GreenLatinos v. Suncor Energy (U.S.A.)

Opinion

Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 1 FILED United States Court of Appeals Tenth Circuit PUBLISH August 3, 2026 UNITED STATES COURT OF APPEALS Christopher M. Wolpert FOR THE TENTH CIRCUIT Clerk of Court _________________________________

GREENLATINOS; 350 COLORADO; SIERRA CLUB,

Plaintiffs - Appellants,

v. No. 25-1243

SUNCOR ENERGY (U.S.A.), INC.,

Defendant - Appellee. _________________________________

Appeal from the United States District Court for the District of Colorado (D.C. No. 1:24-CV-02164-DDD-SBP) _________________________________

Kirti Datla of Earthjustice, Washington, D.C. (Ian Coghill, Rachael Jaffe of Earthjustice, Denver, Colorado, with her on the briefs), for Plaintiffs- Appellants.

Hugh Q. Gottschalk of Wheeler Trigg O’Donnell LLP (Eric L. Robertson of Wheeler Trigg O’Donnell LLP; Carlos R. Romo, John H. Bernetich of Williams Weese Pepple & Ferguson PC, with him on the brief), Denver, Colorado, for Defendant-Appellee. _________________________________

Before BACHARACH, KELLY, and FEDERICO, Circuit Judges. _________________________________

FEDERICO, Circuit Judge. Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 2

Over twenty years ago, the Environmental Protection Agency entered

into consent decrees with the Valero Energy Corporation and Conoco Phillips

to enforce the Clean Air Act (CAA) with respect to petroleum refineries. Those

consent decrees covered refinery plants in Commerce City, Colorado that are

now owned and operated by Suncor Energy (U.S.A.), Inc. In spite of this

decades-old enforcement initiative by EPA, however, the Commerce City

refinery plants continue to emit air pollution. A group of environmental

organizations, on behalf of their members affected by the pollution, sued

Suncor Energy to enforce the CAA’s mandate.

When Congress created a right for citizens to sue for violations of “an

emission standard or limitation” under the CAA, 42 U.S.C. § 7604(a), it also

created an exception for cases where “the [EPA] Administrator or [a] State has

commenced and is diligently prosecuting a civil action in a court of the United

States or a State to require compliance with the standard, limitation, or order,”

id. at § 7604(b)(1)(B). If the exception applies, then citizen suits for the same

enforcement are barred.

The district court held that the environmental organizations’ suit fell

within this exception, known as the diligent prosecution bar, and granted

Suncor Energy’s Rule 12(b)(6) motion to dismiss. The district court’s analysis,

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however, departed from the text enacted by Congress, so exercising our

jurisdiction under 28 U.S.C. § 1291, we reverse.

I

Before we address the merits of this appeal, we will first examine the

factual and procedural history of the case. As usual, on review of a Rule 12(b)(6)

decision, we take “the bulk” of the relevant facts from the complaint itself.

Levorsen v. Octapharma Plasma, Inc., 828 F.3d 1227, 1229 n.2 (10th Cir. 2016).

But we also find relevant the statutory and regulatory circumstances that

underpin the parties’ dispute, and since these details are judicially noticeable

when deciding a Rule 12(b)(6) motion, e.g., United States v. Williams, 442 F.3d

1259, 1261 (10th Cir. 2006), we will attend to these facts as well. We therefore

begin our review of the background with an Act of Congress – the CAA.

A

In 1970, “to protect and enhance the quality of the Nation’s air resources

so as to promote the public health and welfare and the productive capacity of

its population,” Congress passed and President Richard Nixon signed the

amendments that now encompass the modern CAA. 42 U.S.C. § 7401(b)(1). See

generally Brigham Daniels, Andrew P. Follett, & Joshua Davis, The Making of

the Clean Air Act, 71 Hastings L.J. 901, 941–43 (2020). The CAA is scaffolded

by a complex set of authorities (and acronyms) for EPA to promulgate and

enforce emissions limitations.

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At its foundation lie the National Ambient Air Quality Standards

(NAAQS), which EPA sets for certain pollutants at a level that is “requisite to

protect the public health” and “public welfare.” 42 U.S.C. § 7409(b)(1), (2);

Whitman v. American Trucking Associations, 531 U.S. 457, 475–76 (2001). The

CAA then authorizes EPA to set New Source Performance Standards (NSPS)

that establish technology-based standards of performance to achieve emissions

limits. 42 U.S.C. § 7411. It also authorizes states to require regulated entities

to employ reasonably available control technology (RACT) for pollution control

as part of their state implementation plans. 42 U.S.C. § 7502(c).

Separate from the NAAQS, the CAA also regulates certain other,

hazardous air pollutants through EPA-set National Emissions Standards for

Hazardous Air Pollutants (NESHAPS). 42 U.S.C. § 7412(c)(2). For NESHAPS,

EPA establishes technology-based standards for achieving “the maximum

degree of reduction,” known as maximum achievable control technology

(MACT) standards. 42 U.S.C. § 7412(d)(2); see, e.g., 40 C.F.R. § 63.51. Together

with the NAAQS and RACT, this alphabet soup is the principal means by

which the CAA accomplishes its purpose.

To better understand how these laws operate in practice, consider a

regulation found in 40 C.F.R. § 63.670(e), which forms the basis for claim 21 of

the organizations’ complaint. That regulation establishes standards for the

operation of flares “used as a control device for an emission point.” 40 C.F.R.

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§ 63.670. Among other requirements, the flare must be operated “to maintain

the net heating value of flare combustion zone gas (NHVcz) at or above the

applicable limits,” which vary based on the type of flare technology used,

“determined on a 15-minute block period basis when regulated material is

routed to the flare for at least 15-minutes.” Id. at § 63.670(e). In other words,

to ensure that the flare is actually burning off unhealthy emissions, the flare

operator must keep the flare sufficiently hot. This is the MACT requirement

described above. See generally Mossville Environmental Action Now v. EPA,

370 F.3d 1232, 1235–36 (D.C. Cir. 2004) (describing MACT standards).

At the turn of the century, EPA began a programmatic effort to bring its

various authorities to bear on petroleum refineries across the country. See, e.g.,

Notice of Lodging of Proposed Consent Decree Under Clean Air Act, 77 Fed.

Reg. 25750, 25750 (Apr. 30, 2012) (“This is one of numerous national

settlements reached as part of the EPA’s Clean Air Act Petroleum Refinery

Initiative.”). According to EPA’s website, the petroleum refinery initiative

resulted in 37 settlements with companies covering over 95% of the United

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States’ petroleum refining capacity and 112 refineries. 1 This appeal concerns

two of those settlements.

In 2002, EPA entered into a consent decree with Conoco Inc. as part of

the agency’s petroleum refinery initiative. Doc. 29 in United States v. Conoco

Inc., No. 4:01-CV-4430 (S.D. Tex. Apr. 29, 2002); see also Notice of Lodging of

Consent Decree Under the Clean Air Act, 67 Fed. Reg. 3735 (Jan. 25, 2002).

The consent decree required Conoco to take a variety of anti-pollution

measures with respect to refineries the company owned and operated across

the nation, including one refinery just northwest of Denver in Commerce City.

