The Environmental Conservation Organization appeals the dismissal of its Clean Water Act citizen suit against the City of Dallas, Texas, arguing that the district court erroneously concluded that its suit was barred under the doctrine of res judicata. We agree that the citizen suit should have been dismissed, but not for the reasons given by the district court. We hold that the citizen suit became moot prior to the district court’s entry of final judgment; therefore, we vacate the judgment because it was rendered without jurisdiction.
I. Facts and Procedural Background
The City of Dallas, Texas (“City”), operates a municipal separate storm sewer system (“MS4”) that collects, channels and discharges storm water runoff into the Trinity River and its tributaries. The City operates this separate storm sewer system pursuant to a Clean Water Act permit (the “MS4 Permit”). See 33 U.S.C. § 1342. The MS4 Permit delineates permissible and impermissible discharges into the Trinity River watershed and mandates the implementation of a storm water management program (“SWMP”). The SWMP imposes planning and monitoring requirements to ensure that non-storm water and other pollutants are not discharged through the separate storm sewer system. See 33 U.S.C. § 1342(p); 40 C.F.R. § 122.26.
The Environmental Conservation Organization (“ECO”) is a non-profit environmental watch group based in Dallas, Texas. Some of ECO’s members use the Trinity River and its tributaries for recreational purposes. In September 2003, due to problems with the City’s operation of its separate storm sewer system and the polluted appearance of some Dallas-area water bodies, ECO notified the City that it would file a citizen suit in order to remedy alleged violations of the MS4 Permit and the Clean Water Act (“CWA” or “Act”). In accordance with the requirements of the CWA citizen-suit provision, ECO simultaneously notified the Administrator of the Environmental Protection Agency (EPA) and the Texas Commission on Environmental Quality (“TCEQ”) of the alleged violations and ECO’s intent to initiate a citizen suit against the City if no corrective action was taken. See 33 U.S.C. § 1365(b). In October 2003, ECO sent a *523 revised notification letter to the City, the EPA Administrator and the TCEQ, pressing its concerns regarding the alleged violations and reiterating its intent to bring a citizen suit.
In December 2003, over sixty days after sending notice of the alleged violations, ECO filed a CWA citizen suit against the City in federal district court. ECO’s complaint alleged that illicit pollutant discharges from various City-owned facilities violated both the MS4 Permit and the CWA (because they constituted discharges from point sources without a permit). The complaint also alleged that the City violated its MS4 Permit by failing to comply with the SWMP requirements contained therein. ECO sought civil penalties and injunctive relief under the CWA.
While neither the EPA nor the State of Texas commenced an enforcement action against the City prior to ECO’s filing this suit, the EPA had begun to investigate the City’s separate storm sewer system. 1 In February 2004, the EPA issued an administrative compliance order that identified various violations of the City’s MS4 Permit and the CWA. See 33 U.S.C. § 1319(a). The compliance order instructed the City to rectify these violations or arrange a “show cause” meeting with the EPA in order to explain why no enforcement action should be taken in regards to the alleged violations. Shortly after the order was issued, the City and the EPA began negotiating a settlement of the violations noticed therein. The EPA encouraged ECO to participate in the settlement discussions, but ECO declined. Ultimately, the City reached a settlement with the EPA.
In May 2006, the EPA, joined by the State of Texas, filed a CWA enforcement action against the City in federal district court (the “EPA enforcement action”). On the same day, EPA filed a proposed consent decree that contained the terms of the settlement agreement. The filing of the consent decree triggered a public notice and comment period, during which ECO was the only person or organization to submit comments. See 28 C.F.R. 50.7. ECO did not oppose entry of the consent decree, but expressed concern that the punitive provisions and some remedial provisions were inadequate. At the close of the public comment period, the EPA responded to ECO’s comments and moved for entry of the consent decree. In August 2006, the district court granted EPA’s unopposed motion to enter the consent decree. See United States v. City of Dallas, No. 3:06-cv-00845 (N.D.Tex. Aug. 28, 2006). 2
The consent decree is a lengthy and detailed document. It requires, among other things, that the City pay $800,000 in civil penalties, undertake supplemental environmental projects totaling at least $1.2 million, meet minimum staffing requirements in its environmental quality and sewer system monitoring departments, and provide ongoing compliance reports to the EPA. The City is also subject to an audit of its storm water system -within three years of the consent decree’s entry and must pay stipulated penalties if it is found to be in violation of any of its provisions. The decree’s appendices provide strict timetables for satisfaction of these *524 provisions, which are posted on a website and available to the public.
