BOWMAN, Circuit Judge:

The Sierra Club, Inc., and two local environmental organizations (collectively, "Sierra Club") filed this lawsuit against the United States Environmental Protection Agency and its administrators (collectively, "the EPA") alleging that the EPA violated its oversight responsibility under section 303(d) of the Clean Water Act, 33 U.S.C. § 1313(d), by approving Florida's list of impaired waters. The district court, ruling on cross-motions for summary judgment, determined that the EPA's approval was not arbitrary or capricious and entered judgment in the EPA's favor. We affirm in part and reverse and remand in part.

I.

Congress passed the Clean Water Act ("CWA") "to restore and maintain the chemical, physical, and biological integrity of the Nation's waters." 33 U.S.C. § 1251(a). Toward that end, Congress determined that the states and the federal government should work together to combat water pollution. See Arkansas v. Oklahoma, 503 U.S. 91, 101, 112 S.Ct. 1046, 117 L.Ed.2d 239 (1992) ("The Clean Water Act anticipates a partnership between the States and the Federal Government. . . ."). The CWA thus divides between the federal government (via the EPA) and the states many of the duties for monitoring and regulating the nation's waters. Because an understanding of the responsibilities placed on the states and the EPA is essential to resolving this case, we include here a general discussion of the applicable CWA provisions and regulations.

First, the CWA requires states to establish "water quality standards" for waterbodies within their boundaries. 33 U.S.C. § 1313(a)-(c); see also 40 C.F.R. §§ 130.2(d), 131.4(a). To do this, a state must first designate the use (or uses) to be made of a waterbody, such as water supply, fishing, or swimming. 40 C.F.R. § 131.2. Then, the state must determine the water quality criteria necessary to safely permit the designated use. Id. §§ 131.2, 131.3(b). Those criteria become the "water quality standard" for the waterbody. Id. §§ 131.2, 131.3(i). "Water quality standards play an important role in maintaining and improving the cleanliness and safety of the nation's waterbodies, because they are designed to determine which waterbodies are safe enough to support their designated uses." Fla. Pub. Interest Research Group Citizen Lobby, Inc. v. EPA ("FPIRG"), 386 F.3d 1070, 1074 (11th Cir.2004).

Next, each state must compile a list of waterbodies that are not safe enough to support their designated uses, i.e., that do not meet their water quality standards. 33 U.S.C. § 1313(d)(1)(A). This list is known as a state's "impaired waters list" or "303(d) list" (so called because section 303(d) of the CWA, 33 U.S.C. § 1313(d), requires the creation of the list). Each waterbody on the impaired waters list is known as a "water quality limited segment" ("WQLS"). 40 C.F.B. § 130.2(j). The placement of a waterbody on a state's impaired waters list is significant because the CWA requires that states target WQLSs for pollution control.

States undertake the task of decreasing pollution in their WQLSs by establishing a "total maximum daily load" ("TMDL") for pollutants in a designated WQLS. 33 U.S.C. § 1313(d)(1)(C); 40 C.F.R. 40 C.F.R. § 130.7(c)(1). "A TMDL is a specification of the maximum amount of a particular pollutant that can pass through a waterbody each day without water quality standards being violated." Sierra Club v. Meiburg, 296 F.3d 1021, 1025 (11th Cir.2002) (citing 33 U.S.C. § 1313(d)(1)(C)). States must establish a TMDL for every pollutant that prevents or is expected to prevent a waterbody from attaining applicable water quality standards. 40 C.F.R. § 130.7(c)(1)(ii). Once a TMDL is established, the state (as well as the federal government) strives to decrease the amount of the pollutant to which that TMDL applies so that the TMDL is not exceeded. The CWA also requires states to "establish a priority ranking [for WQLSs needing TMDL development], taking into account the severity of the pollution and the uses to be made of such waters." 33 U.S.C. § 1313(d)(1)(A); see also 40 C.F.R. § 130.7(b)(4).

States are required to submit their lists of WQLSs, TMDLs, and priority rankings to the EPA every two years. 40 C.F.R. § 130.7(d)(1). The EPA has the duty of approving or disapproving the lists. 33 U.S.C. § 1313(d)(2) § 130.7(d)(2). If the EPA disapproves a state's impaired waters list or a TMDL, the EPA must issue its own list or TMDL. 33 U.S.C. § 1313(d)(2); 40 C.F.R. § 130.7(d)(2).