App. VI at 8. Three years later, EPA entered into a similar consent decree with

Valero Refining Company, which owned and operated another nearby refinery.

Doc. 37 at 10, United States v. Valero Refining Co., No. 5:05-CV-569 (W.D. Tex.

Nov. 23, 2005); App. VII at 13; see also Notice of Public Comment Period for

Proposed Consent Decree Under the Clean Air Act, 70 Fed. Reg. 36410-01

(June 23, 2005).

1 Petroleum Refinery Nat’l Case Results, EPA, https://perma.cc/TH4Q-

P648. “The Court may take judicial notice of the contents of an agency’s website.” Buhendwa v. Regional Transportation District, 82 F. Supp. 3d 1259, 1262 n.1 (D. Colo. 2015) (citing Coleman v. Dretke, 409 F.3d 665, 667 (5th Cir. 2005)); New Mexico ex rel. Richardson v. Bureau of Land Mgmt., 565 F.3d 683, 703 n.22 (10th Cir. 2009).

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In addition to substantive anti-pollution measures, the consent decrees

also contained a variety of procedural features for their enforcement. For

example, the consent decrees were each entered with respect to a host of state

intervenors, so that enforcement responsibilities would be distributed across

federal and state environmental agencies. App. VI at 4 (listing Colorado,

Louisiana, Oklahoma, and Montana as state intervenors and parties to the

consent decree); App. VII at 4 (listing Colorado, Louisiana, New Jersey,

Oklahoma, and Texas as state intervenors and parties to the consent decree).

The consent decrees also included a variety of reporting requirements, both on

a general periodic basis and for specific incidents. See, e.g., App. VI at 57, 71–

72; App. VII at 97, 123–26. And the consent decrees set forth complex schemes

of stipulated penalties for violations of the consent decrees’ provisions (in

addition to the usual possibility of contempt for violating a court order), and a

procedure for federal and state agencies to impose the penalties. App. VI at

83–92; App. VII at 166–77. Finally, each consent decree confirmed the district

courts’ continuing jurisdiction over the case after the entry of the consent

decree.

In the years since the entry of the consent decrees, EPA and the Colorado

Department of Public Health and Environment have taken a variety of

administrative actions to enforce the anti-pollution objective embodied in the

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consent decrees with respect to the refineries located in Commerce City. 2 For

example, in March 2020, Colorado issued a “compliance order on consent” to

address violations of state and federal law, including the consent decrees. App.

VIII at 4–89. This administrative action imposed stipulated penalties pursuant

to the consent decrees. In June 2023, Colorado issued a “compliance advisory”

to again address alleged violations of state and federal law, including the

consent decrees. App. IX at 88–143. In February 2024, Colorado issued yet

another “compliance order on consent.” App. VIII at 91–163. Like the March

2020 compliance order, this one too imposed stipulated penalties pursuant to

the consent decrees. Later that year in June, EPA got involved and together

with Colorado jointly issued a “notice of violation” alleging violations of state

and federal law, including the consent decrees. App. IX at 4–86.

The acronym-filled discussion of regulations and administrative jargon

in this record should not obscure the material environmental impact that the

refineries in Commerce City were having (and presumably continue to have)

2 In 2003, Suncor purchased the refinery operated by Conoco. In 2005,

Suncor purchased the refinery operated by Valero. The refinery formerly operated by Conoco is sometimes referred to as Plants 1 and 3 and other times as the West Plant. The refinery formerly operated by Valero is sometimes referred to as Plant 2 and other times as the East Plant. For simplicity’s sake, we will refer to each as the West Plant and the East Plant respectively. After Suncor purchased the West and East Plants, the respective consent decrees were amended to include Suncor as a party bound to them.

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on the health and welfare of Coloradans. According to the complaint, which in

turn relied on Suncor’s own filings made pursuant to the consent decrees, the

refineries “emitted approximately 25 tons of hazardous air pollutants, 500 tons

of CO, 650 of NOx, 125 tons of PM, 450 tons of volatile organic compounds

(‘VOCs’), and 230 tons of SO2.” App. I at 26. Unsurprisingly, these emissions

have had a deleterious effect on the health of nearby residents, who have some

of the highest rates of asthma, cardiovascular disease, and diabetes in the

state. The refineries’ neighbors spend less time outdoors, limit the meals they

eat outside, and their daily activities are generally affected by the refineries’

belching odors and fumes. On all thirteen metrics that EPA uses to identify

disparate environmental effects on vulnerable populations, the area

surrounding the Commerce City refineries scored among the highest

percentiles in Colorado, ranking among the most heavily polluted regions in

the state.

B

To staunch this flow of emissions in Commerce City, several

environmental organizations filed suit in the District of Colorado in August

2024. On October 29 that same year, the environmental organizations filed the

first amended and operative complaint. The plaintiffs consisted of three

groups: GreenLatinos, “a national nonprofit organization that convenes a

broad coalition of Latino leaders committed to addressing environmental,

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natural resources, and conservation issues that significantly affect the health

and welfare of the Latino community”; Sierra Club, “a national nonprofit

organization” that “works to promote clean energy, safeguard the health of our

communities, protect wildlife, and preserve our remaining wild places through

grassroots activism, public education, lobbying, and legal action”; and 350

Colorado, “a nonprofit organization and grassroots movement working to build

a fossil-free future powered by 100% renewable energy.” App. I at 19–20. We

refer to the plaintiff organizations collectively as the “environmental

organizations” for the purposes of this opinion.

Together, the environmental organizations sought redress on behalf of

themselves and their members against Suncor Energy, the company that now

owns and operates the Commerce City refineries. They invoked what is known

as the “citizen suit” provision of the CAA, which allows “any person” to

“commence a civil action. . . against any person . . . who is alleged to have

violated (if there is evidence that the alleged violation has been repeated) or to

be in violation of [] an emission standard or limitation under” the Act. 42 U.S.C.

§ 7604(a)(1). The operative complaint made 28 claims for relief, each alleging

various violations of the CAA and its implementing regulations. The complaint

also alleged violations of the federal consent decrees previously mentioned.

To give some texture to the kind of claim that the environmental

organizations are making, consider another example. Claim 25 alleged that a

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heater unit known as H-2101 does not comply with the requirements of the

West Plant Consent Decree. The West Plant Consent Decree required the

refinery operator “to reduce the overall NOx emissions from Controlled

Heaters and Boilers at their refineries in an amount greater than or equal to

1,526 tons per year” by, among other things, installing technologies that would

“reduce NOx emissions to 0.040 lbs per mmBTU or lower.” App. VI at 21.

According to the complaint, however, H-2101 was technologically incapable of

meeting this emissions limit. So the complaint alleges that from the moment

H-2101 was installed in January 2006, Suncor Energy has been in unbroken

violation of the terms of the consent decree.

Suncor Energy filed a motion to dismiss under Rule 12(b)(1) for want of

jurisdiction and Rule 12(b)(6) for failure to state a claim upon which relief could

be granted. Fed. R. Civ. P. 12(b)(1), (6). Suncor argued first that the

environmental organizations could not demonstrate an Article III injury-in-

fact, so they lacked standing to pursue their claims. Suncor next argued that

the environmental organizations’ claims were jurisdictionally barred by 42

U.S.C. § 7604(b)(1)(B), which prevents private parties from suing under the

CAA if a government entity is “diligently prosecuting” CAA violations as well.

