257.53 (defining “owner” and “operator”); see also id.
§ 257.50(b) (Rule applies to “disposal units located off-site of
the electric utility or independent power producer”).
Moreover, the impoundment’s purpose is to “dispose of” CCR
and, accordingly, the disposal process continues so long as the
CCR remains in the pond. Id. § 257.53 (“CCR 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”
(emphasis added)).23

     For the foregoing reasons, and regarding Section IV.B.1
only, I concur in the judgment. Otherwise, I fully concur in
the per curiam opinion.




    23
       The EPA’s regulatory definition of “impoundment” is
consistent with the dictionary definition of the verb “impound,”
which manifests continuing action. See Impound, WEBSTER’S
THIRD NEW INTERNATIONAL DICTIONARY 1136 (3d ed. 1993)
(“[T]o confine or store (water)[.]”).
