Full opinion text
Opinion filed Per Curiam. 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 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 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 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 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 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 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 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. 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. 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). 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. 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. 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 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 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) 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." 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 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 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 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. 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 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. 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 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, 103 S.Ct. 2856, 77 L.Ed.2d 443 (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 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 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 of the W[aste] M[anagement] U[nit]."). 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 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 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 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 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 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/fossil/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. 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 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, 73 S.Ct. 67, 97 L.Ed. 54 (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 challenged eighteen provisions of the Final Rule and questioned the EPA's statutory authority to regulate inactive surface impoundments. 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 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. 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 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, Environment