Suncor pointed to the consent decrees filed in the Texas district courts as the

basis for applying the diligent-prosecution bar, asserting that “[p]ermitting

Plaintiffs to pursue claims alleging the same violations of the Federal Court

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Consent Decrees would undercut the policies” underlying the diligent

prosecution bar, and so the bar applied to the environmental organizations’

citizen suit. App. I at 169.

The district court granted Suncor’s motion to dismiss, but only on Rule

12(b)(6) grounds. On standing, the district court concluded the environmental

organizations had standing to pursue their claims. 3 On diligent prosecution,

however, the district court rejected the environmental organizations’

protestations that the bar did not apply. “Taken together,” wrote the district

court, “the consent decrees, compliance orders, compliance advisories, and

notice of violation serve as ongoing enforcement actions by the EPA and

Colorado against Defendant for alleged violations of the Act. As a result,

Plaintiffs’ citizen suit, which consists of identical or substantially similar

allegations, is barred by statute.” App. X at 295.

The district court therefore dismissed the environmental organizations’

operative complaint and entered final judgment for Suncor on May 23, 2025.

This timely appeal followed.

3 Suncor does not ask that we revisit the environmental organizations’

standing. Our independent review of the record discloses no basis for disturbing the district court’s conclusion.

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II

Before we turn to the substance of this rather technical appeal, we will

first describe our standard of review. We will then return to the CAA itself and

introduce some basic concepts and the key pieces of the statutory text that

frame the parties’ dispute. Finally, we will address the principal issue on

appeal: whether the environmental organizations’ complaint is precluded by

the diligent prosecution bar. We conclude that the district court incorrectly

analyzed the diligent prosecution bar, and its judgment is therefore reversed.

A

“We review a district court’s grant of a motion to dismiss de novo.”

Hennessey v. University of Kansas Hospital Authority, 53 F.4th 516, 527 (10th

Cir. 2016). To survive Rule 12(b)(6) and in turn to obtain relief from a dismissal

on appeal, a “plaintiff must state a claim that is facially plausible; that is, the

plaintiff must ‘plead[ ] factual content that allows the court to draw the

reasonable inference that the defendant is liable for the misconduct alleged.’”

Messerli v. AW Distributing, Inc., 153 F.4th 1077, 1081 (10th Cir. 2025)

(alteration in original, quoting Ashcroft v. Iqbal, 556 U.S. 662, 678 (2009)).

Suncor Energy did not move for Rule 12(b)(6) dismissal on diligent-

prosecution bar grounds in the district court; instead, it argued that the bar

was jurisdictional and so the suit ought to be dismissed on Rule 12(b)(1)

grounds. The district court, however, disposed of the diligent prosecution

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bar under Rule 12(b)(6). Although the distinction rarely matters for

appellate purposes, Colorado Environmental Coalition v. Wenker, 353 F.3d

1221, 1227 (10th Cir. 2004), had the district court granted Suncor’s Rule

12(b)(1) motion, we would have reviewed the district court’s “findings of

jurisdictional fact for clear error.” Stuart v. Colorado Interstate Gas Co., 271

F.3d 1221, 1225 (10th Cir. 2001). The district court did not explain its

reasons for analyzing the Rule 12(b)(1) motion under Rule 12(b)(6).

Nonetheless, we think the district court was correct to do so: the

diligent prosecution bar is not jurisdictional. In the district court, Suncor

relied heavily on cases that applied the bar as jurisdictional with little or

no reasoning, “exactly the sort of ‘drive-by jurisdictional rulings’ the

[Supreme] Court tells us to view with a jaundiced eye.” Hobby Lobby Stores,

Inc. v. Sebelius, 723 F.3d 1114, 1159 (10th Cir. 2013) (en banc) (Gorsuch, J.,

concurring). The latest of those cases, Karr v. Hefner, predated the Supreme

Court’s more recent drum-beat that only a clear statement by Congress can

suffice to render a requirement jurisdictional. Compare 475 F.3d 1192,

1197–98 (10th Cir. 2007) (omitting any discussion of whether the diligent

prosecution bar is jurisdictional), with United States v. Wong, 575 U.S. 402,

409–10 (2015) (noting “procedural rules, including time bars, cabin a court’s

power only if Congress has ‘clearly state[d]’ as much” (quoting Sebelius v.

Auburn Reg’l Med. Ctr., 568 U.S. 145, 153 (2013)) (alteration in original)).

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And we find little in the text of the diligent prosecution provision to satisfy

this “high bar.” Wong, 575 U.S. at 409. The provision does not itself use the

term “jurisdiction,” and indeed, another provision in the same statutory

section does. Compare 42 U.S.C. § 7604(a), with id. at § 7604(b).

Nearly all our sister circuits, so far as we can tell, agree: the diligent

prosecution bar is non-jurisdictional. See, e.g., Group Against Smog &

Pollution, Inc. (GASP) v. Shenango Inc., 810 F.3d 116, 122–26 (3d Cir. 2016)

(holding that the bar is non-jurisdictional even though parties did not raise

the issue); Cebollero-Bertran v. Puerto Rico Aqueduct & Sewer Authority, 4

F.4th 63, 70–72 (1st Cir. 2021) (construing similar language in the Clean

Water Act, 33 U.S.C. § 1365 (CWA)); South River Watershed Alliance, Inc.

v. Dekalb County, 69 F.4th 809, 821–23 (11th Cir. 2023) (CWA); Naturaland

Trust v. Dakota Finance LLC, 41 F.4th 342, 346–48 (4th Cir. 2022) (holding

prior cases construing CWA’s diligent prosecution bar as jurisdictional were

abrogated by “Supreme Court’s current approach to such matters”); Adkins

v. VIM Recycling, Inc., 644 F.3d 483, 491–92 (7th Cir. 2011) (construing

similar language in the Resource Conservation and Recovery Act, 42 U.S.C.

§ 6972(b)(1)(B) (RCRA)).

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“The distinction matters. Jurisdictional requirements cannot be

waived or forfeited, must be raised by courts sua sponte, and . . . do not

allow for equitable exceptions.” Boechler, P.C. v. Commissioner of Internal

Revenue, 596 U.S. 199, 203 (2022); see also Adkins, 644 F.3d at 491 (“The

distinction is vital.”). With respect to the diligent prosecution bar, the

distinction affects which party bears the burden of asserting it and

consequently whether a party’s failure to timely invoke the bar can be held

to have waived it. With these thoughts, we proceed to apply the standard of

review applicable to Rule 12(b)(6) decisions.

B

We turn now to address the historical background that produced the

CAA statutory text. Although the policy debates that animated the CAA era do

not alter our legal analysis, they do appropriately contextualize it.

