Full opinion text
STUART KYLE DUNCAN, Circuit Judge: Steam-electric power plants generate most of the electricity used in our nation and, sadly, an unhealthy share of the pollution discharged into our nation's waters. To control this pollution, the Clean Water Act, 33 U.S.C. § 1251 et seq. , empowers the Environmental Protection Agency to promulgate and enforce rules known as "effluent limitation guidelines" or "ELGs." Id. §§ 1311, 1314, 1362(11). For quite some time, ELGs for steam-electric power plants have been, in EPA's words, "out of date." 80 Fed. Reg. 67,838. That is a charitable understatement. The last time these guidelines were updated was during the second year of President Reagan's first term, the same year that saw the release of the first CD player, the Sony Watchman pocket television, and the Commodore 64 home computer. In other words, 1982. See id. (noting ELGs were "promulgated and revised in 1974, 1977, and 1982"). The guidelines from that bygone era were based on "surface impoundments," which are essentially pits where wastewater sits, solids (sometimes) settle out, and toxins leach into groundwater. Id. at 67,840, 67,851. Impoundments, EPA tells us, have been "largely ineffective at controlling discharges of toxic pollutants and nutrients." Id. at 67,840. Consequently, in 2005 the agency began a multi-year study to bring the steam-electric ELGs into the 21st century. Id. at 67,841. In November 2015, EPA unveiled the final rule: the " Effluent Limitations Guidelines and Standards for the Steam Electric Power Generating Point Source Category," 80 Fed. Reg. 67,838 (Nov. 3, 2015). The rule updates guidelines for six of the wastestreams that issue from plants and foul our waters. Importantly, the Clean Water Act requires setting new ELGs based on the "Best Available Technology Economically Available" or "BAT." 33 U.S.C. § 1314(b)(2)(B). BAT is the gold standard for controlling water pollution from existing sources. By requiring BAT, the Act forces implementation of increasingly stringent pollution control methods. See NRDC v. EPA , 822 F.2d 104, 123 (D.C. Cir. 1987) (describing the Act as "technology-forcing"). We consider a challenge to the final rule brought by various environmental petitioners. They target two discrete parts of the rule: the new ELGs for "legacy wastewater" (wastewater from five of the six streams generated before a specific date) and for "combustion residual leachate" (liquid that percolates through landfills and impoundments). These two categories account for massive amounts of water pollution. For instance, leachate alone would qualify as the 18th-largest source of water pollution in the nation, producing more toxic-weighted pound equivalents than the entire coal mining industry. The environmental petitioners' basic complaint is that EPA set an unlawful BAT for these two categories. Whereas the BAT for the other streams adopts modern technologies, they claim the agency arbitrarily set BAT for legacy wastewater and leachate using the same archaic technology in place since 1982-namely, impoundments. It was as if Apple unveiled the new iMac, and it was a Commodore 64. The environmental petitioners challenge those portions of the rule under the Administrative Procedure Act and the well-worn Chevron test governing review of agency action. See Chevron, USA, Inc. v. Nat. Res. Def. Council, Inc. , 467 U.S. 837, 104 S.Ct. 2778, 81 L.Ed.2d 694 (1984). For the reasons discussed below, we agree that the portions of the rule regulating legacy wastewater and combustion residual leachate are unlawful. Accordingly, we VACATE those portions of the rule and REMAND to the agency for reconsideration. I. BACKGROUND A. The Clean Water Act The Clean Water Act ("CWA" or "Act"), 86 Stat. 833, as amended, 33 U.S.C. § 1251 et seq. , was enacted over President Nixon's veto in 1972. See Train v. City of New York , 420 U.S. 35, 40, 95 S.Ct. 839, 43 L.Ed.2d 1 (1975). Few laws have shouldered a weightier burden-namely, "to restore and maintain the chemical, physical, and biological integrity of the Nation's waters." 33 U.S.C. § 1251(a) ; see also, e.g., City of Milwaukee v. States of Illinois and Michigan , 451 U.S. 304, 318, 101 S.Ct. 1784, 68 L.Ed.2d 114 (1981) ("Congress' intent in enacting [the CWA] was clearly to establish an all-encompassing program of water pollution regulation."); Am. Petroleum Inst. v. EPA , 661 F.2d 340, 343-44 (5th Cir. Unit A Nov. 13, 1981) (" API I ") (noting CWA's "ambitious purpose"). To that end, the Act makes "unlawful" the "discharge of any pollutant by any person" into the nation's "navigable waters," unless otherwise permitted. 33 U.S.C. §§ 1311(a), 1362(7), (12). We have previously detailed the Act's "distinct, though interlocking, regulatory schemes." Chem. Mfrs. Ass'n v. EPA , 870 F.2d 177, 195 (5th Cir. 1989) (" CMA "), clarified on reh'g , 885 F.2d 253. Here we focus on one of the Act's key regulatory tools: "effluent limitation guidelines" ("ELGs" or "guidelines"), which are nationwide standards set by the EPA Administrator to govern pollutant discharges from point sources. See 33 U.S.C. § 1314(b) (authorizing Administrator to set "effluent limitation guidelines" for "classes and categories of point sources"); Tex. Oil & Gas , 161 F.3d at 927 ("ELGs are the rulemaking device prescribed by the CWA to set national effluent limitations for categories and subcategories of point sources"). The Act requires ELGs to be based on technological feasibility rather than on water quality. Id. at 927 (citing E.I. du Pont de Nemours & Co. v. Train , 430 U.S. 112, 130-31, 97 S.Ct. 965, 51 L.Ed.2d 204 (1977) ; API I , 661 F.2d at 343-44 ). That is, the Administrator must "require industry, regardless of a discharge's effect on water quality, to employ defined levels of technology to meet effluent limitations." API I , 661 F.2d at 344 ; see also Tex. Oil & Gas , 161 F.3d at 927 (ELGs are "technology-based rather than harm-based" insofar as they "reflect the capabilities of available pollution control technologies to prevent or limit different discharges rather than the impact that those discharges have on the waters"). The Act therefore mandates a system in which, as available pollution-control technology advances, pollution-discharge limits will tighten. See, e.g., Nat'l Crushed Stone , 449 U.S. at 69, 101 S.Ct. 295 (the Act "provides for increasingly stringent effluent limitations") (citing 33 U.S.C. § 1311(b) ); CMA , 870 F.2d at 196 (the Act requires compliance with "technology-based pollutant-effluent limitations that, in time, will become more stringent") (citing 33 U.S.C. §§ 1311(b), 1314(b) ). The D.C. Circuit accurately described this aspect of the Act's scheme as "technology-forcing," meaning it seeks to "press development of new, more efficient and effective [pollution-control] technologies." NRDC v. EPA , 822 F.2d 104, 123 (D.C. Cir. 1987) (" NRDC I " ); see also, e.g., NRDC v. EPA , 808 F.3d 556, 563-64 (2nd Cir. 2015) (" NRDC II ") (describing ELG scheme as "technology-forcing, meaning it should force agencies and permit applicants to adopt technologies that achieve the greatest reductions in pollution") (citing NRDC I ). The Act prescribes various technological standards to be used in setting effluent limitations. Two are relevant here: "best practicable control technology currently available" ("BPT") and "best available technology economically achievable" ("BAT"). Compare 33 U.S.C. §§ 1311(b)(1)(A) ; 1314(b)(1)(B) (BPT), with id. §§ 1311(b)(2)(A) ; 1314(b)(2)(B) (BAT). The less stringent of these two standards is BPT, which the Supreme Court has described as only "a first step toward [the Act's] goal." Nat'l Crushed Stone , 449 U.S. at 75 n.14, 101 S.Ct. 295 ; see also, e.g., BP Explor. & Oil, Inc. v. EPA , 66 F.3d 784, 789 (6th Cir. 1995) (describing BPT as "the first stage of pollutant reduction"). BPT applied to limitations on direct discharges of pollutants during an interim period (originally slated to end in 1984 but later extended to 1989). 