The present dispute arose out of Florida's 2002 update to its 1998 impaired waters list. In this update, the Florida Department of Environmental Protection ("FDEP") reexamined about twenty percent of Florida's waterbodies, applying Florida's water quality standards and Florida's newly passed Impaired Waters Rule ("IWR"), Fla. Admin. Code Ann. Ch. 62-303. Florida then revised its 1998 list and submitted the new list ("Florida's 2002 List") to the EPA for review on October 1, 2002. The EPA conducted its review of Florida's 2002 List and published the results of the review in a "Decision Document" dated June 11, 2003. Decision Document Regarding Department of Environmental Protection's § 303(d) List Amendment Submitted on October 1, 2002 and Subsequently Amended on May 12, 2003, Admin. R. 2.4 ("Decision Document"). The Decision Document indicates that the EPA approved, in large part, Florida's 2002 List, but disapproved Florida's failure to identify certain waterbodies as impaired and Florida's removal (or "delisting") of certain waterbodies that were on the 1998 list. Ultimately, the EPA added eighty WQLSs to Florida's 2002 List.

Sierra Club brought this suit against the EPA under the Administrative Procedure Act ("APA"), 5 U.S.C. § 706(2)(A), alleging that the EPA's approval of Florida's 2002 List was arbitrary, capricious, and not in accordance with the law. Sierra Club's first claim challenged the EPA's approval of Florida's decision not to list waterbodies when the data showing that those waterbodies were impaired consisted solely of fish consumption advisories for mercury or data that was older than 7.5 years. Sierra Club's second claim challenged the EPA's approval of Florida's priority ranking of waterbodies for TMDL development, arguing that Florida did not take into account statutory standards in setting the rankings. Sierra Club's third and final claim challenged the EPA's approval of Florida's delisting of waterbodies where those waterbodies had exceeded applicable water quality standards at least once in the preceding 7.5 years or were delisted because the violations of water quality standards were due to natural conditions. Shortly after Sierra Club filed its complaint, the FDEP moved to intervene in the lawsuit as a defendant. The district court denied intervention. Sierra Club and the EPA eventually brought cross-motions for summary judgment. The district court granted summary judgment to the EPA on all counts.

The FDEP appealed the denial of intervention, and Sierra Club appealed the entry of summary judgment in favor of the EPA. Because we were alerted that a case on remand from the Eleventh Circuit to the District Court for the Northern District of Florida might affect our analysis of Sierra Club's summary judgment appeal, see FPIRG, 386 F.3d 1070, we held our decision in abeyance pending the outcome of that case. The District Court for the Northern District of Florida issued its opinion on remand in FPIRG on February 15, 2007. FPIRG, No. 4:02cv408-WS, slip op. (N.D.Fla. Feb. 15, 2007). We have reviewed that decision and conclude that it does not affect our ability to rule on the present appeals.

II.

We begin by addressing the FDEP's appeal of the district court's order denying intervention as of right under Federal Rule of Civil Procedure 24(a)(2). Our review of the denial of a motion for intervention as of right is de novo. Georgia v. United States Army Corps of Eng'rs, 302 F.3d 1242, 1249 (11th Cir.2002).

Intervention as of right under Rule 24(a)(2) must be granted when four requirements are met:

(1) the application to intervene is timely; (2) the applicant has an interest relating to the property or transaction which is the subject of the action; (3) the applicant is so situated that the disposition of the action, as a practical matter, may impede or impair his ability to protect that interest; and (4) the applicant's interest will not be represented adequately by the existing parties to the suit.

ManaSota-88, Inc. v. Tidwell, 896 F.2d 1318, 1321 (11th Cir.1990). We need not discuss the first three requirements because we conclude that the FDEP has not met its burden of proof on the fourth requirement.

The FDEP asserts an interest in "the regulation of water quality of Florida's waters," Br. of Intervenor/Appellee at 7, and contends that with each WQLS added to Florida's impaired waters list, a new TMDL is established and the FDEP must expend resources to establish and enforce pollution-discharge permits, id. at 5-6, 15. The FDEP's objective in this litigation, therefore, is to defend Florida's 2002 List — which was revised and approved by the EPA — and prevent the addition of waterbodies to the list. Id. at 15. This objective is the same as the EPA's objective.

"We presume adequate representation when an existing party seeks the same objectives as the would-be interveners." Clark v. Putnam County, 168 F.3d 458, 461 (11th Cir. 1999). While this presumption is "weak," it nonetheless imposes upon the applicant for intervention "the burden of coming forward with some evidence to the contrary." Id. The FDEP has presented no evidence that the EPA may not adequately represent its interests. The FDEP adopts the EPA's brief on appeal in its entirety, and the FDEP does not suggest that it would take a position different from the EPA's in the district court.