Historically, people affected by pollution, like the neighbors of the

Commerce City refinery, might have relied on the common law system of torts

to deter pollution and obtain redress for pollution-related injuries. Mark

Latham, Victor E. Schwartz, & Christopher E. Appel, The Intersection of Tort

and Environmental Law: Where the Twains Should Meet and Depart, 80

Fordham L. Rev. 737, 772 (2011). But over time, it became apparent that the

individualized and causation-bound nature of tort liability could not fully

satisfy the pressing environmental needs of the day. See, e.g., Palma J. Strand,

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Note, The Inapplicability of Traditional Tort Analysis to Environmental Risks:

The Example of Toxic Waste Pollution Victim Compensation, 35 Stan. L. Rev.

575, 577–80, 583–84 (1983). In its place arose a robust statutory regime for

regulatory enforcement, giving administrative agencies authority to prevent,

deter, and remedy environmental harm. Latham, Schwartz, & Appel, The

Intersection of Tort and Environmental Law, at 743–45 (identifying “a series

of modern federal environmental statutes” as ushering in “the new field of

environmental law”). The CAA was one of the first statutes enacted as part of

this experiment with public environmental law. Daniels, Follett, & Davis, The

Making of the Clean Air Act, at 903–04.

Congress did not entirely displace the role of private persons in

vindicating its environmental objectives. The CAA provides that “any person

may commence a civil action on his own behalf [] against any person . . . who

is alleged to have violated (if there is evidence that the alleged violation has

been repeated) or to be in violation of (A) an emission standard or limitation

under this chapter or (B) an order issued by the Administrator or a State with

respect to such a standard or limitation.” 42 U.S.C. § 7604(a)(1). This clause is

known as the citizen suit provision. Gwaltney of Smithfield, Ltd. v. Chesapeake

Bay Foundation, Inc., 484 U.S. 49, 58–59 (1987) (describing similar provision

in CWA). Although compensatory damages are generally understood to not be

available under citizen suit provisions, Mulcahey v. Columbia Organic

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Chemicals Co., Inc., 29 F.3d 148, 152 (4th Cir. 1994), plaintiffs can seek

equitable relief, Gwaltney, 484 U.S. at 61–62, and extract civil penalties to

deter pollution, Friends of the Earth, Inc. v. Laidlaw Environmental Services

(TOC), Inc., 528 U.S. 167, 185 (2000).

The citizen suit provision has proven structurally vital for the greater

statutory scheme. Accord Daniels, Follett, & Davis, The Making of the Clean

Air Act, at 930–31. Its most obvious contribution to environmental law has

been as a reserve guarantor of congressional objectives if the executive branch

is unable or unwilling to advance them. Id.; Environmental Conservation

Organization v. City of Dallas, 529 F.3d 519, 526 (5th Cir. 2008); Adkins, 644

F.3d at 499. Relatedly, the specter of citizen suits also serves as a “spur” to

government action. Black Warrior Riverkeeper, Inc. v. Cherokee Mining, LLC,

548 F.3d 986, 992 (11th Cir. 2008) (quoting S. Rep. No. 99-50, 28 (1985)). And

since Congress also required putative plaintiffs to provide EPA with notice of

their anticipated citizen suit 60 days in advance of filing, 42 U.S.C.

§ 7604(b)(1)(A), the citizen suit provision incentivizes private plaintiffs to

investigate CAA violations and then relay that information to EPA –

complementing and assisting the government’s investigatory responsibility.

So, although in form it bears little resemblance to its tort ancestors, in function

the citizen suit builds on a long history of private impulses advancing public

goods.

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Congress also acted, however, to preserve the primacy of government

agencies in environmental law enforcement. Citizen suits are barred under the

CAA “if the [EPA] Administrator or [a] State has commenced and is diligently

prosecuting a civil action in a court of the United States or a State to require

compliance with the standard, limitation, or order, but in any such action in a

court of the United States any person may intervene as a matter of right.” 42

U.S.C. § 7604(b)(1)(B). As already referenced above, this clause is known as

the diligent prosecution bar. GASP, 810 F.3d at 121 n.4. “The bar on citizen

suits when governmental enforcement action is under way suggests that the

citizen suit is meant to supplement rather than to supplant governmental

action.” Gwaltney, 484 U.S. at 60. In other words, although private parties can

play a “supplementary” and “interstitial” role in the environmental legal

universe, administrative agencies retain primacy within that ecosystem. Id. at

60–61.

The diligent prosecution bar shares some distinguishing characteristics

with other preclusive doctrines of the common law. Just as issue preclusion

protects parties “from the expense and vexation attending multiple lawsuits,

conserves judicial resources, and fosters reliance on judicial action by

minimizing the possibility of inconsistent decisions,” Montana v. United States,

440 U.S. 147, 153–54 (1979), so too does the diligent prosecution bar protect

regulated entities from duplicative, expensive, and perhaps inconsistent

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lawsuits. North and South Rivers Watershed Ass’n, Inc. v. Town of Scituate,

949 F.2d 552, 556 (1st Cir. 1991); Friends of Milwaukee’s Rivers v. Milwaukee

Metropolitan Sewerage Dist., 382 F.3d 743, 762–63 (7th Cir. 2004) (likening

diligent prosecution bar to res judicata). And in doing so, the bar incentivizes

regulated entities to cooperate with administrative agencies seeking to enforce

the environmental laws. But as with other preclusive doctrines, tension can

arise and subsist between the principle of access to justice and the principle of

repose. Cf. Montana, 440 U.S. at 164 n.11.

This appeal brings the friction between citizen suits and the diligent

prosecution bar into sharp focus. On the one hand, the environmental

organizations emphasize the care with which Congress balanced the diligent

prosecution bar against the citizen suit provision in the statutory text. On their

theory of the bar, the text sets forth three steps, none of which are satisfied

here, for deciding whether the bar precludes a citizen suit. First, the court must

identify a present judicial action, which the environmental organizations argue

would not include the Texas consent decrees or any subsequent administrative

actions. Second, the court must ensure that the standards and limitations

raised in the potentially preclusive judicial action are the same as the ones in

the citizen suit, which the environmental organizations also argue would

exclude the Texas consent decrees and subsequent administrative actions.

Only after these two steps have been satisfied should the district court consider

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whether the judicial action is being diligently prosecuted. See also South River

Watershed Alliance, 69 F.4th at 824 (describing these elements as “steps,” and

separating diligent prosecution from antecedent questions).

On the other hand, Suncor Energy argues that this lawsuit is “a

duplicative and unnecessary action to enforce alleged violations of the same

emission standards currently being enforced by the federal government and

the State of Colorado.” Resp. Br. at 11. Suncor primarily invokes the “clear

Congressional intent that citizen suits should not ‘intrude’ on EPA’s and

Colorado’s ‘primary enforcement authority.’” Resp. Br. at 28 (quoting Karr, 475

F.3d at 1197).

We largely agree with the environmental organizations. “The issues are

complicated – so for the details, read on.” Haaland v. Brackeen, 599 U.S. 255,

264 (2023). But in a nutshell, we latch onto the environmental organizations’

text-based theory of the statute over Suncor Energy’s purpose-driven

approach. The purpose-driven approach led the district court to apply a

diligent prosecution bar that Congress did not enact in the CAA. We therefore

reverse the judgment and remand for further proceedings.