33 U.S.C. § 1311(b)(1)(A) ; Tex. Oil & Gas , 161 F.3d at 927-28 ; CMA , 870 F.2d at 196. Instead of defining BPT, the Act lists various factors the Administrator must consider in determining it-including an explicit cost/benefit analysis: "the total cost of application of technology in relation to the effluent reduction benefits to be achieved from such application." 33 U.S.C. § 1314(b)(1)(B). We have explained that "BPT limitations are intended to represent the average of the best levels of performance by existing plants of various sizes, ages, and unit processes within the category or subcategory for control of conventional pollutants." CMA , 870 F.2d at 203 (citing 52 Fed. Reg. 42,525) ; see also, e.g., Nat'l Crushed Stone , 449 U.S. at 75-76, 101 S.Ct. 295 (discussing BPT). The stricter of the two standards is BAT, which has applied to existing, direct discharges of toxic and non-conventional pollutants since March 31, 1989. See 33 U.S.C. §§ 1311(b)(2)(A) ; 1314(b)(2)(A); Tex. Oil & Gas , 161 F.3d at 927-28 ; see also BP Explor. , 66 F.3d at 790 (describing BAT as "the second stage" of pollutant reduction). When pollutants are regulated under this standard, the EPA "must set discharge limits that reflect the amount of pollutant that would be discharged by a point source employing the best available technology that the EPA determines to be economically feasible across the category or subcategory as a whole." Tex. Oil & Gas , 161 F.3d at 928. We have held that BAT limitations must "be based on the performance of the single best-performing plant in an industrial field." CMA , 870 F.2d at 226. In describing the relationship between BAT and BPT, the Supreme Court has explained that a BAT must achieve "reasonable further progress" towards the Act's goal of eliminating pollution, and BPT serves as the "prior standard" for measuring that progress. See Nat'l Crushed Stone , 449 U.S. at 75, 101 S.Ct. 295 (explaining that "BPT serves as the prior standard with respect to BAT['s]" reasonable further progress requirement). As with BPT, the Act lists factors the Administrator must consider in determining BAT. 33 U.S.C. § 1314(b)(2)(B). The Administrator has "considerable discretion" in weighing those factors. Tex. Oil & Gas , 161 F.3d at 928 (citation omitted). Unlike BPT, however, the BAT factors omit a cost/benefit analysis and replace it with a requirement to consider only "the cost of achieving such effluent reduction." Id ;see also, e.g., Nat'l Crushed Stone , 449 U.S. at 71, 101 S.Ct. 295 (BPT and BAT factors are "similar ... except that in assessing BAT total cost is no longer to be considered in comparison to effluent reduction benefits"). Indeed, the Supreme Court has explained that, unlike BAT, "BPT limitations do not require an industrial category to commit the maximum economic resources to pollution control, even if affordable." Nat'l Crushed Stone , 449 U.S. at 75, 101 S.Ct. 295. B. The Final Rule The rule at issue in this case regulates effluent discharges from steam-electric power plants. Those plants burn nuclear or fossil fuels to heat water in boilers, generating steam that drives turbines connected to electric generators. 80 Fed. Reg. 67,839 n.1. This process produces something nearly everyone regards as good: electricity. Indeed, the plants regulated by the rule provide most of the electricity annually produced in the United States. But the process also produces something everyone regards as bad: pollution. According to EPA, discharges from these plants account for "about 30 percent of all toxic pollutants discharged into surface waters by all industrial categories regulated under the CWA." Id. at 67,839 -40; see also, e.g., Michigan v. EPA , --- U.S. ----, 135 S.Ct. 2699, 2705, 192 L.Ed.2d 674 (2015) (addressing regulation of air pollution from power plants under the Clean Air Act); ConocoPhillips Co. v. EPA , 612 F.3d 822, 826 (5th Cir. 2010) (addressing regulation of cooling water systems at power plants). For instance, power plant discharges contain toxic metals such as mercury, arsenic, lead, and selenium, which bioaccumulate in fish, accumulate in lake and reservoir sediment, and pollute drinking water supplies. People who eat the tainted fish or drink the tainted water can suffer negative health consequences such as cancer, cardiovascular disease, neurological disorders, kidney and liver damage, and lowered IQs (in children). Id. at 67,840. EPA first promulgated and then revised ELGs for steam-electric power plants in 1974, 1977, and 1982. See id. ; see also 39 Fed. Reg. 36,186 (Oct. 8, 1974) ; 42 Fed. Reg. 15,690 (Mar. 23, 1977) ; 47 Fed. Reg. 52,290 (Nov. 19, 1982). Those guidelines are now, in the agency's words, "out of date," because "[t]hey do not adequately control the pollutants (toxic metals and other[s] ) discharged by this industry, nor do they reflect relevant process and technology advances that have occurred in the last 30-plus years." 80 Fed. Reg. 67,840. The old rules and the processes they regulated are relics of the past: The processes employed and pollutants discharged by the industry look very different today than they did in 1982. Many plants, nonetheless, still treat their wastewater using only surface impoundments, which are largely ineffective at controlling discharges of toxic pollutants and nutrients. Id. ("Surface impoundments" are ponds designed to allow particulates to settle out of wastewater by force of gravity. See infra . ) Happily, though, EPA reports that, "[i]n the several decades since the steam electric ELGs were last revised," technologies that are more effective, "affordable," and "widely available" have "increasingly been used at plants." Id. Thus, EPA began a new rulemaking to update power plant ELGs. The agency conducted a detailed industry study from 2006-2009 and on June 7, 2013 issued a proposed rule, 78 Fed. Reg. 34,432, 34,439, that generated over 200,000 comments. 80 Fed. Reg. 67,844. On November 3, 2015 the agency issued a final rule entitled " Effluent Limitations Guidelines and Standards for the Steam Electric Power Generating Point Source Category" ("final rule" or "rule"). 80 Fed. Reg. 67,838. The rule addresses these six streams produced by power plants: 1. Flue gas desulfurization (FGD) wastewater 2. Fly ash transport wastewater 3. Bottom ash transport wastewater 4. Flue gas mercury control (FGMC) wastewater ("Hg control waste") 5. Combustion residual leachate (or "Leachate") 6. Gasification wastewater (not depicted in figure below). 