United States Court of Appeals

FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued January 27, 2023 Decided May 9, 2023

No. 20-1335

NATURAL RESOURCES DEFENSE COUNCIL,

PETITIONER

v.

MICHAEL S. REGAN, ADMINISTRATOR, U.S. ENVIRONMENTAL 

PROTECTION AGENCY AND ENVIRONMENTAL PROTECTION 

AGENCY,

RESPONDENTS

AMERICAN WATER WORKS ASSOCIATION,

INTERVENOR

On Petition for Review of a Final Action

of the Environmental Protection Agency

Sarah V. Fort argued the cause for petitioner. With her on 

the briefs was Charles R. Corbett.

David J. Berger was on the brief for amici curiae

Academic Scientists in support of petitioner.
2

Sarah A. Buckley, Senior Attorney, U.S. Department of 

Justice, argued the cause for respondents. With her on the brief 

was Todd Kim, Assistant Attorney General.

Corinne Snow argued the cause for respondent-intervenor 

American Water Works Association. With her on the brief was 

Ronald J. Tenpas. Jeremy C. Marwell entered an appearance.

Annie S. Amaral and Thomas C. Roberts were on the brief 

for amici curiae American Chemistry Council and Western 

Growers in support of respondents. 

Before: PAN, Circuit Judge, and SENTELLE and TATEL, 

Senior Circuit Judges.

Opinion for the Court filed by Senior Circuit Judge

SENTELLE.

Opinion concurring in the judgment filed by Circuit Judge 

PAN.

SENTELLE, Senior Circuit Judge: In 2011, the 

Environmental Protection Agency (“EPA”) issued its “final 

determination to regulate perchlorate in drinking water” under 

the Safe Drinking Water Act. Drinking Water: Regulatory 

Determination on Perchlorate, 76 Fed. Reg. 7,762, 7,762 (Feb. 

11, 2011). That determination started a clock under the Safe 

Drinking Water Act requiring EPA to propose regulations

within twenty-four months and promulgate regulations within 

eighteen months of the proposal. See 42 U.S.C. § 300g1(b)(1)(E). But EPA never promulgated perchlorate 

regulations. Instead, nine years later, the agency purported to 

withdraw its regulatory determination. See Drinking Water: 

Final Action on Perchlorate, 85 Fed. Reg. 43,990, 43,991 (July 

21, 2020). Natural Resources Defense Council (“NRDC”) 
3

petitions for review of this action, arguing that EPA lacks the

authority to withdraw a regulatory determination under the Act

and that, even if EPA possesses such authority, it acted 

arbitrarily and capriciously by doing so. EPA, joined by 

Intervenor American Water Works Association, defends its 

action. Because the Safe Drinking Water Act does not permit 

EPA to withdraw a regulatory determination, we grant 

NRDC’s petition, vacate EPA’s withdrawal of its regulatory 

determination, and remand to the agency for further 

proceedings. 

I. Background 

a. Statutory Framework 

The Safe Drinking Water Act authorizes EPA to regulate

potentially harmful contaminants in the nation’s drinking 

water. See 42 U.S.C. § 300g-1(b)(1)(A). First enacted in 1974, 

the Act has since undergone several amendments. The 1986 

amendments required EPA to select at least twenty-five new 

contaminants for regulation every three years. Pub. L. No. 99-

339, § 101(b)(3)(C), (D), 100 Stat. 642, 644 (1986). Congress 

apparently created this strict regulatory scheme, at least in part, 

because it believed EPA had failed to regulate a sufficient 

number of contaminants under the Act’s prior structure. See S. 

Rep. No. 104-169, at 8, 12 (1995). When Congress amended 

the Act in 1996 to create the present scheme, it replaced the 

strict three-year regulatory requirement with a discretionary 

scheme that allows EPA to determine when contaminants 

warrant regulation. 42 U.S.C. § 300g-1(b)(1)(A); S. Rep. No. 

104-169, at 12–13.

Under the current Act, every five years EPA must publish 

a list of unregulated contaminants that may require future 

regulation (the “Contaminant Candidate List”) and make a 
4

preliminary determination, subject to notice and comment, of

whether to regulate at least five listed contaminants. 42 U.S.C.

§ 300g-1(b)(1)(B)(i)(I), (ii)(I). After the comment period ends, 

EPA must make its final regulatory determination. Id.

§ 300g-1(b)(1)(B)(ii)(I). The agency can only determine to 

regulate a contaminant if it finds, based upon the “best 

available public health information,” id.

§ 300g-1(b)(1)(B)(ii)(II), that:

(i) the contaminant may have an adverse effect on the 

health of persons; (ii) the contaminant is known to 

occur or there is a substantial likelihood that the 

contaminant will occur in public water systems with a 

frequency and at levels of public health concern; and 

(iii) in the sole judgment of the Administrator, 

regulation of such contaminant presents a meaningful 

opportunity for health risk reduction for persons served 

by public water systems,

id. § 300g-1(b)(1)(A)(i)–(iii).

The Act frontloads EPA’s discretion, allowing the agency

to create the list of contaminants that may require future 

regulation, id. § 300g-1(b)(1)(B)(i)(I), select which of those

listed contaminants to consider for regulation, id.

§ 300g-1(b)(1)(B)(ii)(I), and determine whether the selected 

contaminants meet the statutory criteria for regulating, id.

§ 300g-1(b)(1)(A)(iii). Once EPA makes its regulatory 

determination, however, the Act balances that discretion with 

a strict, mandatory scheme governing the regulatory process. 

It instructs that, after determining the statutory criteria are met, 

the EPA Administrator “shall, in accordance with the 

procedures established by this subsection, publish a maximum 

contaminant level goal and promulgate a national primary 

drinking water regulation.” Id. § 300g-1(b)(1)(A) (emphasis 
5

added). The maximum contaminant level goal (“MCLG”) is

an unenforceable, aspirational level and is defined as “the level 

at which no known or anticipated adverse effects on the health 

of persons occur and which allows an adequate margin of 

safety.” Id. § 300g-1(b)(4)(A). The national primary drinking 

water regulations also normally include an enforceable 

maximum contaminant level (“MCL”) that must be set “as 

close to the maximum contaminant level goal as is feasible.” 

Id. § 300g-1(b)(4)(B). In limited circumstances, EPA can issue 

an alternative enforceable standard. See id. § 300g-1(b)(6)(A), 

(7)(A). EPA must propose the MCLG and national primary 

drinking water regulations within twenty-four months of 

making its determination to regulate and must publish the 

MCLG and promulgate the regulations within eighteen months 

of the proposal, subject to a nine-month extension. Id.

§ 300g-1(b)(1)(E).