A similar situation was addressed by the Fifth Circuit in Associated Industries of Alabama, Inc. v. Train, 543 F.2d 1159 (5th Cir. 1976). In Train, the Attorney General of Alabama sought to intervene as a defendant in a case challenging as arbitrary and capricious the EPA's disapproval of Alabama's water quality standards and substitution of EPA-promulgated standards. The Court affirmed the denial of intervention as of right, reasoning that although the state had an interest in protecting its citizens from overly lax water quality standards, "[w]hen the issue for decision in this case is stated in terms of the legality of the EPA promulgation under § 303 of the [CWA], it is apparent . . . that Alabama's interests in the litigation are adequately represented by existing parties." Id. at 1161 n. 7 (internal quotations omitted). This reasoning applies with equal force here: the FDEP's and the EPA's mutual interest in this case is to defend the legality of the EPA's approval of Florida's 2002 List. As in Train, nothing in the record before us "cast[s] doubt upon the will of the EPA to defend the legality of [its] promulgation." Id. at 1161. Likewise, "[n]o claim or defense on behalf an of [Florida] has been suggested which is mi not or will not be asserted by the EPA R. defendants." Id. While the FDEP hypothesizes that the EPA might have an increased incentive to compromise with Sierra Club in settlement discussions because Florida, not the EPA, bears the brunt of the costs that accompany TMDL development, nothing in the record supports the suggestion that the EPA is more likely to settle. Cf. Clark, 168 F.3d at 462 (granting intervention where a "discussion of settlement that was mentioned on the record at [a] hearing" indicated that the proposed defendant-interveners and the defendants had divergent positions on settlement). Moreover, the Train Court rejected a similar argument — "that the effect of a plaintiffs victory in this suit will be greater upon the State than upon the EPA" — because the argument did not re-fleet upon the vigor of the EPA's defense of its actions. Train, 543 F.2d at 1160.

We conclude that the interests of the FDEP are adequately represented by the pr EPA, and we affirm the district court's de denial of intervention.

III.

We review the district court's grant of summary judgment de novo, applying the same legal standards as the district court. See Nat'l Parks Conservation Ass'n v. Norton, 324 F.3d 1229, 1236 (11th Cir. 2003). Summary judgment is proper if, when viewing the evidence in the light most favorable to the non-moving party, there is no genuine issue of material fact and the moving party is entitled to judgment as a matter of law. Id. (citing Fed.R.Civ.P. 56(c)).

Under the APA, a court shall "set aside agency action, findings, and conclusions found to be arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law." 5 U.S.C. § 706(2)(A). Agency action is considered arbitrary or capricious if the agency has

relied on factors which Congress has not intended it to consider, entirely failed to consider an important aspect of the problem, offered an explanation for its decision that runs counter to the evidence before the agency, or is so implausible that it could not be ascribed to a difference in view or the product of agency expertise.

Motor Vehicle Mfrs. Ass'n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 43, 103 S.CT. 2856, 77 L.Ed.2d 443 (1983.)

When an agency interprets a statute that the agency is responsible for administering, courts must give the agency's interpretation due deference if (1) Congress has delegated interpretive authority to the agency, (2) the statute is silent or ambiguous with respect to the issue at hand, and (3) the agency's interpretation of the statute is reasonable. Chevron, U.S.A., Inc. v. Natural Res. Def. Council, Inc., 467 U.S. 837, 843, 104 S.Ct. 2778, 81 L.Ed.2d 694 (1984); United States v. Mead Corp., 533 U.S. 218, 229, 121 S.Ct. 2164, 150 L.Ed.2d 292 (2001). "[A] court may not substitute its own construction of a statutory provision for a reasonable interpretation made by the administrator of an agency." Chevron, 467 U.S. at 844, 104 S.Ct. 2778; see also Sierra Club v. Johnson, 436 F.3d 1269, 1274 (11th Cir.2006). Likewise, courts must give deference to an agency's reasonable interpretation of its own regulations. Sierra Club v. Johnson, 436 F.3d at 1274. "We will uphold the agency's interpretation of its regulations so long as it is reasonable, that is, so long as the interpretation sensibly conforms to the purpose and wording of the regulations." Id. (internal quotation marks and citation omitted).

IV.