80 Fed. Reg. 67,846 -47. The rule treats another category ("legacy" wastewater), which is a subset of five other streams. Infra I.B.1. This diagram illustrates how such streams are produced: When EPA originally regulated steam-electric effluents in the 1970s and 1980s, it did so under the less-stringent BPT standard, see supra I.A, and set BPT for bottom ash transport water and leachate as surface impoundments. See 80 Fed. Reg. 67,848 -49. Surface impoundments, or "ash ponds," are essentially watery pits that "rely on gravity to remove particulates from wastewater" and were "the technology basis for the previously promulgated BPT effluent limitations for low volume waste sources." Id. at 67,840, 67,851. As the new rule describes, however, the ensuing three decades have rendered that BPT standard "out of date," because it "do[es] not adequately control the pollutants (toxic metals and other[s] ) discharged by this industry, nor do[es] [it] reflect relevant process and technology advances that have occurred in the last 30-plus years." Id. at 67,840. Moreover, the Act required that the new guidelines for existing direct discharges conform to the stricter BAT standard. See 33 U.S.C. § 1311(b)(2)(A), 80 Fed. Reg. 67,848 -49; see supra I.A. EPA thus considered more advanced control methods, which it notes are "affordable technologies that are widely available and already in place at some plants." 80 Fed. Reg. 67,840. The agency describes those methods as follows: • Chemical precipitation means treating wastewater by introducing chemicals that will react with substances currently dissolved or suspended in the water to produce a solid, non-soluble precipitate , which then can be filtered out or left to settle to the bottom of the wastewater. EPA Wastewater Technology Fact Sheet , EPA 832-F-00-018 (Sept. 2000). • Biological treatment means introducing bacteria or other microorganisms to remove pollutants, specifically "heavy metals, selenium, and nitrates." 80 Fed. Reg. 67,850. • Dry handling , for fly ash, means "a dry vacuum system that employs a mechanical exhauster to pneumatically convey the fly ash (via a change in air pressure) from hoppers directly to a silo," without getting the ash wet. Id . at 67,852. For bottom ash, dry handling refers to "a system in which bottom ash is collected in a water quench bath and a drag chain conveyor (mechanical drag system) then pulls the bottom ash out of the water bath on an incline to dewater the bottom ash." Id. • Evaporation , for FGD wastewater and gasification wastewater, means using "a falling-film evaporator (also known as a brine concentrator) to produce a concentrated wastewater stream (brine) and a distillate stream." Id. at 67,838, 67,853. From those options EPA selected the following technologies as BAT for the various wastestreams: Technology basis for the main Wastestreams BAT/NSPS/PSES/PSNS regulatory options FGD Wastewater Chemical Precipitation + Biological Treatment Fly Ash Transport Water Dry handling Bottom Ash Transport Water Dry handling/Closed loop FGMC Wastewater Dry handling Gasification Wastewater Evaporation Leachate Impoundment (Equal to BPT) 80 Fed. Reg. 67848-49 (adapted from Table VIII-1- FINAL RULE: STEAM ELECTRIC MAIN REGULATORY OPTIONS ). As shown, the rule set more advanced technologies as BAT for five of the six wastestreams. See also id. at 67,850, 67,852, 67,853 (explaining selection for each stream). For leachate and "legacy" wastewater, however, the rule selected "impoundment" as BAT, the same technology set as BPT in 1982. Id. at 67,854. Our focus is on the rule's treatment of those streams, and so we provide additional detail below. 1. Legacy Wastewater Legacy wastewater is not a distinct type of wastestream. Instead, as the final rule explains, the term describes wastewater from five of the streams (FGD, fly ash, bottom ash, FGMC, and gasification wastewater) that is "generated prior to" a future date. 80 Fed. Reg. at 67,854. That date, which is determined by the permitting authority, is required to be "as soon as possible beginning November 1, 2020 but no later than December 31, 2023." 82 Fed. Reg. 43,496. Wastewater from streams generated before that date is denominated "legacy" wastewater and is not subject to the stricter BAT applicable to those streams. Id. Instead, the BAT for legacy wastewater is "equal to the previously promulgated BPT regulations" in effect since 1982-namely, impoundments. Id. This means that legacy wastewater is allowed by the final rule to contain the same quantity of toxic pollutants allowed since 1982. See id. (setting BAT for legacy wastewater "equal to the previously promulgated BPT limitations on [total suspended solids] in the discharge of fly ash transport water, bottom ash transport water, and low volume waste sources"); see also EPA Study Report , EPA 821-R-09-008, at 5-20 (table listing pollutant concentrations at several individual impoundments studied during the rulemaking process). The "legacy" category will thus encompass a massive amount of wastewater from the five composite streams. For instance, according to the EPA's Study Report, in 2008 alone the average plant produced over 2.7 billion gallons of fly ash transport water per year, as well as over 1.1 billion gallons of bottom ash transport water. Id. at 5-6, 5-7. The rule imposes much more stringent limits on wastewater from these same streams generated after the date to be set by the permitting authority (again, between November 1, 2020 and December 31, 2023). For instance, EPA found that a combination of chemical precipitation and biological treatment was the BAT for treating pollution from non-legacy FGD wastewater, and that "dry handling" (a technique for disposing of fly ash and bottom ash without adding water) was the BAT for non-legacy ash wastestreams. Id. at 67,850 -53. These technologies are significantly newer than surface impoundments, and EPA concluded they were the superior option for treating pollution in non-legacy wastewater. Id. The rule accounts for the discrepancy between legacy and non-legacy wastewater regulations in various ways. For instance, it explains that legacy wastewater "already exists in wet form" and would thus not be amenable to dry handling, and also that EPA lacked data on whether legacy wastewater could be "reliably incorporated" into a closed-loop process "given the variation in operating practices among surface impoundments containing legacy wastewater." Id. at 67,854 -55. The rule also asserts that EPA lacked sufficient data to determine whether chemical or biological treatment would be effective on legacy wastewater. Legacy wastewater, the agency explained, is often "commingled"-meaning different streams are mixed together in an impoundment-making testing and data collection difficult. Id. at 67,855. For instance, commingling may result in varying the concentration and "flow rate" of pollutants in an impoundment. Id. The rule acknowledges that multiple plants are in fact using chemical precipitation to treat commingled wastewater, but it nonetheless asserts that EPA lacks the requisite data from those plants. Id. at 67,855 n.29. Finally, the rule also acknowledges that a few plants discharge from impoundments containing non-commingled FGD legacy wastewater, but it nonetheless declines to establish a stricter BAT for that stream as well. Id. at 67,855. The rule explains that, in the agency's view, imposing the stricter technologies even on non-commingled legacy wastewater would create bad "incentives"-for instance, encouraging plants to begin commingling FGD with other wastewaters or to release FGD wastewater from impoundments on an "accelerated schedule" prior to the compliance date. Id. 2. Leachate The final rule describes leachate as follows: Leachate includes liquid, including any suspended or dissolved constituents in the liquid, that has percolated through or drained from waste or other materials placed in a landfill, or that passes through the containment structure (e.g. , bottom, dikes, berms) of a surface impoundment. 