During the time in which EPA was investigating and negotiating with the City, ECO’s citizen suit remained pending on the district court’s docket. After entering the consent decree in the EPA enforcement action, the district court directed the City to file a motion for summary judgment in ECO’s action so that the court could determine whether the consent decree precluded ECO’s citizen suit. The City moved for summary judgment, arguing that ECO’s suit should be dismissed because the final judgment in the EPA enforcement action precluded re-litigation of the same claims in a citizen suit. Alternatively, the City argued that ECO’s claims were mooted by the resolution of the EPA enforcement action. ECO opposed the motion to dismiss, arguing first that the language and statutory scheme of the CWA indicated that the res judicata doctrine and principles of mootness could not be invoked to dismiss a prior-filed citizen suit. Alternatively, ECO argued that neither the elements of res judicata nor the test for mootness had been satisfied.
The district court relied on res judicata to dismiss ECO’s citizen suit, • rejecting arguments that both the privity and similarity of claims requirements were not met in its case. The court noted that even though “the Fifth Circuit has never considered whether res judicata applies to a citizen enforcement action under the CWA,” precedents from other circuits had applied res judicata to citizen suits under similar circumstances.
Envtl. Conservation Org. v. City of Dallas,
II. Discussion
A. Standard of Review
We review the district court’s grant of summary judgment
de novo,
applying the same standard as the district court.
Greenwell v. State Farm Mut. Auto. Ins. Co.,
This case also requires us to examine the district court’s jurisdiction. The City argues that ECO’s claims were moot at the time that the district court entered summary judgment. We review questions of federal jurisdiction
de novo,
including arguments that a case or controversy has become moot.
In re Scruggs,
B. Standing
“Mootness is ‘the doctrine of standing in a time frame. The requisite personal interest that must exist at the commencement of litigation (standing) must continue throughout its existence (mootness).’ ”
Ctr. for Individual Freedom v. Carmouche,
The U.S. Department of Justice, participating in this appeal as
amicus curiae,
urges us to resolve this ease on res judica-ta grounds and argues that recent Supreme Court precedent permits us to do so, pretermitting the mootness issue.
See Sinochem Int’l Co. v. Malaysia Int'l Shipping Corp.,
— U.S. -,
We recognize that, in some limited instances, “a federal court has leeway to choose among threshold grounds for denying audience to a case on the merits.”
Sinochem,
This is not “a textbook case for immediate [res judicata] dismissal.”
Sinochem,
C. Mootness Analysis
1. A citizen suit may be dismissed as moot
Congress enacted the CWA with the express purpose of “restoring] and maintaining] the chemical, physical, and biological integrity of the Nation’s waters.” 33 U.S.C. § 1251(a). Among the myriad of mechanisms for achieving this goal, Con
*526
gress empowered private citizens to bring suit in federal court against alleged violators of the Act. 33 U.S.C. § 1365. Under the CWA citizen-suit provision, federal courts are authorized to enter injunctions and assess civil penalties, payable to the United States Treasury, against any person found to be in violation of “an effluent standard or limitation” under the Act. § 1365(a);
Friends of the Earth, Inc. v. Laidlaw Envtl. Servs. (TOC), Inc.,
However, Congress placed some limitations on the commencement of citizen suits under the CWA. First, a citizen may not commence suit prior to sixty days after giving notice of the alleged violation to the EPA, the State, and the alleged violator. § 1365(b)(1)(A). Second, no citizen suit may be brought if the EPA or State “has commenced and is diligently prosecuting a civil or criminal action” against the alleged violator. § 1365(b)(1)(B). The Act is silent as to which mechanisms may be invoked to dispense with citizen suits — like ECO’s— that have been properly commenced under Section 1365(b).
ECO argues that “courts should not employ judicially-created mootness or res judicata doctrines” to dismiss a properly filed citizen suit because this would conflict with Congress’s statutory scheme under the CWA. ECO invokes the canon of statutory construction
(expressio unius est exclusio
alterius) that instructs “[wjhen a statute limits a thing to be done in a particular mode, it includes a negative of any other mode.”
