Affirmed by published opinion. Judge SHEDD wrote the opinion, in which Judge GREGORY and Judge DUFFY joined.
OPINION
Piney Run is a small stream with its headwaters near the border of Carroll and Baltimore counties in Maryland. For the second time, the Piney Run Preservation Association (“the Association”) has filed a citizen suit under the Clean Water Act (“CWA” or “the Act”) alleging that the County Commissioners of Carroll County (“the County”) are violating the CWA by discharging treated wastewater (i.e., effluent) from the Hampstead Wastewater Treatment Plant (“the Plant”) into Piney Run. The Association specifically contends that the temperature of the discharged effluent at times exceeds the thermal limitation set forth in the County’s National Pollutant Discharge Elimination System (“NPDES”) permit. 1 On the County’s motion to dismiss pursuant to Rule 12(b)(1) of the Federal Rules of Civil Procedure, the district court found that the Maryland Department of the Environment (“MDE”) was “diligently prosecuting” a CWA enforcement action against the County for violating the Permit; consequently, the court held that the Association was barred by 33 U.S.C. § 1365(b)(1)(B) from maintaining this suit. 2 The Association now appeals the order of dismissal, arguing that the district court erred in its “diligent prosecution” determination. We affirm.
I
Congress enacted the CWA “to restore and maintain the chemical, physical, and biological integrity of the Nation’s waters.” 33 U.S.C. § 1251. “To serve those ends, the Act prohibits ‘the discharge of any pollutant by any person’ unless done in compliance with some provision of the Act.”
S. Fla. Water Mgmt. Dist. v. Miccosukee Tribe of Indians,
“Although the primary responsibility for enforcement rests with the state and federal governments, private citizens provide a second level of enforcement and can serve as a check to ensure the state and federal governments are diligent in prosecuting Clean Water Act violations.”
Sierra Club v. Hamilton Cty. Bd. of Cty. Comm’rs,
II
As we explained in the prior CWA litigation between these parties, MDE has classified Piney Run as a Class III-P stream, meaning that it is protected as a source of public drinking water and as a body capable of supporting a self-sustaining trout population. The Plant is subject to the CWA and since 1975 has operated under a series of NPDES permits granted by MDE, the most recent one having been issued in 1990. That permit contains express limitations on the amount of certain pollutants that the Plant can discharge, but heat — which is a statutory pollutant— was not originally listed in it.
See Piney Run Pres. Ass’n,
*457 In March 2000, as part of ongoing administrative litigation involving the Plant, MDE modified the County’s permit to include a thermal limitation for the Plant’s discharged effluent and procedures for measuring ambient conditions in Piney Run. The thermal limitation provides that the effluent cannot exceed the higher of 20°C or the ambient temperature of the stream. Following contested proceedings before an administrative law judge, the modified permit became effective in early 2004. Several months later, the County applied to MDE for an alternate effluent level (“AEL”) — i.e., a less stringent temperature restriction — and submitted a Study Plan for Alternate Effluent Limitation for MDE’s review. 3 During the pen-dency of this application, the County has provided MDE with supplemental reports and appears to have cooperated with MDE. MDE has not yet made a final determination on this application.
Apart from its AEL application, the County petitioned the state circuit court for review of MDE’s decision to modify the 1990 permit. The County also moved to stay enforcement of the modified permit pending the outcome of the petition for review. In turn, MDE filed a complaint seeking injunctive relief and civil penalties against the County for, inter alia, violating the thermal limitation in the modified permit (“the MDE enforcement action”). Eventually, the circuit court denied the County’s petition for judicial review, but it stayed enforcement of the modified permit until a final decision was rendered on the County’s petition for review or November 20, 2005, whichever came first. In light of the circuit court’s stay of enforcement of the modified permit and the County’s appeal of the denial of its petition for review, MDE and the County jointly moved the circuit court to stay the MDE enforcement action until the stay of the modified permit was lifted. The circuit court granted the joint motion and stayed the MDE enforcement action. On November 20, 2005, the stay of enforcement of the modified permit expired, and that permit again became effective. Subsequently, MDE’s final decision modifying the permit was judicially affirmed. 4
In July 2006, MDE and the County entered into a Consent Judgment in the
*458
MDE enforcement action reflecting their “agreement as to appropriate long-term and interim measures to insure compliance with the Permit.” J.A. 373. The circuit court entered the Consent Judgment on July 25, 2006, and its terms became the order of the circuit court.