80 Fed. Reg. 67,847. Where leachate occurs in a lined landfill or impoundment, it is typically collected and transported to an impoundment, where it is either "discharge[d] ... directly to receiving waters" or recycled to another impoundment prior to discharge. Id. Unlined landfills or impoundments simply "allow the leachate to potentially migrate to nearby ground waters, drinking water wells, or surface waters." Id. The rule explains that "surface impoundments are the most widely used systems to treat ... leachate." Id. Elsewhere, the rule acknowledges that "[g]round water contamination from surface impoundments" containing power plant wastewater "threatens drinking water, as evidenced by more than 30 documented cases." Id. at 67,840 ; see also EPA Study Report , EPA 821-R-09-008, at 3-24 (landfill leachate diagram). The EPA study detailed the size of leachate pollution: Given plants using current technologies (mostly surface impoundments), leachate pollution amounts to 70,300 toxic-weighted pound equivalents per year. See Technical Development Document ("TDD") , EPA-821-R-15-007, at 10-39. Leachate thus accounts for more equivalent pollution than the entire coal mining industry. Id. ; Annual Effluent Guidelines Review Report , EPA-821-R-16-002, at 2-26 (listing pollution from other industries). The final rule sets BAT for leachate equal to the previous BPT standard established in 1982. Id. at 67,854. The agency offers two primary justifications for its decision not to regulate leachate with any of the more advanced control technologies now available. First, the rule explains that EPA called for comments on leachate regulation during notice-and-comment rulemaking, but that "[c]ommenters did not provide information that the EPA could use to establish BAT limitations" for leachate. Id. at 67,854. Second, the rule asserts that leachate forms "a very small portion of the pollutants discharged collectively by all steam power plants." Id. The agency reasons that, because the new BAT limits established for wastewater from other streams will substantially curtail total power plant pollution, the new rule "represents reasonable further progress toward the CWA's goals" even without establishing any stricter controls on leachate. Id. II. PROCEDURAL HISTORY Four separate lawsuits challenging the final rule were originally brought in the Second, Fifth, Eighth, and Ninth Circuits. Different groups of petitioners challenged different parts of the rule. Various power companies ("Industry Petitioners") challenged the regulation of non-legacy FGD and gasification wastewater. Two water company associations ("Water Company Petitioners"), challenged the non-legacy FGD wastewater regulation. Finally, various environmental groups ("Environmental Petitioners" or "petitioners") challenged the regulation of legacy wastewater and leachate. The four cases were consolidated by the United States Judicial Panel on Multidistrict Litigation and randomly assigned to our court. The Utility Water Act Group ("UWAG") has since intervened to defend those portions of the rule challenged by the Environmental Petitioners. In August 2017, we granted EPA's motion to sever and hold in abeyance the Industry Petitioners' and Water Company Petitioners' challenges to the final rule. In September 2017, EPA announced it would reconsider the rule's regulations concerning non-legacy FGD and bottom ash transport water. See 82 Fed. Reg. 43,494. As a result of these procedural developments, the challenges to the final rule raised by the Industry Petitioners and the Water Company Petitioners are not before us. We address only the challenges brought by the Environmental Petitioners. III. STANDARD OF REVIEW The Environmental Petitioners challenge the legacy wastewater regulation under the Administrative Procedure Act ("APA"). As relevant here, a court "shall ... hold unlawful and set aside" agency action under the APA if it finds such action was "arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law." 5 U.S.C. § 706(2)(A). Under this "highly deferential" standard, Avoyelles Sportsmen's League, Inc. v. Marsh , 715 F.2d 897, 904 (5th Cir. 1983), we are "not empowered to substitute [our] judgment for that of the agency." Citizens to Preserve Overton Park, Inc. v. Volpe , 401 U.S. 402, 416, 91 S.Ct. 814, 28 L.Ed.2d 136 (1971). This is particularly so where the agency's decision turns on "its evaluation of complex scientific data within its technical expertise." BCCA Appeal Grp. v. EPA , 355 F.3d 817, 824 (5th Cir. 2003) (citing Baltimore Gas & Elec. Co. v. NRDC , 462 U.S. 87, 103, 103 S.Ct. 2246, 76 L.Ed.2d 437 (1983) ). Indeed, "[i]f the agency's reasons and policy choices conform to minimal standards of rationality, then its actions are reasonable and must be upheld." Tex. Oil & Gas Ass'n , 161 F.3d at 934. Furthermore, the "EPA's choice of analytical methodology [in setting and enforcing standards] is entitled to a presumption of regularity," leaving challengers with a "considerable burden" to carry. Am. Petroleum Inst. v. EPA , 787 F.2d 965, 983 (5th Cir. 1986). Our review under the APA is not toothless, however. We must set aside agency action if the agency "entirely failed to consider an important aspect of the problem, offered an explanation for its decision that runs counter to the evidence before the agency, or is so implausible that it could not be ascribed to a difference in view or the product of agency expertise." Motor Vehicle Mfrs. Ass'n of U.S., Inc. v. State Farm Mut. Auto. Ins. Co. , 463 U.S. 29, 43, 103 S.Ct. 2856, 77 L.Ed.2d 443 (1983) (" State Farm "); see generally, e.g., Atchafalaya Basinkeeper v. U.S. Army Corps of Eng'rs , 894 F.3d 692, 697 (5th Cir. 2018) (reciting State Farm standard). "[W]e must also ensure that the agency 'examine[d] the relevant data and articulate[d] a satisfactory explanation for its action,' " and assess " 'whether the [agency's] decision was based on a consideration of the relevant factors[.]' " 10 Ring Precision, Inc. v. Jones , 722 F.3d 711, 723 (5th Cir. 2013) (quoting State Farm , 463 U.S. at 43, 103 S.Ct. 2856 ); see also, e.g., Michigan v. EPA , --- U.S. ----, 135 S.Ct. 2699, 2706, 192 L.Ed.2d 674 (2015) (explaining that "agency action is lawful only if it rests 'on a consideration of the relevant factors' ") (quoting State Farm , supra ); U.S. Chamber of Commerce v. U.S. Dep't of Labor , 885 F.3d 360, 382 (5th Cir. 2018) ("Illogic and internal inconsistency are characteristic of arbitrary and unreasonable agency action."); Illinois Pub. Telecom. Ass'n v. FCC , 117 F.3d 555, 566 (D.C. Cir. 1997), decision clarified on reh'g , 123 F.3d 693 (unexplained and "seemingly illogical" decisions are arbitrary and capricious). Furthermore, we "may uphold agency action only on the grounds that the agency invoked when it took the action." Michigan v. EPA , 135 S.Ct. at 2712 (citing SEC v. Chenery Corp. , 318 U.S. 80, 87, 63 S.Ct. 454, 87 L.Ed. 626 (1943) ). The Environmental Petitioners challenge the leachate regulation under the two-step framework articulated in Chevron, USA, Inc. v. Natural Resources Defense Council, Inc. , 467 U.S. 837, 104 S.Ct. 2778, 81 L.Ed.2d 694 (1984), governing judicial review of agency interpretations of statutes. See generally, e.g., BCCA Appeal Grp. , 355 