Before discussing the specific arguments raised by Sierra Club and decided by the district court, we address Sierra Club's more general and further-reaching argument — raised for the first time on appeal — that Florida's 2002 List must be invalidated in its entirety because it was developed under the IWR, portions of which were later abrogated. See FPIRG, 386 F.3d at 1075-77; FPIRG, No. 4:02cv408-WS, slip op. at 12 (N.D.Fla. Feb. 15, 2007). "As a general rule, an appellate court will not review a legal issue or theory not presented to the trial court, unless the issue is a pure question of law and the court's failure to consider it would result in a miscarriage of justice." NAACP v. Hunt, 891 F.2d 1555, 1563 (11th Cir.1990). Because this argument does not involve a pure question of law and our failure to consider it will not result in a miscarriage of justice, we decline to review it.

V.

We turn now to issues that Sierra Club raised in the district court. Sierra Club first alleges that the EPA's decision to approve Florida's 2002 List was arbitrary and capricious because the list was missing waterbodies for which available data indicated the presence of dangerous levels of mercury. The CWA requires that states identify all waterbodies within their boundaries that do not meet or are not expected to meet water quality standards. See 33 U.S.C. § 1313(d)(1)(A); 40 C.F.R. §§ 130.2(1), 130.7(b)(1). EPA regulations require states to "assemble and evaluate all existing and readily available water quality-related data and information to develop [their impaired waters lists]." 40 C.F.R. § 130.7(b)(5) (emphasis added). Sierra Club asserts that the FDEP violated this mandate when it: (A) applied a provision of the IWR that prohibited the use of "data more than 7.5 years old" in developing Florida's 2002 List, Fla. Admin. Code Ann. r. 62-303.400; and (B) failed to consider fish consumption advisories for mercury. As a result, Sierra Club claims, Florida's 2002 List violates the CWA because it does not include all waterbodies in the state that do not meet or are not expected to meet water quality standards.

A.

The district court concluded that Florida and the EPA were permitted to use a 7.5-year data cutoff. In reaching this conclusion, the district court relied on 40 C.F.R. § 130.7(b)(6)(iii). That regulation requires that states, when submitting their impaired waters lists to the EPA for approval, include with their submission "[a] rationale for any decision to not use any existing and readily available data and information." 40 C.F.R. § 130.7(b)(6)(iii). The district court considered Florida's rationale for the 7.5-year data cutoff (i.e., that data from the Everglades indicated a decrease in mercury accumulation and that Florida desired to make listing decisions based on current conditions) and the EPA's reasons for finding the data cutoff "'reasonable'" (discussed below), and apparently found § 130.7(b)(6)(iii) satisfied. Order on Summary Judgment at 13 (quoting Decision Document at 23). We believe that the district court's reliance on § 130.7(b)(6)(iii) was misplaced.

While § 130.7(b)(6)(iii) implies that Florida has a right to decide not to use certain data, it does not obviate the requirement in § 130.7(b)(5) that Florida evaluate all existing and readily available data. By taking the hard-line approach of not considering any data older than 7.5 years — even when there is no more current data for a particular waterbody — Florida has not fulfilled § 130.7(b)(5)'s evaluation requirement. Moreover, states are required by the CWA to identify all waterbodies that fail to meet water quality standards, 33 U.S.C. § 1313(d)(1)(A); states cannot shirk this responsibility simply by claiming a lack of current data. The district court misinterpreted the CWA's statutory and regulatory scheme when it held to the contrary, and we must therefore remand this issue for an analysis under the correct legal standard.

On remand, of course, the district court will evaluate the EPA's decision to approve Florida's 2002 List, not the methodology underlying Florida's creation of the list. In the EPA's review, the EPA recognized Florida's failure to fully comply with § 130.7(b)(5), noting that the IWR "only provides for assessment when . . . data meets certain temporal and spacial guidelines set out in the rule." Decision Document at 23-24. Nevertheless, "[i]n reviewing data as evidence of water quality for purposes of identifying water quality limited segments," the EPA "chose to look only at data collected within the past 7.5 years." Id. at 23. Thus, the EPA adopted the same 7.5-year data cutoff applied by the FDEP. The EPA reasoned that the data cutoff was reasonable "since older data can be less reliable in representing current conditions." Id. at 23. While the EPA's data-cutoff decision contradicts the CWA's statutory and regulatory language such that it is not entitled to deference, see Chevron, 467 U.S. at 842-43, 104 S.Ct. 2778, the EPA asserts that this decision did not lead to any impaired waterbodies being left off Florida's 2002 List. Sierra Club disagrees with the EPA's assertion. According to Sierra Club, sixty-one waterbodies — listed in Attachment A to the Amended Complaint — were excluded from Florida's 2002 List because of the 7.5-year data cutoff. This is a factual dispute that we leave for the district court to address on remand. Unless the dispute is resolved or found not material, it may preclude the entry of summary judgment on Sierra Club's first claim.