C

We now address the core substance of this appeal – the meaning of the

diligent prosecution bar in the CAA. As foreshadowed, we agree with the

environmental organizations that the diligent prosecution bar sets forth

21 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 22

distinct elements (or “steps,” as some courts frame them). South River

Watershed Alliance, 69 F.4th at 823–24; Ohio Valley Environmental Coalition,

Inc. v. Maple Coal Co., 808 F. Supp. 2d 868, 883 (S.D. W. Va. 2011); Connecticut

Fund for Environment v. Contract Plating Co., Inc., 631 F. Supp. 1291, 1293

(D. Conn. 1986). We take each step in turn.

The environmental organizations argue first that the diligent

prosecution bar can be triggered only by a civil judicial action that is presently

being enforced. We agree, but we also reject the environmental organizations’

argument that consent decrees do not satisfy this requirement.

The text of the statute would appear to admit of little else. The diligent

prosecution bar is triggered when EPA or a state “has commenced and is

diligently prosecuting a civil action in a court.” 42 U.S.C. § 7604(b)(1)(B). This

court and the Supreme Court have repeatedly stressed that “Congress’ use of

a verb tense is significant in construing statutes.” United States v. Wilson, 503

U.S. 329, 333 (1992); see also Carr v. United States, 560 U.S. 438, 448 (2010);

United States v. Husted, 545 F.3d 1240, 1243–44 (10th Cir. 2008). Congress’

choice of verb tense here unambiguously refers to something that is current or

present, even if it began at some point in the past. The term “has commenced”

is phrased in the present perfect tense, see 42 U.S.C. § 7604(b)(1)(B), which

usually “conveys to a listener that the event in question continues to be true or

22 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 23

valid.” Hewitt v. United States, 606 U.S. 419, 429 (2025). And that phrase is

followed by the present tense “is.” See 42 U.S.C. § 7604(b)(1)(B). Congress was

thus focused on actions that are presently being prosecuted. Indeed, the

Supreme Court reached a similar conclusion when construing the Clean Water

Act’s citizen suit provision as not reaching wholly past actions. Gwaltney, 484

U.S. at 59 (“One of the most striking indicia of the prospective orientation of

the citizen suit is the pervasive use of the present tense throughout § 505.”).

Turning from verbs to objects, the phrase “civil action in a court of the

United States or a State” is likewise unambiguous. 42 U.S.C. § 7604(b)(1)(B).

The term “civil action” refers to “a proceeding in a judicial court, not an

administrative” forum. Western Watersheds Project v. U.S. Dept. of the Interior,

677 F.3d 922, 926 (9th Cir. 2012); see also Patham Const. Co. v. Knox County

Hospital Ass’n, 326 N.E.2d 844, 854 (Ind. Ct. App. 1975) (“The term ‘action’ in

its usual sense, at least its usual legal sense, means a suit brought in court, a

formal complaint within the jurisdiction of a court of law.”). The word “court”

also usually excludes administrative forums. Baughman v. Bradford Coal Co.,

Inc., 592 F.2d 215, 217 (3d Cir. 1979) (“Generally, the word ‘court’ in a statute

is held to refer only to the tribunals of the judiciary and not to those of an

executive agency with quasi-judicial powers.”). Also, the statute expressly

provides for otherwise-barred citizen suit plaintiffs to intervene in the pending

government-initiated action – a mechanism which does not exist, so far as we

23 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 24

know, in administrative proceedings. Student Public Interest Research Group

of N.J., Inc. v. Fritzche, Dodge & Olcott, Inc., 759 F.2d 1131, 1138–39 (3d Cir.

1985). 4

Putting these phrases together, only one thing can trigger the diligent

prosecution bar: an action in a judicial forum that is presently being enforced.

Other tools that are related to that judicial action (administrative actions, for

example) may well be relevant to determining whether that presently enforced

judicial action is being diligently prosecuted. But only the standards and

limitations found in that presently enforced judicial action can preclude a

citizen suit under the diligent prosecution bar.

Our conclusion is buttressed by the fact that other diligent prosecution

bars found in other environmental statutes enumerate administrative actions

explicitly as a basis for preclusion. 33 U.S.C. § 1319(g)(6) (CWA); 42 U.S.C.

§ 6972(b)(2)(B)(iv) (RCRA); 42 U.S.C. § 11046(e) (Emergency Planning &

4 Some room for uncertainty can be found on the outer boundaries of

the phrase “civil action,” and particularly with respect to state-level bodies. See, e.g., Baughman v. Bradford Coal Co., Inc., 592 F.2d 215 (3d Cir. 1979) (deciding whether Pennsylvania Environmental Hearing Board is a court for the purposes of § 7604(b)(1)(B)). We have no occasion in this case to reach those outer boundaries, however, for no one argues that EPA agency proceedings, or those of the Colorado Department of Public Health and Environment, are court proceedings.

24 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 25

Community Right-to-Know Act). 5 The omission of similar language in the CAA

– arguably the foundational environmental statute – speaks volumes. Applying

this judicial-action-only standard, the only actions or activities in the record

that would trigger the bar in this case are the two Texas consent decrees.

Suncor does not really dispute any of this on appeal. But its position with

respect to this first element or step of the diligent prosecution bar is hard to

pin down. At times Suncor appears to acknowledge that the relevant question

is whether the consent decrees, not the later administrative actions, are being

diligently enforced. But Suncor’s position shifts slightly when it argues that

“the Court must consider any agency enforcement after entry of the Consent

Decrees – not just the standards enforced in the Consent Decrees themselves

– to determine the extent of overlap between allegations in enforcement

proceedings and in a citizen suit.” Resp Br. at 49. In other words, Suncor asks

us to consider the standards and limitations being enforced in administrative

actions, rather than simply the actions pending in court. This argument or rule

does not reflect the text Congress enacted. The CAA does not ask whether EPA

5 The Comprehensive Environmental Response, Compensation, & Liability Act (CERCLA) does not explicitly refer to administrative actions – instead, it refers only to “an action.” 42 U.S.C. § 9659(d)(2). We attribute significance to Congress’ adornment of the CAA’s diligent prosecution bar with the adjective “civil” before the noun “action” while deciding not to make a similar choice in CERCLA.

25 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 26

is diligently prosecuting a standard or limitation in any forum at all; it asks

instead whether EPA is diligently prosecuting a standard or limitation “in a

court.” 42 U.S.C. § 7604(b)(1)(B).

The environmental organizations, for their part, make an equally bold

argument for evading the diligent prosecution bar. They turn to the verb

phrase “has commenced and is” to argue that consent decrees are not presently

ongoing and therefore do not satisfy the first step of the diligent prosecution

bar analysis. To better understand the nature of the environmental

organizations’ argument, we must briefly examine the nature of consent

decrees before rejecting their argument.