F.3d at 824 (discussing Chevron in context of challenge to Clean Air Act regulations). At step one, the court considers "whether Congress has directly spoken to the precise question at issue." Chevron , 467 U.S. at 842, 104 S.Ct. 2778. If Congress has directly spoken on an issue, that settles the matter: "[T]he Court, as well as the agency, must give effect to the unambiguously expressed intent of Congress." Id. at 842-43, 104 S.Ct. 2778. Only if the statutory text is ambiguous can the court proceed to step two, asking whether the agency's construction of the statute is "permissible." Id. at 843, 104 S.Ct. 2778. If the construction is permissible, it should be upheld. "[A] court may not substitute its own construction of a statutory provision for a reasonable interpretation made by the administrator of an agency." Id. " Chevron review and arbitrary and capricious review overlap at the margins," specifically at Chevron step two. Indep. Petroleum Ass'n of Am. v. Babbitt , 92 F.3d 1248, 1258 (D.C. Cir. 1996) ; see also Nutraceutical Corp. v. Von Eschenbach , 459 F.3d 1033, 1038 (10th Cir. 2006). IV. ANALYSIS A. Challenge to Legacy Wastewater Regulation We first consider petitioners' challenge to the final rule's regulation of legacy wastewater. As already explained, legacy wastewater is not a separate wastewater stream but instead a subset of five of the other streams. Supra I.B.1. Specifically, legacy wastewater is defined by when wastewater is generated: Wastewater "generated prior to" the compliance date set for the new rule by a permitting authority is denominated "legacy" wastewater. See 80 Fed. Reg. 67,854. Instead of subjecting legacy wastewater to the more advanced and effective technologies that kick in after the rule's compliance date (i.e. , chemical precipitation, biological treatment, dry handling, or evaporation), the rule sets BAT for legacy wastewater as equal to the BPT previously set in 1982 (i.e ., surface impoundments). See id. at 67,854 -55. 1. Petitioners challenge that decision as arbitrary and capricious under the APA on two grounds. First, they claim the Act does not grant the Administrator authority to base BAT limits on when waste is generated, but instead requires setting BAT limits for "categories and classes of point sources" regardless of when waste is generated. See, e.g. , State Farm , 463 U.S. at 43, 103 S.Ct. 2856 (explaining that, "[n]ormally, an agency rule would be arbitrary and capricious if the agency has relied on factors which Congress has not intended it to consider"). Second, they claim that the rulemaking record refutes the conclusion that surface impoundments are BAT for legacy wastewater because, among other things, the final rule itself demonstrates that impoundments are ineffective at removing toxic pollutants. See, e.g., id. (an agency rule would also be arbitrary and capricious if it "offered an explanation for its decision that runs counter to the evidence before the agency"). We need not reach petitioners' first argument, because we conclude for multiple reasons that EPA acted arbitrarily and capriciously by setting a BAT limit for legacy wastewater equal to the outdated BPT standard of surface impoundments. First, the final rule repeatedly recognizes that impoundments are "largely ineffective" at removing toxins from wastewater. 80 Fed. Reg. 67,840. Impoundments "rely on gravity to remove particulates from wastewater," but the rule explains that "gravity in surface impoundments" fails to "effectively or reliably" remove "[p]ollutants ... present mostly in soluble (dissolved) form, such as selenium, boron, and magnesium," and also fails to "effectively" remove the dissolved portion of "metals present in both soluble and particulate forms (such as mercury)." Id . at 67,851. When ingested by humans either through drinking water or through seafood, these metals can lead to serious harms including "cancer, cardiovascular disease, neurological disorders, kidney and liver damage, and lowered IQs in children." Id. at 67,840. Additionally, the rule informs us that various factors can alter chemical conditions in impoundments and thus compromise their effectiveness: For instance, low pH in the impoundment environment can convert particulate metals to soluble form, reducing the "settling efficiency in the impoundments" and "leading to increased levels of dissolved metals and high concentrations of metals in discharges from surface impoundments." Id. at 67,851. Even the changing seasons-in an effect called "seasonal turnover"-impair impoundments by cooling the upper layer of water and causing it to sink, resulting in "resuspension of solids ... and a consequent increase in the concentrations of pollutants discharged from the impoundment." Id. These conceded defects in impoundments are in critical tension with EPA's choosing them as BAT for legacy wastewater. After all, BAT is supposed to be "the CWA's most stringent standard" for setting discharge limits for existing sources. Tex. Oil & Gas , 161 F.3d at 928 ; see also 33 U.S.C. §§ 1311(b)(2), 1314(b)(2). We are rightfully skeptical when EPA specifies impoundments as BAT while, in the same breath, detailing how bad those impoundments are in stemming the discharge of toxic pollution. See, e.g. , CMA , 885 F.2d at 265 (remanding because EPA "failed ... [to] demonstrate a reasonable basis for its conclusion" that its chosen BAT was as effective as a proposed alternative). Second, as the rule also recounts, the flaws of impoundments are precisely why EPA refused to set them as BAT for five of the six wastewater streams at issue here. See 80 Fed. Reg. 67,851 -53. For instance, the rule states that "EPA did not select surface impoundments as the BAT technology for FGD wastewater because it would not result in reasonable further progress toward eliminating the discharge of all pollutants, particularly toxic pollutants ." Id. at 67,851 (emphasis added); see also id. (explaining that EPA declined to set impoundments as BAT "[b]ecause many of the pollutants of concern in FGD wastewater are present in dissolved form and would not be removed by surface impoundments ") (emphasis added). EPA likewise declined to set impoundments as BAT for fly ash transport water, bottom ash transport water, FGMC wastewater, and gasification water, and in each case explained that it did so "for the same reasons ... that EPA did not identify surface impoundments as BAT for FGD wastewater." Id. at 67,852 (fly ash), 67,853 (bottom ash, FGMC, and gasification wastewater). In other words, for five of the six wastewater streams regulated by the final rule (the one exception is leachate, discussed in V.B. infra ), EPA affirmatively rejected surface impoundments as BAT "because [they] would not result in reasonable further progress toward eliminating the discharge of all pollutants, particularly toxic pollutants." Id. at 67,851. And yet, having rejected impoundments as BAT because they would not achieve "reasonable further progress" toward eliminating pollution from those streams, EPA turned around and chose impoundments as BAT for each of those same streams generated before the compliance date. That paradoxical action signals arbitrary and capricious agency action. See, e.g., Chamber of Commerce , 885 F.3d at 382 ("Illogic and internal inconsistency are characteristic of arbitrary and unreasonable agency action."); see also, e.g., GameFly, Inc. v. Postal Regulatory Comm'n , 704 F.3d 145, 148 (D.C. Cir. 2013) (explaining that agency action " 'illogical on its face' " may be arbitrary and capricious) (quoting Am. Fed'n of Gov't Emps., Local 2924 v. Fed. Labor Relations Auth. , 470 F.3d 375, 380 (D.C. Cir. 2006) ). It also strongly suggests that EPA has contravened the plain language of the CWA, which defines BAT as the technology that "will result in reasonable further progress" toward pollutant discharge elimination. 