B.

The district court also rejected Sierra Club's argument that Florida and the EPA must place waterbodies subject to statewide fish consumption advisories on the impaired waters list because such waterbodies do not meet or are not expected to meet water quality standards. See 33 U.S.C. § 1313(d)(1)(A); 40 C.F.R. §§ 130.2(j), 130.7(b)(1). The district court deferred to the EPA's position, originally set forth in an EPA "guidance letter," that only waterbody-specific data (such as waterbody-specific fish consumption advisories) constitute "existing and readily available [water quality-related] data and information" that states must consider in developing their impaired waters lists. EPA guidance letter of Oct. 24, 2000, Admin. R. 1.8 at 3.

Because there is no indication that the EPA's guidance letter was the product of a "formal [agency] adjudication," "notice-and-comment rulemaking," or "any other circumstances reasonably suggesting that Congress ever thought of [guidance letters] as deserving . . . deference," the EPA's guidance letter is not entitled to Chevron deference. Mead Corp., 533 U.S. at 230-31, 121 S.Ct. 2164; see also Christensen v. Harris County, 529 U.S. 576, 587, 120 S.Ct. 1655, 146 L.Ed.2d 621 (2000) (ruling that agency interpretations in opinion letters, policy statements, agency manuals, and enforcement guidelines are not entitled to Chevron deference). That is not to say, however, that the EPA's guidance letter is entitled to no deference at all. Rather, the "'specialized experience'" of the EPA and "the value of uniformity in [the EPA's] administrative . . . understandings of what a national law requires" entitles the guidance letter to "a respect proportional to its `power to persuade.'" Mead Corp., 533 U.S. at 234-35, 121 S.Ct. 2164 (quoting Skidmore v. Swift Co., 323 U.S. 134, 139-40, 65 S.Ct. 161, 89 L.Ed. 124 (1944)).

Like the district court, we are persuaded by the guidance letter' and the EPA's position on statewide fish consumption advisories. In the guidance letter, the EPA directs that for the purpose of compiling impaired waters lists, fish consumption advisories constitute data demonstrating a waterbody's non-attainment of a "fishable" use only when: "the advisory . . . is based on [fish] tissue data, the data are from the specific waterbody in question, and the risk assessment parameters of the advisory . . . are cumulatively equal to or less protective than those in the water quality standards." EPA guidance letter of Oct. 24, 2000, Admin. R. 1.8 at 6 (emphasis added). The EPA warns that statewide fish consumption advisories do not demonstrate the need to place a waterbody on a state's impaired waters list because the criteria applicable to statewide fish consumption advisories do not parallel the criteria for forming an impaired waters list:

For example, a State may have issued a statewide or regional warning regarding fish tissue contaminated with a bioaccumulative pollutant, based on data from a subset of waterbodies. A State may use a higher fish consumption value in determining the need for an advisory compared to the value used in establishing water quality criteria for the protection of human health. . . . In such instances . . . they need not be listed as impaired under section 303(d) unless there are waterbody-specific data . . . showing non-attainment of section 101(a) uses.

Id. at 4. We find the informed judgment of the EPA thorough and reasonable, and entitled to some deference. See Skidmore, 323 U.S. at 140, 65 S.Ct. 161.

In reviewing Florida's 2002 List, the EPA noted that Florida's listing methodology for use of fish consumption advisories followed the EPA's guidance letter. Decision Document at 41. This listing methodology states that fish consumption advisories may only be used if the data underlying the fish consumption advisory "is based on the statistical evaluation of fish tissue data from at least twelve (12) fish collected from the specific water segment or water body to be listed." Fla. Admin. Code Ann. r. 62-303.470(1)(a) (emphasis added) (discussed in Decision Document at 38). The EPA concluded that "the State's process, if properly applied, is reasonable for identifying applicable water quality limited segments." Decision Document at 41. We cannot find this decision arbitrary or capricious. The EPA gave a rational explanation, based on the opinion of its experts, as to why state-wide fish consumption advisories (to the extent that they are not based on data collected from a specific waterbody in question) are not reliable indicators of water quality in a particular waterbody for the purpose of forming an impaired waters list. We therefore uphold the EPA's decision that Florida was required to consider only waterbody-specific data in developing its 2002 List.