“A consent decree is a settlement agreement subject to continued judicial

policing.” United States v. Board of County Commissioners of Hamilton County,

937 F.3d 679 (6th Cir. 2019) (alterations adopted, internal quotations omitted).

“A consent decree is a hybrid; it is both a settlement and an injunction,”

Conservation Northwest v. Sherman, 715 F.3d 1181, 1185 (9th Cir. 2013), “a

dual character that has resulted in different treatment for different purposes,”

Local No. 93, Intern. Ass’n of Firefighters, AFL-CIO C.L.C. v. City of Cleveland,

478 U.S. 501, 519 (1986). Whatever the precise ontology, the Supreme Court

has consistently admonished that consent decrees possess at least two defining

characteristics: the imprimatur of the judicial court, and the power of that

court to review the decree on an ongoing basis. United States v. Swift & Co.,

26 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 27

286 U.S. 106, 114–15 (1932); Rufo v. Inmates Suffolk County Jail, 502 U.S.

367, 378 (1992). So, the question of whether consent decrees can (at least in

some circumstances) trigger the diligent prosecution bar as an ongoing civil

action would seem to answer itself – by definition, they can.

The environmental organizations rely on one case where this court said,

“a district court’s order approving the consent decree is tantamount to a final

judgment on the merits.” Jackson v. Los Lunas Community Program, 880 F.3d

1176, 1198 (10th Cir. 2018). But “[t]he fact that a consent decree looks like a

judgment” does not render it a closed thing of the past for all purposes under

the law. Local No. 93, 478 U.S. at 523. The statement in Jackson came in the

context of assessing this court’s jurisdiction over a final judgment and whether

to apply a flexible standard for seeking modification to a consent decree, as

done with respect to injunctions. Jackson, 880 F.3d at 1198. That context (and

the statutory language and history) is quite distinct from the diligent

prosecution bar context. And in any event, Jackson itself contemplated that in

at least some circumstances, the final judgment in a case can result in an order

that continues to be enforced by the parties and the court. See, e.g., id. at 1199

(discussing applicability of Rule 60 to consent decrees).

The environmental organizations cite no authority, and we have found

none, for the proposition that consent decrees cannot trigger the diligent

prosecution bar. To the contrary, the majority of cases cited by both parties

27 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 28

uncontroversially apply the diligent prosecution bar’s analysis to consent

decrees. See, e.g., Piney Run Preservation Ass’n (Piney Run II) v. County

Comm’rs, 523 F.3d 453, 459 (4th Cir. 2008). And the CAA confers a right on

citizen plaintiffs to intervene in an environmental suit where the diligent

prosecution bar would otherwise apply, 42 U.S.C. § 7604(b)(1)(B), which some

courts have applied even after a consent decree is adopted. See, e.g., United

States v. City of Detroit, 712 F.3d 925, 933 (6th Cir. 2013); United States v.

Pacific Gas & Elec., 776 F. Supp. 2d 1007 (N.D. Cal. 2011). Assuming without

deciding that these cases are correct, it would be anomalous to say that the

diligent-prosecution bar does not apply so the citizen suit can go forward, but

simultaneously say that the citizen plaintiffs can intervene in the original suit.

The CAA contemplates citizen suits and intervention as mutually exclusive

possibilities, not overlapping ones. 42 U.S.C. § 7604(b)(1)(B).

The environmental organizations do cite several cases that found no

diligent prosecution in the context of various negotiated or settled agreements.

See, e.g., Citizens for a Better Environment-California v. Union Oil Co. of

California, 83 F.3d 1111, 1118 (9th Cir. 1996); Knee Deep Cattle Co., Inc. v.

Bindana Inv. Co. Ltd., 94 F.3d 514, 516 (9th Cir. 1996). But none of them held

that such agreed resolutions can never satisfy the diligent prosecution bar.

Instead, they appear at most to hold that the agreements in those cases were

not being diligently enforced.

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The environmental organizations also cite Judge Newsom’s concurrence

in South River Watershed Alliance, where he saw “reasonable arguments going

both ways” on this issue. 69 F.4th at 830 (Newsom, J., concurring). However,

we conclude the unbroken practice of federal courts confirms that a civil action

can continue to be diligently prosecuted even after final judgment, so long as

that judgment contemplates future conduct by the parties and the

participation of the courts. Consent decrees fit this bill. Swift, 286 U.S. at 114–

15. 6 Our conclusion is confirmed by the fact that in other diligent prosecution

bars, Congress clarified that consent decrees are part of the universe of judicial

actions that can preclude citizen suits, rather than enumerating them as a

separate category altogether. See, e.g., 42 U.S.C. § 6972(b)(2)(B)(iv) (RCRA).

In sum, only the standards or limitations being diligently prosecuted “in

a court” are preclusive. 42 U.S.C. § 7604(b)(1)(B). The judicially approved and

monitored consent decrees count for this purpose. The administrative actions

upon which Suncor Energy relies, however, are not actions which alone trigger

6 We do not mean to say that every consent decree automatically satisfies the present prosecution requirement. We deal here with consent decrees entered over twenty years ago. What matters is whether a consent decree is presently being enforced, either through the procedural mechanisms in the decree or through separate administrative actions (such as the enforcement of stipulated penalties provided in the consent decree). A consent decree that has lain dormant for years does not indefinitely satisfy present prosecution and may also fail the separate diligence requirement.

29 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 30

the diligent prosecution bar. Though, they may be relevant to determining

whether the other steps in the preclusion analysis are met.

We turn now to the second step in the diligent prosecution bar analysis:

identifying which standards or limitations are preclusive. According to the

environmental organizations, the standard or limitation in the judicial action

must be the same as the one in the citizen suit complaint in order to preclude

the citizen suit. Again, we agree, but will remand for the district court to apply

the correct standard in the first instance.

When inquiring into the nature of a noun in statutory text, the article

preceding the noun can shed light on the question. See, e.g., Niz-Chavez v.

Garland, 593 U.S. 155, 160–65 (2021). The nouns “standard” and “limitation”

in the diligent prosecution bar are preceded by the word “the.” 42 U.S.C. §

7604(b)(1)(B). “The” is a definite article, The Chicago Manual of Style § 5.75

(18th ed. 2024), and definite articles usually refer to something already

“specifically provided for.” Work v. United States ex rel. McAlester-Edwards

Coal Co., 262 U.S. 200, 208 (1923); Nielsen v. Preap, 586 U.S. 392, 408 (2019);

United States v. Lopez, 131 F.4th 1114, 1132–33 (10th Cir. 2025). Here, only

the standards, limitations, or orders that are raised in the complaint could

qualify, because those are the only ones already “specifically provided for” in

the cross-referenced cause of action. 42 U.S.C. § 7604(b) (“No action may be

30 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 31

commenced [] under subsection (a)(1). . .”). We hold that the statutory text

requires the standards and limitations in the citizen suit complaint must be

the same as the ones raised in the proffered judicial action in order for the

diligent prosecution bar to apply.

Our conclusion aligns with courts that have considered the same or

similar questions. California Sportsfishing Protection Alliance v. Chico Scrap

Metal, Inc., 728 F.3d 868, 874 (9th Cir. 2013) (CWA); In re Volkswagen “Clean

Diesel” Marketing, Sales Practices, and Products Liability Litigation, 894 F.3d

1030, 1039 (9th Cir. 2018) (“The explicit textual cross-references between

subsections (a) and (b), and the use of the definite article (‘the standard,

limitation, or order’), signify with precision that the diligent prosecution bar

forecloses only citizen suits that seek to enforce the same ‘standard, limitation,

or order’ as the government enforcement action.” (footnote and emphases

omitted, quoting statute)). We find these decisions from the Ninth Circuit

persuasive.