33 U.S.C. § 1311(b)(2)(A) (emphasis added); see also, e.g., Nat'l Crushed Stone , 449 U.S. at 74-75, 101 S.Ct. 295 (discussing "reasonable further progress" component of BAT). Third, the final rule explains that the shortcomings of surface impoundments were a key factor in motivating EPA to conduct the 2006-2009 study and revise water pollution regulations for power plants in the first place. The rule describes the previous ELGs from 1974, 1977, and 1982 as "out of date," because they failed to "adequately control the pollutants ... discharged by this industry" and failed to "reflect relevant process and technology advances that have occurred in the last 30-plus years." 80 Fed. Reg. 67,840 ; see also id. (stating that, "[i]n the several decades since the steam electric ELGs were last revised, [more effective] technologies have increasingly been used at plants"). And the rule minces no words in laying the shortcomings of the prior ELGs at the feet of surface impoundments: The processes employed and pollutants discharged by the industry look very different today than they did in 1982. Many plants, nonetheless, still treat their wastewater using only surface impoundments, which are largely ineffective at controlling discharges of toxic pollutants and nutrients. Id. Thus, the final rule describes impoundments as an outdated and ineffective pollution control technology, and yet the same rule chooses to freeze impoundments in place as BAT for legacy wastewater. That is inconsistent with the "technology-forcing" mandate of the CWA. NRDC II , 808 F.3d at 563-64 (citing NRDC I , 822 F.2d at 123 ). To that point, the Supreme Court has explained that BAT has an inbuilt "reasonable further progress" standard and that "BPT serves as the prior standard with respect to BAT." Nat'l Crushed Stone , 449 U.S. at 75, 101 S.Ct. 295. Yet here EPA appears to have conflated the prior standard with the advanced one: It has selected as BAT the same three-decades-old technology previously set as BPT-a technology the current rule condemns as anachronistic and ineffective at eliminating pollution discharge. In other words, EPA asks us to believe that impoundments are both archaic and cutting-edge at the same time. That we cannot do. See GameFly, 704 F.3d at 148 ; Chamber of Commerce , 885 F.3d at 382. Fourth, the final rule strongly indicates that other available technologies are far better than impoundments at removing pollutants from the various streams that comprise legacy wastewater. For instance, after explaining why impoundments are ineffective at removing toxic metals from FGD wastewater, the rule states that a combination of chemical precipitation and biological treatment is better at removing those pollutants. Id. at 67,850-51. Importantly, the rule explicitly concludes that "[c]hemical precipitation and biological treatment are more effective than surface impoundments at removing both soluble and particulate forms of metals." Id. at 67,851 (emphasis added). The rule also relies on that reasoning to justify rejecting impoundments as BAT for fly ash transport water, bottom ash transport water, FGMC wastewater, and gasification wastewater. Id. at 67,852-53. Moreover, the rule categorically states that more advanced control methods, such as chemical and biological methods, "are affordable technologies that are widely available, and already in place at some plants." Id. at 67,840. These affirmative findings are difficult, if not impossible, to square with EPA's decision nonetheless to set 1980s-era impoundments as the BAT for legacy wastewater. To be sure, the agency's statements in the final rule do not prove that chemical precipitation or biological treatment (or some combination of the two) are BAT for legacy wastewater. That is for the agency to decide. But those statements do cast grave doubt on the agency's selection of impoundments as BAT for legacy wastewater. See, e.g., Nat'l Crushed StoneAss'n , 449 U.S. at 74, 101 S.Ct. 295 (BAT calls for the "maximum use of technology within the economic capability of the [plant] owner or operator"). Once again, the EPA's own rule strongly indicates that it was arbitrary and capricious in doing so. Fifth and finally, our court has long recognized that " 'Congress intended [BAT] limitations to be based on the performance of the single best-performing plant in an industrial field.' " Tex. Oil & Gas , 161 F.3d at 928 (quoting CMA , 870 F.2d at 226 )); see also, e.g., Kennecott v. EPA , 780 F.2d 445 (4th Cir. 1985) ("In setting BAT, EPA uses not the average plant, but the optimally operating plant, the pilot plant which acts as a beacon to show what is possible.") (citing A Legislative History of the Water Pollution Control Act Amendments of 1972 , 93d Cong., 1st Sess. (Comm. Print 1973), at 798). Yet here the rule says nothing to indicate that the choice of impoundments as BAT for legacy wastewater was based on anything like "the performance of the single best-performing plant" in the field. To the contrary, everything the rule says about the record of impoundments over the past three decades indicates that their performance in controlling discharges has been distinctly poor. See, e.g., 80 Fed. Reg. 67,840 (stating that "impoundments ... are largely ineffective at controlling discharges of toxic pollutants and nutrients"); id. (stating that "[g]round water contamination from surface impoundments ... threatens drinking water, as evidenced by more than 30 documented cases"); id. at 67,851 (declining to set impoundments at BAT for FGD wastewater because various dissolved toxic metals "are not effectively and reliably removed by gravity in surface impoundments"). Moreover, the rule also states that multiple plants are in fact treating legacy wastewater using chemical precipitation, id. at 67,855 n.29, a method the rule concedes is "more effective than surface impoundments at removing both soluble and particulate forms of metals[.]" Id. at 67,851. Yet the rule merely states-without explanation-that it lacks "data to characterize the effluent from these systems." Id. at 67,855 n.29. That unexplained assertion casts grave doubt on the agency's BAT decision. See also infra V.A.2 (discussing additional problems created by agency's lack of data excuse). These shortcomings in the agency's explanations strongly indicate that its BAT decision simply defaults to the outdated BPT standard that has been demonstrated to be a poor performer by the agency's own analysis. That is antithetical to the statutorily-mandated BAT standard. See, e.g. , Nat'l Lime Ass'n v. EPA , 233 F.3d 625, 634 (D.C. Cir.), as amended on denial of reh'g (2001) (remanding EPA rule for failure in agency's "clear statutory obligation to set emission standards" for various air pollutants); NRDC v. EPA , 863 F.2d 1420, 1433 (9th Cir. 1988) (despite EPA's asserted lack of "complete information" on availability of technology, declaring BAT limitation invalid because "Congress has demonstrated its intent to require industry to do as much as possible to control toxic discharges") (citing 33 U.S.C. § 1311(b)(2)(A)(i) ). In sum, we conclude that the EPA's decision to set surface impoundments as BAT for legacy wastewater was arbitrary and capricious. Far from demonstrating that impoundments are the "best available technology economically achievable" for treating legacy wastewater, the evidence recounted in the final rule shows that impoundments are demonstrably ineffective at doing so and demonstrably inferior to other available technologies. In light of this record, we cannot accept that an outdated, ineffective and inferior technology is BAT when applied to legacy wastewater. No record evidence affirmatively makes that case and, as we have explained, the evidence recounted in the final rule runs in the opposite direction. 