There is more to the matter than this, however. Sierra Club argues that even accepting the EPA's position, Florida and the EPA did not adhere to the guidance letter's mandate that states consider waterbody-specific data in developing their impaired waters lists. According to Sierra Club, "[a]t least 46 of the waters named in this lawsuit are under waterbody-specific fish consumption advisories issued by the Florida Department of Health; and numerous waters have fish tissue testing by the Department of Health supporting the advisory. Thus, under the Guidance Document, they were required to be on the § 303(d) list." Br. for Plaintiffs-Appellants at 44. This is another factual matter that we leave to the district court's consideration in its evaluation of whether the EPA's approval of Florida's 2002 List was arbitrary or capricious.

Finally, we reject Sierra Club's claim that the EPA violated the CWA when it allegedly added to Florida's 2002 List eleven waters under fish consumption advisories for mercury that had been delisted from Florida's 1998 List. Sierra Club argues that when the EPA determines that a state's list is incomplete, the EPA's "only option under CWA § 303(d)(2) is to disapprove that list and establish a complete list of its own." Br. for Plaintiffs-Appellants at 52. The EPA objects to the factual premise of Sierra Club's claim, but even if the facts are as Sierra Club alleges, the claim has no merit. The CWA directs that if the EPA disapproves a state's identification of impaired waters and the TMDLs established for them, the EPA must "identify such waters in such State and establish such loads for such waters as [it] determines necessary to implement the water quality standards applicable to such waters." 33 U.S.C. § 1313(d)(2). We do not read this provision to require the EPA to develop an entirely new list each time it partially disapproves a state's list. Nothing in the CWA prohibits the EPA's current practice of adding waterbodies to an impaired waters list already prepared by a state; to require more would be impractical and a waste of resources.

We therefore reverse the entry of summary judgment on Sierra Club's first claim, but uphold the district court's holdings deferring to the EPA's positions that (1) Florida was only required to consider fish consumption advisories based on waterbody-specific data and (2) the EPA may add waterbodies under fish consumption advisories to a state's impaired waters list. We remand this claim for further proceedings.

VI.

In Sierra Club's second claim for relief, it alleges that the EPA's approval of Florida's priority ranking of WQLSs for TMDL development was arbitrary and capricious. Under the CWA, Florida was required to "establish a priority ranking for [impaired] waters, taking into account the severity of the pollution and the uses to be made of such waters." 33 U.S.C. § 1313(d)(1)(A); see also 40 C.F.R. § 130.7(b)(4) (requiring states to include in their impaired waters lists "a priority ranking for all listed water quality-limited segments still requiring TMDLs, taking into account the severity of the pollution and the uses to be made of such waters") and (d)(1) (requiring states to submit to the EPA "the list of waters, pollutants causing impairment, and the priority ranking including waters targeted for TMDL development within the next two years"). Florida established its priority ranking, designating each WQLS as "high," "medium," or "low" priority. Sierra Club argues that in establishing this priority ranking, Florida did not adequately take into account the statutory factors of "severity of the pollution" and "uses to be made of such waters." Florida categorically designated as "low" priority all "[w]ater segments that are listed before 2010 due to fish consumption advisories for mercury (due to the current insufficient understanding of mercury cycling in the environment)." Fla. Admin. Code Ann. r. 62-303.500(3)(a).

According to Sierra Club, mercury in fish tissue presents a serious health risk, such that mercury-impaired waters would be designated "high" priority if the relevant statutory factors were given adequate consideration. Sierra Club argues that Florida's categorical designation of mercury-impaired waters as "low" priority both violated the mandate of the CWA (requiring states to take into account the severity of pollution and uses to be made of the waterbody) and was based on a faulty premise (the alleged insufficient understanding of mercury cycling). Thus, Sierra Club asserts, the EPA was wrong to approve Florida's priority ranking.

The district court did not address the underlying merits of Sierra Club's argument. Rather, the district court ruled that the argument failed because the EPA had no duty to approve Florida's priority ranking:

Because there is no requirement that the EPA actually approve or disapprove of a state's priority rankings, the Court finds that Defendants have met their burden of showing the absence of a genuine issue of material fact on this matter, and summary judgment is granted in favor of Defendants as to Count II.

Order on Summary Judgment at 18. The district court seems to have construed Sierra Club's argument to have been that the EPA had a duty to approve the specific level of ranking assigned by Florida to each WQLS. The district court rejected that argument, holding that the EPA had no such duty. We need not address that issue, however, because we conclude that the district court misconstrued Sierra Club's argument.