Recall that the district court applied the diligent prosecution bar to the

environmental organizations’ suit because it “consists of identical or

substantially similar allegations.” App. X at 295. Our holding does not permit

application of the diligent prosecution bar to merely “substantially similar”

standards or limitations. The district court’s application of the diligent

prosecution bar to all of the environmental organizations’ claims, even those

31 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 32

that are not identical to the standards and limitations enforced in the consent

decree, was error.

Suncor’s response is to argue purpose over text. The portions of their

brief dedicated to the sameness requirement contain no reference to the text of

the statute whatsoever. Instead, Suncor makes the more general point that the

purpose of the diligent prosecution bar would be compromised if we did not

construe it broadly to include related, but not identical, standards and

limitations. Suncor principally relies on two cases, one from the Supreme Court

and one from this circuit, to build a theory of statutory purpose, but neither

case moves the needle in the direction of Suncor.

First, Suncor quotes extensively from the Supreme Court’s decision in

Gwaltney of Smithfield, Ltd. v. Chesapeake Bay Foundation, Inc., 484 U.S. 49

(1987). Suncor’s argument goes something like this: EPA has discretion to

choose which standards and limitations to enforce and the Supreme Court in

Gwaltney warned against second-guessing that discretion through citizen

suits. But as an initial matter, the Supreme Court dealt there with a question

quite distinct from the one here, whether the Clean Water Act’s citizen suit

provision allowed suits “for wholly past violations.” Id. at 52. The Court

therefore focused on the language of the citizen suit provision, not the diligent

32 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 33

prosecution bar, and addressed the latter only to shed light on the former. Id.

at 60. 7 Gwaltney is therefore of limited value for Suncor.

More fundamentally, Suncor quotes those portions of Gwaltney that rely

heavily on legislative history and purpose rather than text. See, e.g., id. at 60

(“The bar on citizen suits when governmental enforcement action is under way

suggests that the citizen suit is meant to supplement rather than to supplant

governmental action. The legislative history of the Act reinforces this view of

the role of the citizen suit.”). Even setting aside the well-known problems with

relying on legislative history to discern a statutory purpose, Congress itself

rejected Gwaltney’s line of reasoning when it amended the CAA in 1990 to

permit citizen suits for past violations. Environment Texas Citizen Lobby, Inc.

v. ExxonMobil Corp., 824 F.3d 507, 529 n.18 (5th Cir. 2016). So even for those

who may find it useful to peruse legislative history in search of a statutory

purpose, Gwaltney is unhelpful to their perusal.

Of course, this court has relied on Gwaltney before. In Karr v. Hefner,

which Suncor also quotes extensively, this court broadened the reach of

Gwaltney’s already broad language. 475 F.3d 1192, 1196–98 (10th Cir. 2007)

(“Congress did not intend for them [citizen suits] to be even ‘potentially

7 Indeed, so far as we know, the Supreme Court has never squarely

addressed the diligent prosecution bar of any environmental statute.

33 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 34

intrusive’ on agency discretion.” (quoting Gwaltney, 484 U.S. at 61)). As with

Gwaltney, we are of course bound by Karr’s holdings. But again, we do not

think those holdings get Suncor over the line. Karr’s principal holding was that

“our evaluation of the EPA’s diligence is quite deferential.” Id. at 1198. But

Karr barely addressed the antecedent question: what must EPA be diligent

about?

On that score, Karr quoted the District of Connecticut: “[A] federal court

ought not to allow a citizens’ suit to proceed merely because a prior pending

state suit has not alleged as many separate violations of the Act as has the

citizens’ suit and therefore seeks to impose a less substantial civil penalty on

the defendant.” Connecticut Fund, 631 F. Supp. at 1293. But four paragraphs

prior to the quoted language, the same district court applied this rule: “the

court must determine whether any suit by the state (or the EPA Administrator)

to enforce the same ‘standard, order, or limitation’ was pending in federal or

state court on the date that the citizens’ suit was commenced.” Id. (emphases

added) (quoting Clean Water Act). We therefore read the language from

Connecticut Fund, quoted in Karr, to mean that while a pending government-

initiated suit need not state the same number of violations as the citizen suit,

it must state at least one violation of the same standard or limitation in order

34 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 35

to be preclusive. 8 Our holding today is therefore in harmony with both

Connecticut Fund and Karr. We apply the presumption of diligent enforcement

only after identifying, at steps one and two of the analysis, the same standard

or limitation in a presently enforced judicial action. Id.

There are other arguments Suncor raises, but none persuade. For

example, Suncor points us to Group Against Smog and Pollution, Inc. v.

Shenango Inc., where the Third Circuit rejected “a literal, inflexible, or

grammatical interpretation” of the CAA. 810 F.3d 116, 129 (3d Cir. 2016).

Respectfully, we disagree with this atextual approach and instead embrace the

8 To better illustrate our understanding of Karr and Connecticut Fund, consider two hypotheticals. In the first example, EPA promulgates Regulation 1 that sets standards and limitations for both heaters and boilers. EPA then enters into a consent decree enforcing Regulation 1 as to a polluter’s boilers, but not their heaters. Thereafter, a citizen plaintiff comes along, seeking to enforce Regulation 1 as to the heaters. The diligent prosecution bar applies (assuming EPA’s prosecution of the consent decree is diligent), because although EPA and the citizen plaintiff are seeking to remedy different violations, they are both seeking to enforce the same standard or limitation. See 42 U.S.C. § 7604(f). But let’s change the facts slightly for the second example: this time, the citizen plaintiff comes along and wants to enforce Regulation 2, as to boilers, heaters, or whatever else. The diligent prosecution bar would not apply, because the citizen suit is seeking to enforce a different standard or limitation altogether.

Our construction of the text therefore reflects the balance struck by Congress in barring certain citizen suits. EPA’s bargaining discretion is preserved up to a point: the decision to forgo enforcing a particular standard or limitation up to its maximum enforcement capacity will be preclusive, but the decision to entirely forgo enforcing a standard or limitation will not.

35 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 36

meaning fixed by Congress in the text. From the text, we discern that meaning

by reference to what a reasonably well-informed reader of English would

understand the words Congress chose. “If, in the process of discerning that

meaning, we happen to consult grammar and dictionary definitions – along

with statutory structure and history – we do so because the rules that govern

language often inform how ordinary people understand the rules that govern

them.” Niz-Chavez, 593 U.S. at 169.

At bottom, Suncor’s position on appeal is one that asks us to vindicate

the purpose of the diligent prosecution bar at the expense of its text. But

Congress does not pursue one purpose at the expense of all others. “Deciding

what competing values will or will not be sacrificed to the achievement of a

particular objective is the very essence of legislative choice – and it frustrates

rather than effectuates legislative intent simplistically to assume that

whatever furthers the statute’s primary objective must be the law.” Rodriguez

v. United States, 480 U.S. 522, 526 (1987) (per curiam) (emphasis omitted).

Taking Suncor’s arguments to their logical horizon, we see no principle limiting

us from merely extinguishing the citizen suit provision altogether out of fear

that all citizens suits could potentially infringe on EPA’s discretion. That is not

the judicial function.