The statute also contains an “anti-backslide” provision that 

ensures that, once issued, a regulation can only be revised in a 

way that will “maintain . . . or provide for greater” health 

protections. Id. § 300g-1(b)(9). In all decisions the agency 

makes that are based on science, EPA is instructed to use “the 

best available, peer-reviewed science.” Id.

§ 300g-1(b)(3)(A)(i).

b. Factual and Procedural Background 

Perchlorate, the contaminant at issue in this case, is a 

naturally occurring and manufactured chemical commonly 

used in the aerospace and defense sectors. When ingested,

perchlorate can inhibit the thyroid’s ability to absorb iodide. 

85 Fed. Reg. at 43,994. An iodide-deficient thyroid, in turn, 

disrupts the production of thyroid hormones. Id. And 

disruptions in thyroid hormone production can lead to adverse 

neurodevelopmental outcomes in sensitive populations whose 
6

brains are still developing, including fetuses and children of

lactating women. Id.

Recognizing the potential health risks associated with 

perchlorate, EPA added perchlorate to its Contaminant 

Candidate List in 1998, categorizing it as a chemical “needing 

additional health effects, treatment research, and occurrence 

information.” Announcement of the Drinking Water 

Contaminant Candidate List, 63 Fed. Reg. 10,274, 10,275, 

10,282 (Mar. 2, 1998). The agency published its first

Unregulated Contaminant Monitoring Rule (“UCMR-1”) in 

1999, requiring all large water systems and a sample of small 

water systems to collect data on perchlorate contamination 

between 2001 and 2005. 85 Fed. Reg. at 43,993.

In 2008, after applying a health reference level (the “level 

of concern”) of 15 μg/L and evaluating the frequency of 

perchlorate contamination at that level using the UCMR-1 data, 

EPA issued a preliminary determination not to regulate 

perchlorate after determining that regulation “would not 

present a meaningful opportunity for health risk reduction” and 

sought comment on that proposed action. Drinking Water: 

Preliminary Regulatory Determination on Perchlorate, 73 Fed. 

Reg. 60,262, 60,269, 60,280–81. (Oct. 10, 2008). EPA issued 

a supplemental notice seeking comment on alternative health 

reference levels the following year. See Drinking Water: 

Perchlorate Supplemental Request for Comments, 74 Fed. Reg. 

41,883, 41,889 (Aug. 19, 2009). Deviating from its 

preliminary determination, the agency issued its “final 

determination to regulate perchlorate” in 2011. 76 Fed. Reg.

at 7,762. That determination “initiate[d] the process to develop 

a national primary drinking water regulation . . . for 

perchlorate” and started the clock for EPA to propose the 

MCLG and regulations within twenty-four months and to 

promulgate the final MCLG and regulations within the 
7

following eighteen months. 76 Fed. Reg. at 7762–63; 42 

U.S.C. § 300g-1(b)(1)(A), (E). 

Consistent with its statutory obligation, see id.

§ 300g-1(e), EPA consulted with the Science Advisory Board 

as it worked to develop perchlorate regulations, National 

Primary Drinking Water Regulations: Perchlorate, 84 Fed. 

Reg. 30,524, 30,527–28 (June 26, 2019). At the Board’s 

urging, EPA revised the model it used to predict the effects of 

perchlorate exposure, developing a “broader and more 

comprehensive framework” that directly links iodide uptake 

inhibition to changes in thyroid hormone levels, allowing the 

agency to better analyze the neurodevelopmental effects 

caused by perchlorate exposure. Id.

After EPA missed the statutory deadlines for proposing 

and promulgating the MCLG and regulations, NRDC sued the 

agency in 2016, seeking to compel the agency to regulate. 84 

Fed. Reg. at 30,526. The parties entered a consent decree 

requiring EPA to propose and promulgate the MCLG and final

regulations by 2020. NRDC v. EPA, No. 1:16-cv-01251-ER, 

ECF Nos. 38, 60 (S.D.N.Y.); 84 Fed. Reg. at 30,526. In June 

2019, EPA proposed setting the MCLG and MCL at 56 μg/L

or, in the alternative, at 18 μg/L or 90 μg/L. 84 Fed. Reg. at

30,525. Alternatively, the agency also considered withdrawing 

its 2011 regulatory determination and not promulgating an 

MCLG or national primary drinking water regulations. Id. The 

agency sought comment on its proposal and the three 

alternatives. Id.

In July 2020, after the comment period ended, EPA 

announced it was withdrawing its determination to regulate and 

issuing a “final determination not to regulate perchlorate.” 85 

Fed. Reg. at 43,991. The agency explained that it had “reevaluated” whether perchlorate satisfied the statutory criteria 
8

for regulating using its updated model and concluded that 

“perchlorate does not occur at a frequency and at levels of 

public health concern” and that regulation “does not present a 

meaningful opportunity for health risk reduction.” Id. at 

43,998. In EPA’s view, because its “re-evaluat[ion]” showed 

that the statutory criteria were not met, it lacked the authority 

to regulate. Id.

II. Analysis

a. Statutory Authority 

EPA makes two primary arguments in support of its 

authority to withdraw a regulatory determination. First, it 

argues that its withdrawal of the 2011 regulatory determination 

was consistent with the statute. Its second argument is 

premised on its “inherent authority” to change positions and 

withdraw a determination to regulate, which it claims the Safe 

Drinking Water Act does not abrogate. Resp. Br. at 22. 

We start with EPA’s second argument, which rests on a 

faulty premise. While we have often referred to agencies’ 

“inherent authority,” see, e.g., Ivy Sports Med., LLC v. Burwell, 

767 F.3d 81, 86 (D.C. Cir. 2014), the term “inherent” is 

misleading because “it is ‘axiomatic’ that ‘administrative 

agencies may act only pursuant to authority delegated to them 

by Congress,’” Clean Air Council v. Pruitt, 862 F.3d 1, 9 (D.C. 

Cir. 2017) (quoting Verizon v. FCC, 740 F.3d 623, 632 (D.C. 

Cir. 2014)). Thus, “the more accurate label” for the power EPA 

describes “is ‘statutorily implicit.’” HTH Corp. v. NLRB, 823 

F.3d 668, 679 (D.C. Cir. 2016) (quoting Ivy Sports Med., 767 

F.3d at 93 (Pillard, J., dissenting)). And although the power to 

decide is normally accompanied by the power to reconsider, 

“Congress . . . undoubtedly can limit an agency’s discretion to 

reverse itself.” New Jersey v. EPA, 517 F.3d 574, 582–83 
9

(D.C. Cir. 2008); see Ivy Sports Med., 767 F.3d at 86 (“[A]ny 

inherent reconsideration authority does not apply in cases 

where Congress has spoken.”).