2. EPA defends its choice of impoundments as BAT for legacy wastewater by asserting that "it does not have the data" to justify choosing more advanced pollution control technologies. 80 Fed. Reg. 67,855. The agency explains that most plants "combine some of their legacy wastewater with each other and with other wastestreams," and that this "commingling" can meaningfully alter the characteristics (specifically, the "flow rate and pollutant concentration") of the impoundment water. Id. Because EPA lacked adequate examples of plants treating commingled wastewater "using anything beyond the surface impoundment itself," the agency concluded it lacked data to evaluate the performance of other technologies and therefore defaulted to "the previously promulgated BPT limitations" (i.e. , impoundments). Id. We are unpersuaded. First, EPA's arguments about the characteristics of commingled wastewater glide past the key issue before us, which is whether the agency arbitrarily chose impoundments as BAT. The agency may lack data on how other technologies interact with commingled wastewater, but it assuredly does not lack data on impoundments. To the contrary, we know that impoundments are ineffective at removing toxic pollutants from the various wastewater streams because the agency's own rule tells us so, repeatedly, based on over three decades of observation and analysis. See supra V.B.1. Nor does EPA's criticism of impoundments distinguish "legacy" from "non-legacy" wastewater: instead, the agency categorically states that "surface impoundments ... are largely ineffective at controlling discharges of toxic pollutants and nutrients" from "wastewater." 80 Fed. Reg. 67,840. To be sure, we do not pretend to second-guess EPA's assertions about the pertinent "flow rate" and "pollutant concentration" in commingled wastewater, id. at 67,855, matters beyond our expertise and authority. But those assertions side-step the legal issue we must decide, which is whether the agency was arbitrary in selecting impoundments as BAT. Once again, the agency's own pointed criticism of impoundments indicates that it was. Second, the agency's "lack of data" excuse is untenable on its own terms. In a footnote, the rule concedes that multiple power plants have in fact been using chemical precipitation to treat commingled legacy wastewater. See id. at 67,855 n.29 (stating "EPA identified fewer than ten plants that use chemical precipitation to treat waster that contains, among other things, ash transport water"). Yet, the agency baldly asserts that it "does not have any data to characterize the effluent from these systems" and it raises this dearth of information to justify not regulating legacy wastewater under the same BAT standards as non-legacy wastewater (and to justify a demonstrably outdated technology as BAT). Id. We have previously rejected EPA's argument that an asserted lack of "sufficient data" justified the agency's failure to regulate. See API I , 661 F.2d at 357 (rejecting EPA's argument that its failure to regulate was justified by lack of "sufficient data" where EPA had failed to investigate "in light of ... new information" (internal quotation marks omitted)); see also, e.g., NRDC II , 808 F.3d at 573 (concluding that EPA's failure to gather data can be arbitrary and capricious when the lack of data is "a problem of EPA's own making"); see also infra V.B.2 (further discussing API I and NRDC II ). We reject the argument again here. The final rule recounts (1) the long-recognized deficiencies of impoundments in controlling toxic discharges, 80 Fed. Reg. 67,840 ; (2) the demonstrated superiority of more advanced technologies in doing so, id. at 67,851 -53; (3) the availability of those technologies in the industry, id. at 67,840, 67,844 ; and (4) multiple plants' actual use of one of those advanced technologies (chemical precipitation) to treat commingled legacy wastewater, id. at 67,855 n.29. Given those undisputed statements drawn from EPA's own rule, the agency cannot simply plead a lack of data to justify its decision to set impoundments as BAT. Again, we do not purport to tell the agency what technology it should choose as BAT for legacy wastewater. We decide only that, given the agency's own statements and evidence, it acted arbitrarily in selecting as BAT a pollution control method that decades of data have shown to be ineffective at controlling pollution. Third, given EPA's heavy reliance on the characteristics of commingled legacy wastewater as a reason for declining stricter regulation, one would expect a different policy for non- commingled legacy wastewater. See 80 Fed. Reg. 67,855 & n.28 (asserting as key reason for defaulting to impoundments for legacy wastewater the fact that wastewater at the "vast majority" of plants is "commingled" with other streams). Yet we find the opposite: When EPA identified plants that discharge non-commingled legacy wastewater from impoundments, it still declined to impose more stringent controls and still defaulted to impoundments as BAT. See id. at 67,855 (declining to impose controls "other than surface impoundments" on plants that "discharge from an impoundment containing only legacy FGD wastewater "); id. at 67,855 n.30 (discussing three plants that use impoundments "where the FGD wastewater is not commingled with other process wastewaters in the impoundment") (emphases added). That striking inconsistency undercuts the agency's "commingling" rationale for not imposing the more stringent non-legacy BAT standards. See, e.g., Am. Fed'n of Gov't Emps., Local 2924 v. Fed. Labor Relations Auth. , 470 F.3d 375, 380 (D.C. Cir. 2006) (an agency's decision is "arbitrary and capricious" if "illogical on its own terms"); see also Chamber of Commerce , 885 F.3d at 382 ("Illogic and internal inconsistency are characteristic of arbitrary and unreasonable agency action."). What is more, by selecting impoundments as BAT for any kind of FGD wastewater ("legacy" or not), the rule flatly contradicts itself: The rule states without exception that for FGD wastewater "[c]hemical precipitation and biological treatment are more effective than surface impoundments at removing" toxic pollutants. 