While Sierra Club's argument does implicate the ultimate priority-ranking designation (i.e., "low," "medium," or "high") that Florida gave to some WQLSs, Sierra Club's true challenge is to the factors that Florida considered in assigning the designations. Sierra Club contends that Florida did not consider the "severity of the pollution" and the "uses to be made" of the WQLSs as required by 33 U.S.C. § 1313(d)(1)(A). The EPA acknowledges that "it must ensure that the state has in fact ranked its priorities taking into account the statutory factors." Br. for Federal Defendants-Appellees at 45 n. 14 (emphasis added); see also Decision Document at 8 ("In prioritizing and targeting waters, states must, at a minimum, take into account the severity of the pollution and the uses to be made of such waters. See § 303(d)(1)(A). As long as these factors are taken into account, the Act provides that states establish priorities."). In its review of Florida's 2002 List (which included the priority ranking), the EPA concluded that Florida "prioritized water quality limited segments for TMDL development according to the severity of the impairment and the designated uses of the segment." Decision Document at 49. It is this conclusion that Sierra Club challenges.

The resolution of this issue will involve an examination of whether the EPA's decision is supported by the evidence in the administrative record. We believe that the district court should consider the question in the first instance, particularly in light of the factual matters that it raises. We therefore vacate the entry of summary judgment on Sierra Club's second claim and remand the claim to the district court for further proceedings.

VII.

In Sierra Club's third claim for relief, Sierra Club challenges the EPA's approval of Florida's delisting of forty-five water-body/pollutant combinations that had been on Florida's 1998 List. According to Sierra Club, (A) thirty-eight of the challenged combinations involved water segments that had exceeded applicable water quality standards at least once in the preceding 7.5 years, and (B) seven of the challenged combinations involved waterbodies that were delisted because their violations of water quality standards were deemed the result of natural conditions.

Both sides cite an EPA guidance document and agree that a state may remove a waterbody from its impaired waters list if the "waterbody is meeting all applicable water quality standards . . . or is expected to meet these standards in a reasonable timeframe" or if "the original basis for listing is determined to be inaccurate." Guidance for 1994 Section 303(d) Lists, Nov. 26, 1993, Supp. Admin. R. G at 7. The parties disagree, however, about whether the delisted combinations fall under either of these two categories — Sierra Club argues that they do not; the EPA argues that they do.

A.

First, we address Sierra Club's assertion that the EPA acted arbitrarily or capriciously in approving Florida's delisting of thirty-eight waterbody/pollutant combinations in the face of data showing that the waterbodies involved were not meeting water quality standards. Under Florida's water quality standards, the water quality criteria applicable to a waterbody are "not to be exceeded at any time." Fla. Admin. Code Ann. r. 62-302.530. Sierra Club asserts that the challenged waterbody/pollutant combinations exceeded water quality criteria and standards several times in the 7.5 years immediately preceding the development of Florida's 2002 List. Thus, according to Sierra Club, the EPA erred in approving the delisting of the thirty-eight waterbody/pollutant combinations.

In the Decision Document, the EPA recognized that some of Florida's water quality criteria are "not to be exceeded at any time," but determined that it was reasonable for Florida to interpret that regulatory phrase in concert with the legislation authorizing the creation of Florida's water quality standards. Decision Document at 20. Specifically, the relevant statute states:

It is the intent of the Legislature that water quality standards be reasonably established and applied to take into account the variability occurring in nature. The [FDEP] shall recognize the statistical variability inherent in sampling and testing procedures that are used to express water quality standards. The [FDEP] shall also recognize that some deviations from water quality standards occur as the result of natural background conditions. The [FDEP] shall not consider deviations from water quality standards to be violations when the discharger can demonstrate that the deviations would occur in the absence of any human-induced discharges or alterations to the water body.

Fla. Stat. § 403.021(11). The EPA noted that because Florida does not have a monitoring program that continuously measures all points in its waterbodies (and thus the FDEP could never determine that a waterbody had not exceeded water quality criteria "at any time"), Florida must use statistical sampling to estimate a waterbody's compliance with water quality standards. Florida's Legislature recognized that sampling introduces variability into the testing process — some due to natural variability and some associated with sample collection and analysis. Thus, the EPA concluded, a single sample does not determine whether a waterbody fails to meet water quality standards. Instead, the EPA "considered a number of factors" in reviewing whether a waterbody was impaired. Decision Document at 21. "These factors included whether more recent data show attainment that renders earlier data suspect (trends); the magnitude of exceedance; the frequency of exceedance; pollutant levels during critical conditions; and any other site-specific data and information such as biological monitoring, whether new controls have been implemented on the water, etc." Id.