EPA, then, has no inherent authority. It has only the 

authority given it by the Safe Drinking Water Act. The 

question, then, is whether that authority includes the authority 

to withdraw a regulatory determination.

To answer that question, we look to the statutory text. The 

Act instructs that “[t]he [EPA] Administrator shall, in 

accordance with the procedures established by this subsection, 

publish a maximum contaminant level goal and promulgate a 

national primary drinking water regulation for a contaminant 

. . . if the Administrator determines that” the statutory criteria 

have been met. 42 U.S.C. § 300g-1(b)(1)(A) (emphasis 

added). Elsewhere in the statute, Congress repeated the 

directive three additional times that EPA “shall” regulate. “For 

each contaminant that the Administrator determines to regulate 

under subparagraph (B), the Administrator shall publish 

maximum contaminant level goals and promulgate, by rule, 

national primary drinking water regulations . . . .” Id.

§ 300g-1(b)(1)(E) (emphasis added). Within 24 months of 

making the determination to regulate, “[t]he Administrator 

shall propose the maximum contaminant level goal and 

national primary drinking water regulation.” Id. (emphasis 

added). And within 18 months of proposing the MCLG and 

regulations, “[t]he Administrator shall publish a maximum 

contaminant level goal and promulgate a national primary 

drinking water regulation.” Id. (emphasis added). 

It is well established that “[t]he word ‘shall’ generally 

indicates a command that admits of no discretion on the part of 

the person instructed to carry out the directive.” Assoc. of 

Civilian Technicians, Mont. Air Chapter No. 29 v. FLRA, 22 
10

F.3d 1150, 1153 (D.C. Cir. 1994). That is even more true 

where, as here, the statute explicitly grants the agency

significant discretion at the outset but later instructs the agency 

that it “shall” act. Cf. Anglers Conservation Network v. 

Pritzker, 809 F.3d 664, 671 (D.C. Cir. 2016) (“[W]hen a

statutory provision uses both ‘shall’ and ‘may,’ it is a fair 

inference that the writers intended the ordinary distinction.”).

In 2011, EPA determined that perchlorate satisfied the 

statutory criteria for regulating. 76 Fed. Reg. at 7,763. Under 

the statute, then, EPA has one authorized course of action: it 

“shall” propose and promulgate the MCLG and regulations,

and it “shall” do so by the statutory deadlines. See 42 U.S.C. 

§ 300g-1(b)(1)(A), (E). EPA recognized as much when it 

issued its 2011 “final regulatory determination.” See 76 Fed. 

Reg. at 7,763 (“Once EPA makes a determination to regulate a 

contaminant in drinking water, [the Act] requires that EPA 

issue a proposed [regulation] within 24 months and a final 

[regulation] within 18 months of proposal.” (emphasis added)).

Intervenors likewise admit the statute imposes “a duty to issue 

regulations where the Administrator has determined to 

regulate.” Intervenor’s Br. at 35. To read into the statute 

another course of action—one that allows EPA to withdraw its 

regulatory determination entirely and decide that it “shall not” 

regulate—would be to contravene the statute’s clear language 

and structure and “nullif[y] textually applicable provisions 

meant to limit [EPA’s] discretion.” New Jersey v. EPA, 517 

F.3d 574, 583 (D.C. Cir. 2008) (quoting Whitman v. Am. 

Trucking Ass’ns, 531 U.S. 457, 485 (2001)). Because EPA 

lacked the authority to withdraw its regulatory determination 

in the first instance, its argument that its authority to do so is 

not time-limited by the statutory deadlines for proposing and 

promulgating regulations is of no import.

EPA urges the Court to overlook the statute’s clear 

language, first by arguing that its 2011 determination to 
11

regulate was only a preliminary, interlocutory step in the 

regulatory process that did not bind the agency to issue future 

regulations. This argument contradicts the plain language of 

the statute and is easily disposed of. The Safe Drinking Water 

Act does anticipate that the agency will make a preliminary 

regulatory determination. See 42 U.S.C. 

§ 300g-1(b)(1)(B)(ii)(I) (“[T]he Administrator shall, after 

notice of the preliminary determination and opportunity for 

public comment, . . . make determinations of whether or not to 

regulate such contaminants.”). But the preliminary 

determination precedes the notice and comment period. Once 

that period ends, the agency makes its regulatory 

determination, and that determination is final. See id.

§ 300g-1(b)(1)(E). EPA issued its preliminary determination 

in 2008 and then, after the period for public comments, issued 

its final determination in 2011. Compare 73 Fed. Reg. at 

60,263 (presenting “EPA’s preliminary regulatory 

determination on perchlorate”), with 76 Fed. Reg. at 7,762 

(presenting “EPA’s final determination to regulate 

perchlorate”). 

EPA cites several other provisions of the statute that it 

argues implicitly give the agency the authority to withdraw a 

regulatory determination. None of these provisions, however, 

negate the statute’s clear directive that the agency “shall” 

propose and promulgate regulations after making a regulatory 

determination. The first provision EPA cites requires the 

agency to consider the “best available public health 

information” when evaluating whether the statutory criteria for 

regulating are met, id. § 300g-1(b)(1)(B)(ii)(II), while a similar

provision requires it to apply “the best available, peer-reviewed 

science” to all decisions it makes that are based on science, id.

§ 300g-1(b)(3)(A)(i). EPA argues, and Intervenor agrees, the 

statute required the agency to use its new model, which it 

considers to be the “best available public health information,” 
12

to re-evaluate whether the statutory criteria for regulating were 

satisfied. Because it did so and concluded that two of the 

criteria were no longer met—specifically, that “perchlorate 

does not occur at a frequency and at levels of public health 

concern, and that regulation of perchlorate does not present a 

meaningful opportunity for health risk reduction”—EPA and 

Intervenor argue the agency lacked the authority to regulate 

and had to withdraw its regulatory determination. 85 Fed. Reg.

at 43,998. 

This argument rests on a faulty dichotomy: that EPA must 

either disregard the statutory requirements and issue the 

regulations, or that it can adhere to the statutory requirements 

but is then required to withdraw its regulatory determination. 

Instead, the statute compels a third option. EPA must apply the 

“best available public health information” to determine 

whether the statutory criteria for regulating are satisfied. Id.

§ 300g-1(b)(1)(B)(ii)(II). Once that determination is made, it 

is final. EPA’s obligation then is to consider and apply the 

“best available, peer-reviewed science,” including any new 

developments, to set the substance of the regulations—not to 

reevaluate whether to regulate. See id. § 300g-1(b)(3)(A)(i). 

EPA claims that this reading of the statute will 

“hamstr[i]ng” its decision-making and result in outdated, 

scientifically unsupported regulations. 85 Fed. Reg. at 43,992.