Christensen v. Harris County,
ECO’s argument is flawed for two reasons. First, the “thing to be done” under Section 1365(b) is the
commencement
of a citizen suit, not the resolution of such a suit. Second, and more importantly, mootness is part of the Article III standing inquiry applicable to all suits filed in federal court.
See Laidlaw,
*527 Having established that the jurisdictional mootness inquiry is required in the context of a CWA citizen suit, we turn to an application of mootness principles to ECO’s suit.
2. Standard for determining mootness
As a general rule, “any set of circumstances that eliminates actual controversy after the commencement of a lawsuit renders that action moot.”
Carmouche,
ECO argues that, if the EPA-negotiated consent decree can moot its citizen suit, the test for mootness should be the traditional, stringent standard that was applied by this court in
Carr v. Alta Verde Industries.
According to
Carr,
in order to have a case dismissed as moot, “[t]he defendant must demonstrate that it is
absolutely clear
that the allegedly wrongful behavior could not reasonably be expected to recur.”
However, the test for mootness that ECO urges us to apply is derived from cases in which the defendant argued that its
voluntary
conduct mooted the plaintiffs suit.
See Laidlaw,
*528
Far from voluntary, the City’s compliance with the terms of its MS4 Permit and the CWA has been compelled by an EPA enforcement action and the resulting court-approved consent decree. Further, the actions that allegedly moot ECO’s suit are actions of third parties (the EPA and a federal court), not those of the City. As such, we would not be relying solely on the City’s assurances that it will not “return to [its] old ways.”
United States v. W.T. Grant Co.,
This standard for determining whether a CWA citizen suit has been mooted by a subsequent government enforcement action respects Congress’s intent that citizen suits “supplement rather than ... supplant government action.”
Gwaltney,
Were we to employ the stringent “voluntary cessation” standard urged by ECO to these circumstances — where compliance was brought about as the result of an EPA enforcement action and a court-approved consent
decree
— we would effectively cede primary enforcement authority under the CWA to citizens acting in the role of private attorneys general. Such ceding would discourage defendants in a citizen from entering a consent decree with federal or state enforcement agencies, because defendants would remain exposed to dupli-cative penalties.
See Gwaltney,
Further, the “realistic prospect” mootness standard that we employ today comports with Congress’s policy that only “diligent prosecutions” preempt citizen suits.
See
§ 1365(b)(1)(B). If a citizen-suit plaintiff demonstrates that there is a realistic prospect that the violations alleged in its
*529
complaint will continue notwithstanding the government-backed consent decree, then a less-than-diligent prosecution might have been shown. Placing the burden on the citizen-suit plaintiff to demonstrate that his claims are not mooted by the consent decree is also in step with Congressional policy.
See Karr v. Hefner,
3. ECO’s citizen suit is moot
We now consider whether ECO can satisfy its burden to show a realistic prospect that the CWA violations alleged in its citizen suit will continue notwithstanding the consent decree. The district court found that the consent decree resolved every violation alleged in the ECO citizen suit.
Envtl. Conservation Org.,
ECO relies primarily on an affidavit submitted by Craig T. Maske, a civil engineer with an expertise in water resources whom ECO retained as an expert. Mr. Maske’s affidavit, sworn on November 10, 2006, details various past violations by the City and offers his opinion that these violations will continue. However, Mr. Maske does not mention the consent decree in his affidavit, nor does he offer an explicit opinion that the consent decree will not adequately address the violations discussed therein. Therefore, Mr. Maske’s affidavit does not address the critical fact in our mootness inquiry.
Other evidence is said to be found in the depositions of several City employees. This testimony, given in April and May 2006, provides evidence solely of
past
violations by the City. No employee mentions the consent decree or offers any prediction as to whether the decree will address the relevant violations. ECO asks us to infer from the City’s “distinct track record of failing to comply with CWA requirements” that these violations will continue and suggests that there is authority for drawing such an inference on summary judgment.
See Orange Env’t, Inc. v. County of Orange,
After reviewing the consent decree and comparing it to ECO’s complaint, we agree with the district court that it addresses every MS4 Permit and CWA violation alleged in ECO’s citizen suit. ECO has not pointed to specific facts on appeal that would support an inference that the City will continue to engage in violations that were alleged in ECO’s citizen suit but not addressed by the consent decree.
See Sedgwick James,
(a) Injunctive relief
The consent decree requires the City to take steps to correct the planning, *530 staffing, monitoring, and compliance deficiencies that resulted in the violations alleged in ECO’s suit. ECO argues there is no mootness as to injunctive relief because the immediate cessation of all violations has not been ordered.