At oral argument, both parties agreed that we need not conduct the

sameness inquiry in the first instance, although their rationales were quite

36 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 37

different. Oral Arg. at 8:34 (Counsel for the environmental organizations

arguing that “the simplest way to resolve this case” would be to decide it on

diligence grounds); Oral Arg. at 36:47 (Counsel for Suncor: “None of that needs

to be done.”). We agree, but for a slightly different reason. The error we find in

the district court’s judgment is that it did not apply a sameness inquiry at all,

but rather applied the bar to claims that were merely “substantially similar,”

and which were not found in the consent decrees. App. X at 294. The district

court applied the wrong legal standard before it even got to diligence. It is this

court’s usual practice, when a district court applies the wrong standard rather

than applying the right standard incorrectly, to remand for the district court

to apply the right standard in the first instance. See, e.g., Kent v. Martin, 252

F.3d 1141, 1146 (10th Cir. 2001). This practice conserves appellate resources

while securing to the trial court its appropriate role as the court of first view.

We will adhere to that usual practice here.

At arm’s length, it appears that some, but not all, of the environmental

organizations’ claims will be able to survive the diligent prosecution bar

because they are not the same as anything asserted in the consent decrees.

Although we acknowledge and appreciate the parties’ valiant attempt to assist

us and the district court with a battle of the charts, we suspect that on remand,

the district court will benefit from the parties more precisely identifying the

37 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 38

exact standard or limitation – by regulatory text, if possible – that is at issue

in the environmental organizations’ suit.

Finally, we address “diligent prosecution,” although our discussion will

be brief. As several courts have held, a district court must assess whether the

government’s prosecution has been diligent only after identifying the same

standard or limitation in a present judicial action. South River Watershed

Alliance, 69 F.4th at 823–24; Ohio Valley Environmental Coalition, Inc., 808

F. Supp. 2d at 883; Connecticut Fund, 631 F. Supp. at 1293. The district court

here did not correctly analyze the antecedent issue, so we reverse on that basis.

Nevertheless, because it was fully briefed and presented to us, we take this

opportunity to provide some guidance to the parties and the district court for

how this case should proceed on remand, assuming the district court can

identify claims in the citizen suit that are the same as the standards and

limitations enforced by the Texas consent decrees.

As observed, Karr’s principal holding was that diligent prosecution is

presumed. 475 F.3d at 1198 (“high standard,” “quite deferential”). The parties

are not relieved of this presumption. United States v. Manzanares, 956 F.3d

1220, 1225 (10th Cir. 2020). Nor do we see any issue with the proposition that

when Congress uses the word “diligence,” it does not intend for courts to

38 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 39

second-guess “agency strategy.” 475 F.3d at 1197. 9 Thus, the mere fact that a

defendant has failed to comply with the CAA is insufficient to overcome the

presumption of diligence, Cebollero-Bertran, 4 F.4th at 75, and neither is the

conclusory assertion, without substantiating evidence or argument, that a

stipulated penalties scheme is inadequate, South River Watershed Alliance, 69

F.4th at 827.

The district court appeared to lump together and weigh equally all the

administrative actions taken by EPA and Colorado, even the ones that imposed

no consequences on Suncor and took their existence alone to be sufficient

evidence of diligence. But that casual approach is not the proper inquiry. The

South Carolina district court’s decision in Laidlaw is an illustrative example

that properly balances a presumption of diligence with a nonetheless searching

inquiry. Friends of the Earth, Inc. v. Laidlaw Environmental Services (TOC),

Inc., 890 F. Supp. 470, 489 (D.S.C. 1995) (“[T]he issue presently before the

court involves a delicate balance between the Act’s preference for

governmental enforcement efforts and the recognized policy of allowing private

citizens to participate in the enforcement process. Of course, the overriding

concern is to assure vigorous enforcement of the CWA to achieve the stated

9 To reiterate, however, the presumption of diligence applies only after

the defendant can identify the same standard or limitation in a present judicial action.

39 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 40

goals of the Act.”). In other words, this step requires “more than de minimis

judicial scrutiny of the agency’s performance.” Matthew D. Zinn, Policing

Environmental Regulatory Enforcement: Cooperation, Capture, and Citizen

Suits, 21 Stan. Env’t L.J. 81, 159 (2002); see also id. at 159–60 (suggesting

factors like “the severity of any penalty imposed by the agency; the allowance

of, or failure to allow, pro-regulatory groups to intervene in the enforcement

action; evidence of collusion with the purpose of preempting the suit; and other

evidence of agency somnolence” (footnotes omitted)).

On remand, again assuming that the district court reaches the diligent

prosecution step with respect to at least some of the environmental

organizations’ claims, it should conduct a fact-sensitive inquiry of diligence.

That inquiry permissibly includes the administrative actions taken by

Colorado and EPA after the entry of the consent decrees, but the district court

should grant appropriate weight to those actions that impose actual

consequences for violating the consent decrees (like stipulated penalties) as

opposed to merely threatening consequences, and it should disregard entirely

40 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 41

those actions that are unrelated to violations of the consent decrees. 10 And in

conducting that inquiry, it should be mindful of the appropriate standard

under Rule 12 and the respective burdens of the parties. See Brown v. City of

Tulsa, 124 F.4th 1251, 1263–64, 1269 (10th Cir. 2025).

III

The CAA was the product of extraordinary deliberation and compromise,

and in few places more so than in the section where Congress balanced its

interest in diligent public prosecution with private citizen suit enforcement.

Daniels, Follett, & Davis, The Making of the Clean Air Act, at 930–31. We

respect that deliberation and compromise best when we adhere to the text

enacted in the statute. By deviating from the text in its analysis, we conclude

the district court erred. It applied the diligent prosecution bar to standards

10 On appeal, Suncor heavily emphasizes EPA’s 2024 Notice of Violation (NOV), see, e.g., Resp. Br. at 24 (“There is complete overlap with regard to the emission standards, pollutants, and equipment that form the basis of the violations alleged in the Complaint and in the 2024 NOV.”), but we would be surprised to see the district court place great weight on this NOV. As the environmental organizations note, the NOV was issued only after the environmental organizations gave EPA the statutorily-required notice of their claims and intent to sue. Reply Br. at 25. It would appear counterintuitive to give the NOV, which does not itself appear to impose any consequences, much preclusive effect, when its existence is owed to the environmental organizations’ investigation and litigatory efforts. And in any event, the district court must give preclusive effect only to the same standards or limitations as are found in the consent decrees. The NOV is relevant only insofar as it diligently prosecutes those standards or limitations.

41 Appellate Case: 25-1243 Document: 40-1 Date Filed: 08/03/2026 Page: 42

and limitations not found in the consent decrees, either because they were

found in other, post-consent decree administrative actions or because they

were similar to, but not the same as, the standards or limitations found in both

the judicial and administrative actions. Both bases for applying the diligent

prosecution bar were error.

On remand, the district court should hold the appropriate party to the

burden of identifying, with precision, those claims in the citizen suit that are

the same as the standards and limitations in the consent decree. Only then

may Suncor rely on the presumption of diligence. We REVERSE and REMAND

for further proceedings consistent with this opinion.