But this takes an “all-or-nothing” approach and ignores the 

statutory requirements. If the science changes after the agency 

makes its determination to regulate but before it issues the 

regulations, EPA can—and must—account for those changes

when setting the appropriate regulatory level. 

Undeterred, EPA argues that several other provisions

militate against our understanding that “shall” means “shall.”

One such provision, known as the “anti-backslide” provision,
13

requires EPA to review regulations at least every six years and 

only permits revisions to those regulations that will “maintain, 

or provide for greater, [health] protection.” Id. § 300g-1(b)(9).

In EPA’s view, because this provision applies only to existing 

regulations, the agency is free to withdraw a regulatory 

determination at any time before it promulgates final 

regulations. EPA also argues that another provision that 

renders a determination not to regulate subject to judicial 

review, see id. § 300g-1(b)(1)(B)(ii)(IV), implicitly means that 

a determination to regulate is not subject to judicial review and 

can be withdrawn. Both arguments ignore the rigidity of the 

statute, which creates only two possible courses of action after 

the agency considers a contaminant for regulation: EPA can 

either determine not to regulate, or it can determine to regulate 

and then promulgate the regulations. If EPA takes the first 

approach, there are no regulations to which the anti-backslide 

provision can apply, and its determination is subject to judicial 

review. If EPA takes the second approach, it must promulgate 

regulations, to which the anti-backslide provision applies, and 

which are themselves subject to judicial review. EPA’s attempt

to create a third option, one in which there are no regulations 

to which the anti-backslide provision applies but whereby the 

agency still evades judicial review, is inconsistent with the 

statutory scheme. 

Having run out of provisions in the statute that it views as 

favorable, EPA flips the script and cites to the absence of a 

provision governing how the agency must withdraw a 

regulatory determination, arguing that because the agency did 

not “contravene any express statutory command” or “avoid any 

otherwise applicable statutory process” for withdrawing a 

determination to regulate, its withdrawal was permissible. 

Resp. Br. at 26–27 (internal citation omitted). But this just 

repackages the already rejected argument that the agency 

possesses an “inherent” authority to change its mind. Congress 
14

did not create a process for EPA to withdraw a regulatory 

determination because it seemingly did not want EPA to have 

the power to do so. “Regardless of how serious the [purported] 

problem an administrative agency seeks to address, . . . it may 

not exercise its authority ‘in a manner that is inconsistent with 

the administrative structure that Congress enacted into law.’” 

FDA v. Brown & Williamson Tobacco Corp., 529 U.S. 120, 

125 (2000) (quoting ETSI Pipeline Project v. Missouri, 484 

U.S. 495, 517 (1988)). Further, EPA’s withdrawal of its 

regulatory determination did contravene an express statutory 

command: the command that the agency “shall” regulate. 

EPA eventually abandons the statute altogether, turning 

instead to the Act’s legislative history. In its view, allowing 

the agency to withdraw a regulatory determination would be 

consistent with Congress’s intent in passing the 1996 

amendments, which it argues was to grant the agency 

additional discretion to decide when regulation is warranted 

and to eliminate wasteful spending on regulations without 

significant health benefits. EPA runs into two problems with 

this argument. First, and most fundamentally, EPA’s 

interpretation of what Congress intended in the statute cannot 

overcome the statute’s directive that the agency “shall” 

regulate. See Azar v. Allina Health Servs., 139 S. Ct. 1804, 

1815 (2019) (“[M]urky legislative history . . . can’t overcome 

a statute’s clear text and structure.”). And second, the history 

EPA cites is not inconsistent with our interpretation. The 1996 

amendments undoubtedly gave the agency more discretion in 

determining which contaminants to consider for regulation and 

whether to regulate them in the first instance. But they also 

balanced that discretion with a mandatory scheme requiring 

EPA to regulate after it determines to do so. 

Because the Safe Drinking Water Act requires that the 

agency “shall” regulate after making a regulatory 
15

determination, EPA lacks authority to withdraw that

determination and decide that it “shall not” regulate. 

b. Arbitrary and Capricious Challenges

NRDC also challenges EPA’s decision-making process 

that led to the withdrawal of its regulatory determination, 

arguing that EPA acted arbitrarily and capriciously by setting 

“levels of public health concern” that fail to protect against 

adverse health effects and by relying on selectively updated

data to assess the frequency of perchlorate contamination. 

Because we conclude that EPA lacked statutory authority to 

withdraw its regulatory determination, we do not reach these 

issues. See New York Stock Exch. LLC v. SEC, 962 F.3d 541, 

559 (D.C. Cir. 2020). Having been apprised of NRDC’s 

contentions and our concurring colleague’s views, EPA is, of 

course, free to take those into consideration when it develops 

perchlorate regulations. 

III. Conclusion

We grant NRDC’s petition, vacate EPA’s withdrawal of 

its determination to regulate, and remand to the agency for 

further proceedings. 
PAN, Circuit Judge, concurring in the judgment: In 2011, 

the EPA determined that perchlorate met the statutory factors 

for regulation under the Safe Drinking Water Act. For 

unexplained reasons, the agency missed the 24-month deadline 

to propose the required regulation and the 42-month deadline 

to promulgate it. Eight years passed. When the EPA finally 

commenced the notice-and-comment process to limit

perchlorate in drinking water, the agency proposed establishing 

a Maximum Contaminant Level Goal (“MCLG”) associated 

with a two-point drop in the average IQ of the most sensitive 

population. The agency also sought comments about 

alternative MCLGs associated with one- or three-point drops 

in the benchmark IQ. Moreover, the EPA requested comments 

about whether perchlorate should be regulated at all, “in light 

of new considerations . . . including information on lower 

levels of occurrence of perchlorate than the EPA had 

previously believed to exist and new analysis of the 

concentration [of perchlorate] that represents a level of health 

concern.” National Primary Drinking Water Regulations: 

Perchlorate, 84 Fed. Reg. 30,524, 30,525 (June 26, 2019). 

After the notice-and-comment period expired, the EPA decided 

that the statutory factors for regulation were not met, and 

therefore “withdr[e]w” its 2011 determination to regulate 

perchlorate, based on an updated understanding of the “best 

available public health information.” Drinking Water: Final 

Action on Perchlorate, 85 Fed. Reg. 43,990, 43,992 (July 21, 

2020). The agency’s withdrawal of its regulatory 

determination relied on a MCLG associated with a one-point 

drop in the average IQ of the most sensitive population, and on 

a partial update of the data that the agency used to measure the 

prevalence of perchlorate in the nation’s drinking water. 

In my view, under the circumstances presented, the EPA 

had authority to withdraw its initial regulatory determination

based on changes in the best available, peer-reviewed science. 