80 Fed. Reg. 67,851. Fourth, even assuming a lack of data prevented EPA from determining BAT for legacy wastewater, nothing required the agency simply to set impoundments as BAT. Instead, EPA could have declined to set nationwide effluent guidelines for legacy wastewater and allowed BAT determinations to be made by each facility's permitting authority through the NPDES permitting process on a site-specific basis. See 40 C.F.R. § 125.3(a), (c)(2) ("Technology based treatment requirements may be imposed ... [o]n a case-by-case basis."); Riverkeeper, Inc. v. EPA , 358 F.3d 174, 203 (2nd Cir. 2004) ("We see no textual bar in sections 306 or 316(b) [of the Clean Water Act] to regulating [certain] structures on a case-by-case basis."); Nat'l Wildlife Fed'n v. EPA , 286 F.3d 554, 566-67 (D.C. Cir. 2002) ("We believe EPA acted both reasonably and within its authority in adopting a case-by-case approach" to regulating certain pollutants from paper mills). The agency took that approach in the current rule by deferring setting BAT and other limits for metal cleaning wastes, after determining it lacked the necessary data. See 80 Fed. Reg. 67,863 (directing permitting authorities to "establish such requirements based on [best professional judgment] for any steam electric power plant discharg[ing]" such waste). Instead of deferring a nationwide effluent guideline and allowing case-by-case determination of BAT by permitting authorities, EPA unaccountably defaulted to impoundments-again, which its own rule recognizes as an out-of-date and ineffective pollution control technology. This is further indication that the rule respecting legacy wastewater is arbitrary and capricious. *** In sum, having examined the various justifications set forth for EPA's final rule on legacy wastewater, and finding each of those explanations wanting in light of the agency record, we conclude that EPA's rulemaking was arbitrary and capricious. We therefore set aside that part of the final rule and remand to the agency for reconsideration. See, e.g., Perdue v. FAA , 172 F.3d 866 (5th Cir. 1999) ("This court shall set aside agency action that is 'arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law.' " (quoting 5 U.S.C. § 706(2)(A) )); Checkosky v. SEC , 23 F.3d 452, 491 (D.C. Cir. 1994) (" Section 706(2)(A) [of the APA] provides that a 'reviewing court' faced with an arbitrary and capricious agency decision 'shall'-not may-'hold unlawful and set aside' the agency action."). We recognize that the agency is entitled to considerable deference in setting BAT limitations. See, e.g., BCCA Appeal Grp. , 355 F.3d at 824 (EPA entitled to special deference where its decision turns on "its evaluation of complex scientific data within its technical expertise"); Tex. Oil. & Gas , 161 F.3d at 928 (recognizing EPA's "considerable discretion" in weighing BAT factors). Precisely for that reason, challenges to the agency's BAT determinations often fail because challengers ask the court to elevate itself as an expert over the agency. This case is different. We do not question the scientific or statistical methodologies relied upon by EPA, nor second-guess its weighing of the statutory factors. Instead, we rely on EPA's own scientific conclusions in the rule itself to conclude that its choice of an outdated and ineffective technology as BAT was unlawful under the Act. That is a legal-not a technical or scientific-conclusion that the APA requires us to make. B. Challenge to Leachate Regulation We turn to the challenge to the rule's regulation of combustion residual leachate. As explained, supra I.B.2, leachate consists of liquid that percolates through a landfill or impoundment and is eventually discharged into water. See 80 Fed. Reg. 67,847. The final rule sets BAT for leachate as impoundments, which is the same as the previous BPT for leachate established in 1982. See id. at 67,854 ("This rule identifies surface impoundments as the BAT technology basis for control of pollutants in combustion residual leachate ... establish[ing] a BAT limitation on [total suspended solids] in ... leachate equal to the previously promulgated BPT limitation on [total suspended solids] in low volume waste sources."). EPA offers two justifications for this decision: First, that "[c]ommenters did not provide information that EPA could use to establish [stricter] BAT limitations" for leachate, and, second, that because leachate forms a "very small portion" of overall discharges, the rule's stricter regulation of other wastestreams "already represents reasonable further progress towards the CWA's goals." See id. ; supra I.B.2. The Environmental Petitioners challenge the rule's leachate regulation under the Chevron test for reviewing agency interpretations of statutes. See Chevron , 467 U.S. 837, 104 S.Ct. 2778, 81 L.Ed.2d 694 ; see also generally, e.g., Acosta v. Hensel Phelps Constr. Co. , 909 F.3d 723, 730 (5th Cir. 2018) (summarizing "the two-step framework established in Chevron "); RICHARD J. PIERCE, JR., ADMINISTRATIVE LAW TREATISE § 3.2 ("PIERCE") (discussing "[t]he Chevron Two-Step"). Petitioners assert that EPA's decision to set impoundments as BAT for leachate fails Chevron step one by contravening the plain text and structure of the Clean Water Act. Alternatively, petitioners argue that the agency's decision fails Chevron step two by adopting an impermissible construction of the Act. We address each argument in turn. 1. At Chevron step one, we ask whether the pertinent statute "unambiguously foreclose[s]" the agency's challenged statutory interpretation; if it does, " 'that is the end of the matter; for the court, as well as the agency, must give effect to the unambiguously expressed intent of Congress.' " U.S. Chamber of Commerce v. U.S. Dep't of Labor , 885 F.3d 360, 369 (5th Cir. 2018) (quoting Chevron , 467 U.S. at 842-43, 104 S.Ct. 2778 ); see also, e.g., Acosta , 909 F.3d at 730 (at Chevron step one, the court "must determine 'whether Congress has directly spoken to the precise question at issue' " (quoting City of Arlington v. FCC , 569 U.S. 290, 296, 133 S.Ct. 1863, 185 L.Ed.2d 941 (2013) )). To answer this question, we rely on "the conventional standards of statutory interpretation"-i.e. , "text, structure, and the overall statutory scheme"-as well as "authoritative Supreme Court decisions." U.S. Chamber of Commerce , 885 F.3d at 369 (citing City of Arlington , 569 U.S. at 296, 133 S.Ct. 1863 ; Chevron , 467 U.S. at 843 n.9, 104 S.Ct. 2778 ). We are not to focus myopically on "a particular statutory provision in isolation" because "[t]he meaning-or ambiguity-of certain words or phrases may only become evident when placed in context." FDA v. Brown & Williamson Tobacco Corp. , 529 U.S. 120, 132, 120 S.Ct. 1291, 146 L.Ed.2d 121 (2000). Rather, we must read words " 'in their context,' " interpreting the statute " 'as a symmetrical and coherent regulatory scheme,' " and " 'fit[ting], if possible, all [the statute's] parts into an harmonious whole.' " Id. at 133, 120 S.Ct. 1291 (quoting Davis v. Mich. Dep't of Treasury , 489 U.S. 803, 809, 109 S.Ct. 1500, 103 L.Ed.2d 891 (1989) ; Gustafson v. Alloyd Co. , 513 U.S. 561, 569, 115 S.Ct. 1061, 131 L.Ed.2d 1 (1995) ; FTC v. Mandel Bros., Inc. , 359 U.S. 385, 389, 79 S.Ct. 818, 3 L.Ed.2d 893 (1959) ) (brackets added). Additionally, "we must be guided to a degree by common sense as to the manner in which Congress is likely to delegate a policy decision of such economic and political magnitude to an administrative agency." Texas v. United States , 787 F.3d 733, 760 (5th Cir. 2015) (quoting Brown & Williamson , 529 U.S. at 133, 120 S.Ct. 1291 ). The goal of our Chevron step one inquiry is ultimately to "ascertain whether the statute is silent or ambiguous in addressing the precise question at issue." Tex. Savings & Cmty. Bankers Ass'n v. Fed. Hous. Fin. Bd. , 201 F.3d 551, 554 (5th Cir. 2000) ; see also PIERCE § 3.6 ("The question for the court [at step one] is whether the agency's co