See Long v. State of Md.,
The Consent Judgment initially mandates that upon its effective date, the County “shall achieve compliance with the thermal effluent limitations” set forth in the modified permit. J.A. 374. It also provides that in settlement of all violations of the thermal limitation that occurred between November 20, 2005, and July 25, 2006, the County would pay a civil penalty of $500 for each day that a violation occurred. Pursuant to this provision, the County paid penalties in the amount of $13,500.
Prospectively, the Consent Judgment also imposes stipulated civil penalties of $500 for each day that the County fails to: (1) comply with the thermal limitation of the modified permit; (2) meet any requirement or complete any required work, plan, or report; or (3) adhere to any required milestone date or schedule. In recognition of the County’s AEL request and implementation of an AEL study, the Consent Judgment provides that the stipulated penalties automatically begin to accrue on the day after performance by the County is due or the day a violation occurs, whichever is applicable, and will continue to accrue until the County satisfactorily completes performance or the violation ceases. If MDE approves an AEL and that approval is thereafter challenged, then stipulated penalties will continue to accrue; however, if a final judgment is entered approving an AEL, then stipulated penalties for events that do not exceed the AEL will not be assessed by MDE. Conversely, if an AEL is ultimately disapproved, then stipulated penalties from November 20, 2005, to the date of the final AEL judgment will become due and payable within 30 days from the final judgment date. Additionally, upon final disapproval of an AEL, the County will be required to submit to MDE a plan identifying the measures needed to comply with the thermal limit of the modified permit, and the County will thereafter have 270 days to implement those measures. 5 During that 300-day period, stipulated penalties will continue to accrue for violations of the thermal limitation.
The Consent Judgment also requires the County to implement a multi-phase Environmental Project (“EP”) to “mitigate any water temperature increases caused by storm water runoff or modifications to stream buffers in the Piney Run watershed,” J.A. 376, and it imposes on the County various obligations and deadlines associated with the EP. According to MDE, “implementation of the EP may result in reduced temperature impacts and related environmental benefits to the Piney Run watershed.” J.A. 268.
In late August 2006, the Association’s counsel notified the County and MDE by letter of its intent to file a citizen suit to seek enforcement of the thermal limitation in the modified permit. The Association thereafter filed this lawsuit in November *459 2006. The County responded by moving under Rule 12(b)(1) for dismissal, arguing that the Association is barred from maintaining a citizen suit because the MDE enforcement action constitutes diligent prosecution within the meaning of § 1365(b)(1)(B). The district court granted the motion and dismissed the case, finding “that the consent judgment (even though it does not mandate installation of the chillers that are plaintiffs preferred remedy) and its implementation comfortably come within the relevant standards of ‘diligent prosecution.’ ” J.A. 765.
Ill
We review
de novo
a dismissal under Rule 12(b)(1).
Suter v. United States,
“Section 1365(b)(1)(B) does not require government prosecution to be far-reaching or zealous. It requires only diligence.”
Karr,
The Association argues generally that the district court erred in concluding that MDE has diligently prosecuted the County’s CWA violations “because the Consent Judgment ... provided a benefit to [the] County and failed to ensure that violations would not continue.” Brief of Appellant, at 18-19. As we have noted, a CWA enforcement action will be considered diligent where it is capable of requiring compliance with the Act and is in good faith calculated to do so. Based on the record before us, we hold that the Association has failed to meet its high burden of establishing that the MDE enforcement action does not satisfy this standard.
In reaching this conclusion, we initially note that it cannot seriously be said that the MDE enforcement action is incapable of requiring compliance with the Act. Indeed, the evidence establishes the contrary. MDE modified the County’s permit in 2000 to include the thermal limitation, and the modified permit only became effective in 2004 at the conclusion of contested administrative proceedings. Before then, the County’s discharge of effluent did not violate the terms of its NPDES permit.