But the agency’s ultimate decision not to regulate perchlorate

was arbitrary, capricious, and not in accordance with law,
2

because it relied on a MCLG that did not meet the statutory 

standard, as well as on a biased dataset that was selectively 

updated. I would vacate and remand on those alternative 

grounds, and therefore respectfully concur in the judgment. 

I. Background

In 2011, the EPA published a final determination that 

perchlorate met the requirements for regulation under the Safe 

Drinking Water Act. See Drinking Water: Regulatory 

Determination on Perchlorate, 76 Fed. Reg. 7,762 (Feb. 11, 

2011). Specifically, it found that perchlorate (1) “may have an 

adverse effect on the health of persons”; (2) “is known to occur 

or there is a substantial likelihood that [perchlorate] will occur 

in public water systems with a frequency and at levels of public 

health concern”; and (3) “in the sole judgment of the 

Administrator[ of the EPA], regulation of [perchlorate] 

presents a meaningful opportunity for health risk reduction for 

persons served by public water systems.” Id. at 7,762–63; see 

also 42 U.S.C. § 300g-1(b)(1)(A)(i)–(iii). The statute required 

the agency to propose a MCLG and an accompanying 

regulation that would limit perchlorate in drinking water within 

24 months after making that regulatory determination — i.e., 

by February 11, 2013. See 42 U.S.C. § 300g-1(b)(E). 

But the EPA missed that deadline. In February of 2016, 

the Natural Resources Defense Council (“NRDC”) brought suit 

to compel the agency to issue the tardy perchlorate regulation. 

See Compl., NRDC v. EPA, No. 2:16-cv-1251 (S.D.N.Y. Feb. 

19, 2016), ECF No. 7. The parties entered a consent decree 

that required the EPA to propose the MCLG and accompanying 

regulation by May 28, 2019, and to finalize the MCLG and 

accompanying regulation by June 19, 2020. See Consent 

Decree and Extensions, NRDC v. EPA, No. 2:16-cv-1251 

(S.D.N.Y.), ECF No. 38, 57, 59, 60.
3

Pursuant to the consent decree, the EPA issued its proposal 

for limiting the amount of perchlorate in drinking water in 

2019. See 84 Fed. Reg. at 30,565. The agency proposed setting 

both the MCLG and Maximum Contaminant Level (“MCL”)

at 56 micrograms per liter (μg/L), a level associated with “a 2 

percent decrease in IQ” in the most sensitive population. Id. at 

30,540. The proposal also requested public comments on three 

alternatives: (1) setting the MCLG and MCL at 18 μg/L, a 

level associated with a one-point drop in IQ; (2) setting the

MCLG and MCL at 90 μg/L, a level associated with a threepoint drop in IQ; or (3) withdrawing the agency’s 2011 

determination to regulate perchlorate altogether, “based on new 

information that indicates that perchlorate does not occur in 

public water systems with a frequency and at levels of public 

health concern and there may not be a meaningful opportunity 

for health risk reduction.” Id. at 30,524, 30,541. 

In 2020, the EPA withdrew the 2011 determination to 

regulate perchlorate. The agency based its decision on a new 

understanding of the scientific evidence regarding 

perchlorate’s prevalence in drinking water and its effects on 

human health. The agency “recognize[d] that the [Safe 

Drinking Water Act] does not include a provision explicitly 

authorizing withdrawal of a regulatory determination,” but 

concluded that “such authority is inherent in the authority to 

issue a regulatory determination . . . particularly given the 

requirement that such determination be based on the ‘best 

available public health information.’” 85 Fed. Reg. at 43,992. 

The EPA explained that “new data and analysis developed by 

the Agency as part of the 2019 proposal demonstrate that the 

occurrence and health effects information used as the basis for 

the 2011 determination no longer constitute ‘best available 

information,’ are no longer accurate, and no longer support the 

Agency’s prioritization of perchlorate for regulation.” Id.
4

According to the EPA, the best available scientific 

evidence had changed in two ways. First, the EPA refined its 

understanding of the concentrations at which perchlorate 

causes health problems. In 2011, the agency identified “levels 

of public health concern” that “range[d] from 1 μg/L to 47 

μg/L.” See 76 Fed. Reg. at 7,764. But the publication of 

various studies in the following years shed further light on the 

relationship between exposure to perchlorate and iodide 

deficiencies in pregnant women that cause “a variety of adverse 

neurodevelopmental outcomes” in their fetuses, including 

decreases in IQ. See 84 Fed. Reg. at 30,531; 85 Fed. Reg. at 

44,000 (identifying “the fetus of the iodide deficient pregnant 

mother” as the most sensitive population). Using a new study 

that the agency identified as “the most rigorous analysis 

available in the literature to date,” see 84 Fed. Reg. at 30,534, 

the EPA developed a new model that caused it to reconsider the 

levels at which perchlorate is detrimental to health. Based on 

that model, the agency increased the relevant “levels of public 

health concern” from between 1 and 47 μg/L to between 18 and 

90 μg/L. See 85 Fed. Reg. at 43,992.

Second, the EPA revised its data showing the prevalence 

of perchlorate in the nation’s water supply. The agency based 

its 2011 determination to regulate in part on the UCMR-1 

study, a nationwide survey of perchlorate occurrence in 

drinking water conducted between 2001 and 2005. See 76 Fed. 

Reg. at 7,764–65. After the 2001–2005 period of data 

collection for the UMCR-1 study, California and 

Massachusetts enacted enforceable state-level perchlorate 

drinking-water standards. See 85 Fed. Reg. at 43,995. The 

EPA used compliance-monitoring information from those 

states to update some of the UMCR-1 data points. Id. The 

updates for California, in particular, had the potential to 

disproportionately affect the national picture of perchlorate 

occurrence: “In the original UCMR 1 dataset . . . 320 of 540 
5

samples in which perchlorate was detected were in California.” 

EPA Br. 52; see also Env’t Prot. Agency, EPA 816-R-19-003, 

Perchlorate Occurrence and Monitoring Report App. D at D3 (May 2019). Based on the updated data, the EPA concluded 

that perchlorate did not occur in public water systems at the 

requisite levels of public health concern to justify regulation. 

See 85 Fed. Reg. at 43,992.

II. Authority to Withdraw a Regulatory 

Determination

Under the Safe Drinking Water Act, the EPA’s issuance of 

a regulatory determination triggers a duty to propose and 

promulgate an appropriate regulation. As my colleagues in the 

majority note, the statute imposes that duty by repeatedly using 

the word “shall.” See Maj. Op. 9–10. Specifically, the EPA 

Administrator (1) “shall, in accordance with the procedures 

established by this subsection, publish a [MCLG] and 

promulgate a[n accompanying] regulation for a contaminant . . 