Even had ECO prevailed on its citizen suit, the district court was not bound to order the immediate cessation of all violations. Traditional equitable principles control the decision to enter an injunction, under which the court has broad discretion to balance the interests of the parties. 11A Charles Alan Wright, Arthur R. Miller
&
Mary Kay Lane, Federal Practice and Procedure § 2942 (2d ed.1990). The district court might even have denied in-junctive relief altogether.
See Weinberger v. Romero-Barcelo,
The consent decree achieved some court-ordered mandatory relief that is in-junctive in nature. Because ECO is not entitled to any particular form of injunc-tive relief under the CWA — -and, therefore, was not guaranteed to achieve any other form of relief in its citizen suit than that imposed under the consent decree — its claims for injunctive relief are moot.
(b) Civil penalties
There is some authority that even where a defendant’s
voluntary
acts of compliance are sufficient to moot a citizen’s request for injunctive relief, those voluntary acts will not necessarily moot a related claim for civil penalties.
Comfort Lake,
First, it is significant that the consent decree imposed $800,000 in civil penalties on the City. This does not represent the maximum penalty permissible under the statute.
See
33 U.S.C. § 1319(d). However, even in the event of a successful citizen suit, the district court is not bound to impose the maximum penalty afforded under the statute. In fact, the district court is required to consider a myriad of factors, some of which are mitigating in nature, when determining the appropriate civil penalty under the CWA. § 1319(d);
see Cedar Point,
More importantly, ECO assumed the role of private attorney general in the pursuit of its citizen suit. Any penalty that it achieved would have been paid into the United States Treasury. After ECO initiated its citizen suit, the United States government initiated its own enforcement action and extracted
some
civil penalties from the City. The resulting consent decree (approved by the U.S. Department of Justice) represents the federal government’s discretionary resolution of the level of penalty needed for the same environmental concerns raised by ECO. A private attorney general is no longer needed to raise the issue of the proper civil penalty. That ECO might have sought stiffer penalties against the City does not change the result; ECO is not permitted to upset the primary enforcement role of the EPA by seeking civil penalties that “the Administrator chose to forego .... ”
Gwattney,
III. Conclusion
Underlying our decision today is the recognition that ECO’s concrete interest in the outcome of the litigation was the public’s interest, not an interest specific to ECO or its members.
See Green Forest,
Because the district court’s consent decree in the EPA’s enforcement action addressed all of the CWA violations alleged in ECO’s citizen suit, extracted reasonable civil penalties, and mandated that the City undertake significant corrective measures, ECO’s citizen suit became moot when the consent decree was entered. Accordingly, the district court’s judgment in this case was rendered without jurisdiction and must be vacated.
Goldin v. Bartholow,
We VACATE the district court’s summary judgment in favor of the City and REMAND this case to the district court with instructions that ECO’s citizen suit be dismissed as moot.
Notes
. It appears that EPA’s first physical ''inspections" of the City's separate storm sewer system occurred in November 2003. However, EPA asserts that it began reviewing certain reporting documents filed by the City as early as July 2003.
. ECO had a right to intervene in the EPA enforcement action, see 33 U.S.C. § 1365(b)(1)(B), but declined to do so despite the district court's encouragement.
. Other circuits have reached this conclusion as well.
Comfort Lake Ass'n v. Dresel Con
*527
tracting, Inc.,
. The distinction we draw between the traditional mootness inquiry and the more stringent "voluntary cessation” test is not unique to the circumstances in this case. Federal courts have long-recognized that allegations by a defendant that its voluntary conduct has mooted the plaintiff’s case require closer examination than allegations that "happenstance” or official acts of third parties have mooted the case.
See DeFunis v. Odegaard,
. Under the consent decree, all mandatory planning and any construction must be completed within three to four years. This is not an unreasonably lengthy period for requiring compliance.
Cf. Friends of Milwaukee’s Rivers,
. We are aware that ECO has appealed the denial of its request for costs under Section § 1365(d).
Envt’l Conservation Org. v. City of Dallas,
No. 07-11247 (5th Cir. filed Dec. 10, 2007). Our opinion today should not be read to express any position on the merits of that appeal.
See Murphy v. Fort Worth Indep. Sch. Dist.,