See Piney Run Pres. Ass’n,
The real question raised by the Association is whether the MDE enforcement action, and specifically the Consent Judgment, is in good faith calculated to require the County’s compliance with the Act. 7 In challenging the motivation underlying the MDE enforcement action, the Association points to, inter alia, an alleged request by the County for MDE to file the enforcement action, the fact that the daily fine established in the Consent Judgment is lower than the fine imposed in the prior CWA litigation, and the fact that the Consent Judgment does not establish a final deadline for compliance. In our view, these matters simply do not establish a lack of diligence on MDE’s part.
Even assuming that the County requested MDE to pursue the enforcement action, “there is no incompatibility whatever between a defendant’s facilitation of suit and the State’s diligent prosecution — as prosecutions of felons who confess their crimes and turn themselves in regularly demonstrate.”
Laidlaw Envtl. Servs.,
IV
Based on the foregoing, we affirm the district court’s order of dismissal.
AFFIRMED
Notes
. The prior CWA litigation between these parties also centered on the temperature of the Plant's discharged effluent. In that case, the district court held that although the County’s permit (as it then existed) did not contain a thermal limitation, the County was nonetheless liable under the CWA. Consequently, the court enjoined the County from further CWA violations and assessed civil penalties of $400,000 against it. We vacated the judgment on appeal, concluding
(inter
alia) that the County did not violate the CWA because it "complied with the discharge limitations and reporting requirements of [its] permit” and its effluent discharges "were within the reasonable contemplation of the permitting authority at the time the permit was issued.”
Piney Run Pres. Ass'n v. Cty. Comm’rs of Carroll Cty., Md..,
. Section 1365(b)(1)(B) provides that a citizen suit may not be maintained if the government enforcement agency “has commenced and is diligently prosecuting a civil or criminal 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 citizen may intervene as a matter of right.”
. The applicable regulation provides:
Thermal discharge effluent limitations or standards established in permits may be less stringent than those required by applicable standards and limitations if the dis-charger demonstrates to the satisfaction of the Department that the effluent limitations or standards are more stringent than necessary to assure the protection and propagation of a balanced, indigenous community of shellfish, fish, and wildlife in and on the body of water into which the discharge is made.
Md.Code Regs. 26.08.03.03E(1).
. The primary point of contention concerning the appropriate thermal limitation centers on the effect of the discharged effluent on the Piney Run trout population, and the parties dispute that effect. According to the County, the Plant’s purification process does not involve heating the effluent; rather, it consists of biological and ultraviolet light treatment. Nonetheless, at times the ambient temperature of outdoor air and sunlight exposure can affect both the temperature of the stream and of the effluent before it is discharged. When this occurs, the daytime temperature of the stream often exceeds the effluent; however, at night, the stream cools faster than the stored effluent, causing the effluent to exceed the ambient temperature of the stream. Interestingly, in affirming the administrative decision to modify the permit, the Maryland Court of Special Appeals noted that the administrative law judge found “that there was no evidence the Plant's discharge had any adverse effect on aquatic life that naturally occurring conditions would not have” and that he also indicated that “the discharge may even have a positive effect on the trout population.” Cty. Comm’rs of Carroll Cty. v. Rowland, No. 2338, at 30 (Md.Ct.Spec.App.2006) (J.A. 415).
. In the event that an AEL is disapproved, it appears that the County’s planned means of compliance with the thermal limitation is the installation of mechanical chillers that will cool the effluent before its discharge. The County has been reluctant to utilize chillers based on its concerns that the chillers will impact the area's air and water quality and will cause noise pollution.
. When presented with a Rule 12(b)(1) motion, district courts are permitted to consider materials outside the pleadings,
Suter,
. The Association argues that the presumption of diligence arising from an agency enforcement action can be rebutted "upon a showing that the agency has engaged in a pattern of conduct in its prosecution that could be considered dilatory, collusive or otherwise in bad faith." Brief of Appellant, at 21-22 (internal quotation marks omitted).
. Although Justice Scalia's dissenting opinion discussed the merits of the diligent prosecution question, the
Laidlaw
majority did not decide the issue. Notably, however, the alie-
*461
gation that Laidlaw played a role in facilitating the enforcement action involved more than a mere request that the agency file suit; instead, as the majority noted, the district court found that "Laidlaw drafted the state-court complaint and settlement agreement, filed the lawsuit against itself, and paid the filing fee."