. if the Administrator determines that” the statutory factors are 

met, 42 U.S.C. § 300g-1(b)(1)(A) (emphasis added); (2) “shall 

publish [MCLGs] and promulgate, by rule,” accompanying 

regulations “[f]or each contaminant that the Administrator 

determines to regulate,” id. § 300g-1(b)(1)(E) (emphasis 

added); (3) “shall propose” the MCLG and accompanying 

regulation within 24 months of the determination to regulate, 

id. (emphasis added); and (4) “shall publish” the MCLG and 

accompanying regulation within 18 months of the proposal 

(with a possible nine-month extension), id. (emphasis added).

Based on this language, the majority concludes that when 

the EPA initially determined to regulate perchlorate in 2011, 

the issuance of regulations became mandatory — full stop. See 

Maj. Op. 9–10. But in my view, the question here is not 

whether the existence of a regulatory determination gives rise 
6

to a duty to actually regulate. It undoubtedly does. Instead, the 

salient question is whether the agency may withdraw its 

determination to regulate based on changed circumstances,

thereby vitiating the agency’s obligation to proceed with 

regulation. It is the regulatory determination that kicks off all 

the statutory timelines and imposes on the agency a firm

obligation to regulate; if that determination is withdrawn, those 

attendant requirements are no longer operative. 

The withdrawal of a determination to regulate in this 

context appears to be unprecedented. This may be explained 

by the 24-month statutory deadline to propose a regulation: A

significant change in the underlying science is unlikely to occur 

in that relatively short timeframe. But here, eight years passed 

after the EPA issued its initial determination. At that point,

after undergoing a notice-and-comment procedure, the agency 

made a new determination based on updated information. The 

issue before us is not whether the EPA should have violated the 

Safe Drinking Water Act’s statutory deadline — it should not 

have. But, now that it has, we must consider whether the EPA

has the power to withdraw a regulatory determination when 

changed circumstances justify such a withdrawal. In my view, 

the agency surely has that authority.

Contrary to my colleagues’ view, nothing in the Safe 

Drinking Water Act forbids the EPA from withdrawing a 

determination to regulate. The statute is silent on that issue. 

But reading such a prohibition into the Safe Drinking Water 

Act would force the EPA to violate another statutory provision. 

Specifically, the statute obligates the EPA to use “the best 

available, peer-reviewed science and supporting studies 

conducted in accordance with sound and objective scientific 

practices” in every action under the Safe Drinking Water Act

that “is based on science.” 42 U.S.C. § 300g-1(b)(3)(A)(i). If

new information comes to light before the EPA proposes a 
7

MCLG and accompanying regulation, and the “best available,

peer-reviewed science” makes clear that the initial regulatory 

determination is no longer supported by the evidence, then 

proceeding to regulate despite that new evidence would violate 

this provision. We obviously should not adopt an interpretation 

of the statute that discounts or ignores the EPA’s duty to rely 

on the best available science. See Loughrin v. United States, 

573 U.S. 351, 358 (2014) (explaining the “‘cardinal principle’ 

of interpretation that courts ‘must give effect, if possible, to 

every clause and word of a statute’” (quoting Williams v. 

Taylor, 529 U.S. 362, 404 (2000))); Del. Dep’t of Nat. Res. & 

Env’t Control v. EPA, 895 F.3d 90, 99 (D.C. Cir. 2018) (“[W]e 

strive to construe statutes ‘so that effect is given to all its 

provisions, so that no part will be inoperative or superfluous, 

void or insignificant.’” (quoting Corley v. United States, 556 

U.S. 303, 314 (2009))).

The majority asserts that the agency can reconcile these 

statutory mandates by “apply[ing] the ‘best available, peerreviewed science,’ including any new developments, to set the 

substance of the regulations — not to reevaluate whether to 

regulate.” Maj. Op. 12; see also NRDC Br. 34–35 (making a 

similar argument). But that fails to account for circumstances 

where, as here, the agency concludes that the best available, 

peer-reviewed science does not support regulating a 

contaminant at all. Under the majority’s approach, the agency 

is forced to regulate anyway. The regulation of a contaminant 

entails setting a MCLG and a MCL, which in turn triggers

potentially costly testing requirements. See 42 U.S.C. 

§ 300g-3(c)(1)(A)(i) (requiring public water systems to notify 

customers of “any failure” to “comply with an applicable 

maximum contaminant level”); 40 C.F.R. § 141.23 (requiring 

water systems to “conduct monitoring to determine compliance 

with . . . maximum contaminant levels”). The majority’s 

interpretation gives the agency no choice but to impose a 
8

pointless burden on water systems to test for a substance that

the agency does not even think should be regulated. Congress 

could not have intended “such an illogical result.” Tri-State 

Hosp. Supply Corp. v. United States, 341 F.3d 571, 577 (D.C. 

Cir. 2003); see also Mova Pharm. Corp. v. Shalala, 140 F.3d 

1060, 1068 (D.C. Cir. 1998) (highlighting “the long-standing 

rule that a statute should not be construed to produce an absurd

result,” i.e., a “result [that] is contrary to common sense” and 

“inconsistent with the clear intentions of the statute’s 

drafters”). 

For its part, NRDC contends that the “best available, peerreviewed science” provision requires the EPA to consider only 

the best evidence that was available “at the time” of the original 

determination, i.e., in 2011. See NRDC Br. 35–36. Citing our 

decision in Chlorine Chemistry Council v. EPA, NRDC relies 

on our statement “that the action [should] be taken on the basis 

of the best available evidence at the time of the rulemaking.” 

206 F.3d 1286, 1291 (D.C. Cir. 2000) (emphasis in original). 

But Chlorine Chemistry Council involved very different facts. 

In that case, the EPA refused to establish a MCLG supported 

by the best available evidence “because of the possibility of 

contradiction in the future by evidence unavailable at the time 

of the action.” Id. at 1290–91. In this case, the EPA is not 

arguing that the currently available evidence might be 

contradicted in the future, but that the currently available 

evidence does contradict the agency’s past understanding of the 

science. Moreover, although the EPA made a regulatory 

determination in 2011, the “time of the rulemaking” in this case 

was in 2019, when the EPA sought comments about its 

proposed MCLG and accompanying regulation. See 5 U.S.C. 

§ 551(5) (“‘[R]ule making’ means agency process for 

formulating, amending, or repealing a rule[.]”). If the best 

available evidence at that later time revealed that the statutory 

prerequisites for regulation were not met, then the agency’s 
9

only way forward was to withdraw its earlier decision to the 

contrary. Otherwise, the EPA would be simultaneously 

forbidden yet compelled to rely on “the best available, peerreviewed science.” 42 U.S.C. § 300g-1(b)(3)(A)(i).