Like the district court, we find the EPA's "totality" approach reasonable. Sierra Club has presented no evidence suggesting that the approach is arbitrary, capricious, or in conflict with the CWA. Section 403.021(11) of the Florida Statutes mandates that statistical and natural variability be taken into account in applying water quality standards. Likewise, rule 62-302.530 of the Florida Administrative Code, while stating that the water quality criteria "express the maximum not to be exceeded at any time," also directs that "[i]n applying the water quality standards, the [FDEP] shall take into account the variability occurring in nature and shall recognize the statistical variability in herent [sic] in sampling and testing procedures." Fla. Admin. Code Ann. r. 62-302.530. Rule 62-302.530 of the Florida Administrative Code thus appears consistent with section 403.021(11) of the Florida Statutes, but to the extent the regulation could be read to conflict with the statute, the statute controls. See United States v. Marte, 356 F.3d 1336, 1341 (11th Cir.2004); Fla. Dept. of Revenue v. A. Duda Sons, Inc., 608 So.2d 881, 884 (Fla.Dist.Ct.App. 1992). We affirm the district court's entry of summary judgment for the EPA on the delisting of these thirty-eight waterbody/pollutant combinations.

B.

Next, Sierra Club asserts that the EPA acted arbitrarily or capriciously in approving Florida's delisting of seven waterbodies not meeting water quality standards due to natural conditions. In particular, Sierra Club claims that seven waterbodies with naturally occurring low dissolved oxygen levels were improperly delisted because the CWA does not provide a natural-conditions exception. The EPA disagrees and argues that in accordance with the CWA, it reviewed Florida's delisting decision for consistency with Florida's water quality standards. The CWA does not specifically address whether waterbodies not meeting water quality criteria because of naturally occurring conditions must be included on a state's impaired waters list. The EPA's interpretation of the CWA as not requiring such listings, however, is supported by a careful reading of the CWA and its regulations.

First, the CWA's express purpose is "to restore and maintain the chemical, physical, and biological integrity of the Nation's waters." 33 U.S.C. § 1251(a). The phrase "restore and maintain" indicates that Congress sought to return waterbodies to their natural conditions, not modify waterbodies' natural conditions. Indeed, the House Report on the legislation states that "[t]he word `integrity' as used is intended to convey a concept that refers to a condition in which the natural structure and function of ecosystems is maintained." H.R. Rep. No. 92-911, at 76 (1972).

Second, the provision of the CWA requiring that states compile impaired waters lists directs states to identify waters for which pollutant discharge limits "are not stringent enough to implement any water quality standard applicable to such waters." 33 U.S.C. § 1313(d)(1)(A). As the EPA recognized, to apply this provision, one must examine Florida's water quality standards applicable to the seven waterbodies at issue. Florida's water quality standards provide: "Waters having water quality below the criteria established for them shall be protected and enhanced. However, the [FDEP] shall not strive to abate natural conditions." Fla. Admin. Code. Ann. r. 62-302.300(15). Sierra Club argues that this standard only addresses whether Florida "will work on these waters," not whether the naturally impaired waters should be included on Florida's impaired waters list. Reply Br. for Plaintiffs-Appellants at 26. We do not accept Sierra Club's narrow interpretation of Florida's water quality standards. As noted above, a Florida statute that authorized the creation of Florida's water quality standards directs that the standards be applied in a manner that "take[s] into account the variability occurring in nature." Fla. Stat. § 403.021(11); see also Fla. Admin. Code Ann. r. 62-302.530 ("In applying the water quality standards, the Department shall take into account the variability occurring in nature. . . ."). Because "some deviations from water quality standards occur as the result of natural background conditions," the legislature ordered that the FDEP "shall not consider deviations from water quality standards to be violations when the discharger can demonstrate that the deviations would occur in the absence of any human-induced discharges or alterations to the water body." Fla. Stat. § 403.021(11). When Florida's water quality standards are read in conjunction with this guidance (as well as Florida's entire surface-water scheme), we conclude that waterbodies not meeting water quality standards solely because of natural conditions need not be placed on Florida's impaired waters list. We therefore agree with the district court's holding that the EPA's approval of the delisting of the seven waterbodies was not arbitrary or capricious, and we affirm the entry of summary judgment on Sierra Club's third claim.

VIII.

We affirm the denial of the FDEP's motion to intervene and the entry of summary judgment on Sierra Club's third claim. We reverse and vacate the entry of summary judgment on Sierra Club's first and second claims, and remand those claims for further proceedings consistent with this opinion. We deny the motions of the EPA and the FDEP to strike.

AFFIRMED IN PART; REVERSED, VACATED, AND REMANDED IN PART.