Notably, Congress clearly knew how to limit the agency’s 

ability to change its mind and chose to do so only later in the 

Safe Drinking Water Act’s regulatory process. Specifically, 

the statute’s anti-backsliding provision applies after 

promulgation of the MCLG and accompanying regulation. See 

id. § 300g-1(b)(9). It mandates that “[a]ny revision of a 

national primary drinking water regulation . . . shall maintain, 

or provide for greater, protection of the health of persons.” Id. 

(emphasis added). That provision is inoperative here because 

the EPA made (and then withdrew) only a determination to 

regulate. Nevertheless, Congress’s enactment of a specific 

limitation on the EPA’s ability to revisit and alter drinkingwater regulations under the Safe Drinking Water Act highlights 

the absence of any explicit limitation on changing regulatory 

determinations in this context. The statutory text thus strongly 

suggests that there is no implicit constraint on the agency’s 

ability to reconsider a regulatory determination. See Jama v. 

Immigr. & Customs Enf’t, 543 U.S. 335, 341 (2005) (“We do 

not lightly assume that Congress has omitted from its text 

requirements that it nonetheless intends to apply, and our 

reluctance is even greater when Congress has shown elsewhere

in the same statute that it knows how to make such a 

requirement manifest.”).

My conclusion that the EPA may withdraw a regulatory 

determination is consistent with the ordinary rule that agencies

may “use the same procedures when they amend or repeal a 

rule as they used to issue the rule in the first instance.” Perez 

v. Mortg. Bankers Ass’n, 575 U.S. 92, 101 (2015) (citing FCC 

v. Fox Television Stations, Inc., 556 U.S. 502, 515 (2009)); 
10

Hickman & Pierce, Administrative Law Treatise § 4.5.1 (6th 

ed. 2019) (“In the ordinary course, legislative rules must be 

promulgated using notice and comment procedures and can 

only be modified or replaced using notice and comment 

procedures.”). Indeed, it is a core principle of administrative 

law “that an agency must be given ample latitude to ‘adapt their 

rules and policies to the demands of changing circumstances.’” 

See Motor Vehicles Ass’n of U.S., Inc. v. State Farm Mut. Auto. 

Ins. Co., 463 U.S. 29, 42 (1983) (citation omitted)). If 

Congress had intended to depart from these well-established 

principles in the present context, it would have spoken to that 

issue directly. Cf. Ivy Sports Med., LLC v. Burwell, 767 F.3d 

81, 86 (D.C. Cir. 2014). As it did not, the EPA changed its 

mind in the way that agencies routinely do: It made its 2011 

determination to regulate after notice and comment; then, eight 

years later, it went through another round of notice and 

comment before deciding to change course.

1

Finally, it bears mention that if, after “employing 

traditional tools of statutory construction,” the Safe Drinking 

Water Act remained ambiguous about whether the EPA can

withdraw a regulatory determination, the agency’s 

interpretation ordinarily would be entitled to deference. See 

1 This case does not fall within the exception that applies when 

Congress, by providing an alternative statutory mechanism to correct 

mistakes, restricts the means through which an agency can change 

course. See, e.g., Ivy Sports, 767 F.3d at 86 (holding that agency 

lacked authority to reconsider prior decision where Congress 

“creat[ed] . . . a specific statutory mechanism to correct alleged . . . 

errors”); New Jersey v. EPA, 517 F.3d 574, 582–83 (D.C. Cir. 2008) 

(similar); Am. Methyl Corp. v. EPA, 749 F.2d 826, 835 (D.C. Cir. 

1984) (similar). To the contrary, denying the EPA the ability to 

withdraw its regulatory determination under the present 

circumstances leaves the agency with no mechanism at all to alter 

what it later concluded was an incorrect decision.
11

Chevron, U.S.A., Inc. v. NRDC, 467 U.S. 837, 843 n.9 (1984). 

But because the EPA did not cite Chevron in its brief and 

avoided relying on it at oral argument, see Oral Arg. Tr. 27:15–

18, I decline to consider the applicability of Chevron here. See 

HollyFrontier Cheyenne Refin., LLC v. Renewable Fuels 

Ass’n, 141 S. Ct. 2172, 2180 (2021) (“[T]he government does 

not [invoke Chevron.] . . . We therefore decline to consider 

whether any deference might be due its regulation.”).

In short, under the circumstances presented, it was

permissible for the EPA to reconsider and withdraw a

determination to regulate a contaminant under the Safe 

Drinking Water Act. The agency had not yet proposed and 

promulgated a final regulation when it made a new finding that 

the best available, peer-reviewed science no longer supported 

its prior regulatory determination. In my view, the EPA may 

appropriately reverse a decision to regulate based on a change 

in scientific evidence, after engaging in notice-and-comment 

procedures. 

III. Withdrawal of the 2011 Determination to 

Regulate Perchlorate

Although the EPA was empowered to reconsider its initial 

regulatory determination based on changes in the best 

available, peer-reviewed science, the agency’s ultimate 

decision not to regulate perchlorate in drinking water was 

arbitrary, capricious, and otherwise not in accordance with law. 

See 5 U.S.C. § 706(2)(A); see also Fox Television Stations, 556 

U.S. at 514–16; Nat’l Ass’n of Home Builders v. EPA, 682 F.3d 

1032, 1037–38 (D.C. Cir. 2012). An agency action is arbitrary 

or capricious “if the agency has relied on factors which 

Congress has not intended it to consider, entirely failed to 

consider an important aspect of the problem, offered an 

explanation for its decision that runs counter to the evidence 
12

before the agency, or is so implausible that it could not be 

ascribed to a difference in view or the product of agency 

expertise.” State Farm, 463 U.S. at 43. An agency action that 

“violates [a statute] is ‘not in accordance with law’ within the 

meaning of 5 U.S.C. § 706(2)(A).” Chrysler Corp. v. Brown, 

441 U.S. 281, 318 (1979); see also Ethyl Corp. v. EPA, 306 

F.3d 1144, 1150 (D.C. Cir. 2002) (holding that agency action 

that violated authorizing statute was “not in accordance with 

law”).

A. Proposed MCLGs

The Safe Drinking Water Act requires the EPA to “set [the 

MCLG] at the level at which no known or anticipated adverse 

effects on the health of persons occur and which allows an 

adequate margin of safety.” 42 U.S.C. § 300g-1(b)(4)(A) 

(emphasis added). But the EPA sought comments on MCLGs 

that permitted levels of perchlorate associated with one-, two-, 

and three-point decreases in the average IQ of the most 

sensitive population; and then used those MCLGs as the “levels 

of public health concern” by which it evaluated the need to 

limit perchlorate in drinking water. See 85 Fed. Reg. at 43,995 

(“[T]he EPA used these potential MCLGs as the levels of 

public health concern in assessing the frequency of occurrence 

of perchlorate in this regulatory determination.”).

2 The EPA 

2 The EPA characterizes its proposed MCLGs of 18 μg/L, 

56 μg/L, or 90 μg/L as the levels of perchlorate that “avoid” IQ 

decreases of one, two, or three points, respectively. See 85 Fed. Reg. 

at 43,994, 43,995, 43,999; EPA Br. 2, 11, 16, 33, 34, 39, 45. That 

characterization is at best confusing and at worst misleading. The 

proposed MCLGs are the levels of perchlorate associated with

decreases in IQ of one, two, or three points — not the levels at which 

those cognitive harms do not occur. See 84 Fed. Reg. at 30,536 

(explaining that the agency based its calculations on the daily dose 

of perchlorate “associated with a 1, 2, or 3 point decrease from the 
13

found that “even at the most stringent regulatory level 

considered in the 2019 proposal” — i.e., the level associated 

with a one-point drop in IQ — “perchlorate does not occur in 

public water systems ‘with a frequency and at levels of public 

health concern.’” Id. at 43,992.

The EPA’s proposed MCLGs plainly violated the statutory 

mandate to reflect “the level at which no known or anticipated 

adverse effects on the health of persons occur.” 42 U.S.C. 

§ 300g-1(b)(4)(A). A decrease in average IQ of even one point 

is undoubtedly an “adverse effect[] on the health of persons.” 

Id. Rather than debate that self-evident conclusion, the EPA 

chooses the path of obfuscation, essentially arguing that in this 

“complicated technical area,” the court must defer to the 

agency’s chosen approach to regulation. EPA Br. 34, 37. 

Specifically, the EPA says that we should defer to its reliance 

on the agency’s “‘Benchmark Dose Guidance,’ which 

supported using a 1% effect [on IQ] as a starting point.” Id. at 

34. That guidance focuses not on “what individual levels [of a 

contaminant] can be considered adverse” but instead on “the 

level of change in the endpoint [here, IQ] at which the effect is 

considered to become biologically significant (as determined 

by expert judgment or relevant guidance documents).” See 

Env’t Prot. Agency, Benchmark Dose Technical Guidance x

(June 2012). In other words, the agency relied on its own 

judgment about whether an adverse health effect is 

standardized mean IQ”). At oral argument, the EPA contended that 

setting MCLGs at these levels would “avoid” the relevant IQ 

decreases because regulating at those levels of perchlorate would 

prevent water from containing those levels of perchlorate. See Oral

Arg. Tr. 37:24–38:14, 39:7–9. But the truth appears to be the 

opposite: Setting a maximum of 18 μg/L would not avoid water 

having that much perchlorate, it would permit that level of 

perchlorate — a level that, according to the EPA’s own data, is 

associated with a one-point decline in IQ.
14

“biologically significant” instead of adhering to the statutory 

standard, which requires setting the MCLG “at the level at 

which [there are] no known or anticipated adverse effects on 

the health of persons.” 42 U.S.C. § 300g-1(b)(4)(A). Notably, 

the EPA used the Benchmark Dose Guidance to replace the 

“NOAEL/LOAEL [no or low observed adverse effect level] 

approach,” which had been “used for many years” and which

mirrors what the statute requires. See Benchmark Dose 

Technical Guidance viii. One need not be a scientist to 

understand that by rejecting the “no observed adverse effect 

level” approach, the EPA eschewed what the Safe Drinking 

Water Act demands. Compare id., with 42 U.S.C. 

§ 300g-1(b)(4)(A).

Beneath the technical jargon and puffery about agency 

expertise, the EPA is not really arguing that it complied with 

the statute. Instead, the agency appears to contend that the 

statute’s requirements are not the best way to go about making 

policy in this area, and that its own judgment should control. 

Of course, that position finds no support in the law. Congress 

directed the EPA to set MCLGs “at the level at which no known 

or anticipated adverse effects on the health of persons occur.” 

42 U.S.C. § 300g-1(b)(4)(A). The agency did not act in 

accordance with that law when it used a MCLG associated with 

a one-point drop in IQ — which plainly is an “adverse effect[] 

on the health of persons,” id. — as the basis for withdrawing 

its determination to regulate perchlorate. 

B. UCMR-1 Data

As previously noted, the EPA relied on data from the

UCMR-1 — a nationwide sampling of public water systems for 

perchlorate — when it made its determination to regulate in 

2011. See 76 Fed. Reg. at 7,764. The UCMR-1 survey 

detected “perchlorate at levels greater than or equal to 4 μg/L . 
15

. . [in] approximately 1.9 percent of the” samples collected 

during the 2001–2005 study period. Id. After the UCMR-1 

study, however, Massachusetts and California passed 

enforceable state-level standards on perchlorate in drinking 

water. As a result, statewide monitoring data of perchlorate 

have become available in those states. See 85 Fed. Reg. at 

43,995. 

When the EPA reconsidered its determination to regulate 

in 2019, it updated the UCMR-1 data to reflect some, but not 

all, of the newly available information from Massachusetts and 

California. Instead of replacing all the data points from 

Massachusetts and California in the UCMR-1 dataset, the 

agency updated only those samples where perchlorate was 

detected during the 2001–2005 data collection. See id. In other 

words, it revised any samples that were positive for perchlorate 

in the 2001–2005 data, but left untouched those samples that 

were negative for perchlorate. Thus, as NRDC argues, the 

“EPA set up a one-way ratchet: [C]ontaminated water could 

become clean, but clean water could not become 

contaminated.” NRDC Br. 62. If the purpose of adjusting the 

UCMR-1 data was to obtain the most current systematic picture 

of perchlorate occurrence, there was no apparent reason (and 

the agency proffers none) to selectively update the data from 

Massachusetts and California instead of using all the new data 

from those states. The EPA’s evident failure “to consider [that]

important aspect of the problem” was arbitrary and capricious. 

State Farm, 463 U.S. at 43. 

IV. Conclusion

For the foregoing reasons, I would hold that the EPA has 

authority to withdraw a determination to regulate a 

contaminant under the Safe Drinking Water Act, prior to the 

promulgation of a MCLG and accompanying regulation, when 
16

the best available science supports the agency’s conclusion that 

the required factors for regulation are no longer met. But in my 

view, the EPA’s 2020 decision not to regulate perchlorate was 

arbitrary, capricious, and contrary to law because it was based 

on a MCLG that did not comply with a statutory directive, and 

relied on selectively updated data concerning the prevalence of 

perchlorate in drinking water. On those grounds, I would 

vacate the agency’s withdrawal of its 2011 regulatory 

determination and remand for further proceedings. I therefore 

concur in the judgment.