JUSTICE WHITE delivered the opinion of the Court.
These cases present the question whether the Environmental Protection Agency (EPA) may issue certain variances from toxic pollutant effluent limitations promulgated under the Clean Water Act, 86 Stat. 816, as amended, 33 U. S. C. § 1251 et seq.1
As part of a consolidated lawsuit, respondent Natural Resources Defense Council (NRDC) sought a declaration that § 301(l) of the Clean Water Act, 33 U. S. C. § 1311(l), prohibited EPA from issuing "fundamentally different factor" (FDF) variances for pollutants listed as toxic under the Act.2 Petitioners EPA and Chemical Manufacturers Association (CMA) argued otherwise. To understand the nature of this controversy, some background with respect to the statute and the case law is necessary.
The Clean Water Act, the basic federal legislation dealing with water pollution, assumed its present form as the result of extensive amendments in 1972 and 1977. For direct dischargers — those who expel waste directly into navigable waters — the Act calls for a two-phase program of technology-based effluent limitations, commanding that dischargers comply with the best practicable control technology currently available (BPT) by July 1, 1977, and subsequently meet the generally more stringent effluent standard consistent with the best available technology economically achievable (BAT).3
Indirect dischargers — those whose waste water passes through publicly owned treatment plants — are similarly required to comply with pretreatment standards promulgated by EPA under § 307 of the Act, 33 U. S. C. § 1317(b), for pollutants not susceptible to treatment by sewage systems or which would interfere with the operation of those systems. Relying upon legislative history suggesting that pretreatment standards are to be comparable to limitations for direct dischargers, see H. R. Rep. No. 95-830, p. 87 (1977), and pursuant to a consent decree,4 EPA has set effluent limitations for indirect dischargers under the same two-phase approach applied to those discharging waste directly into navigable waters.
Thus, for both direct and indirect dischargers, EPA considers specific statutory factors5 and promulgates regulations creating categories and classes of sources and setting uniform discharge limitations for those classes and categories. Since application of the statutory factors varies on the basis of the industrial process used and a variety of other factors, EPA has faced substantial burdens in collecting information adequate to create categories and classes suitable for uniform effluent limits, a burden complicated by the time deadlines it has been under to accomplish the task.6 Some plants may find themselves classified within a category of sources from which they are, or claim to be, fundamentally different in terms of the statutory factors. As a result, EPA has developed its FDF variance as a mechanism for ensuring that its necessarily rough-hewn categories do not unfairly burden atypical plants.7 Any interested party may seek an FDF variance to make effluent limitations either more or less stringent if the standards applied to a given source, because of factors fundamentally different from those considered by EPA in setting the limitation, are either too lenient or too strict.8
The 1977 amendments to the Clean Water Act reflected Congress' increased concern with the dangers of toxic pollutants. The Act, as then amended, allows specific statutory modifications of effluent limitations for economic and water-quality reasons in § 301(c) and (g).9 Section 301(l), however, added by the 1977 amendments, provides:
In the aftermath of the 1977 amendments, EPA continued its practice of occasionally granting FDF variances for BPT requirements. The Agency also promulgated regulations explicitly allowing FDF variances for pretreatment standards10 and BAT requirements.11 Under these regulations, EPA granted FDF variances, but infrequently.12
As part of its consolidated lawsuit, respondent NRDC here challenged pretreatment standards for indirect dischargers and sought a declaration that § 301(l) barred any FDF variance with respect to toxic pollutants.13 In an earlier case, the Fourth Circuit had rejected a similar argument, finding that § 301(l) was ambiguous on the issue of whether it applied to FDF variances and therefore deferring to the administrative agency's interpretation that such variances were permitted. Appalachian Power Co. v. Train,620 F.2d 1040, 1047-1048 (1980). Contrariwise, the Third Circuit here ruled in favor of NRDC, and against petitioners EPA and CMA, holding that § 301(l) forbids the issuance of FDF variances for toxic pollutants. National Assn. of Metal Finishers v. EPA,719 F.2d 624 (1983). We granted certiorari to resolve this conflict between the Courts of Appeals and to decide this important question of environmental law. 466 U.S. 957 (1984). We reverse.
Section 301(l) states that EPA may not "modify" any requirement of § 301 insofar as toxic materials are concerned. EPA insists that § 301(l) prohibits only those modifications expressly permitted by other provisions of § 301, namely, those that § 301(c) and § 301(g) would allow on economic or water-quality grounds. Section 301(l), it is urged, does not address the very different issue of FDF variances. This view of the agency charged with administering the statute is entitled to considerable deference; and to sustain it, we need not find that it is the only permissible construction that EPA might have adopted but only that EPA's understanding of this very "complex statute" is a sufficiently rational one to preclude a court from substituting its judgment for that of EPA. Train v. NRDC,421 U.S. 60, 75, 87 (1975); see also Chevron U. S. A. Inc. v. NRDC,467 U.S. 837 (1984). Of course, if Congress has clearly expressed an intent contrary to that of the Agency, our duty is to enforce the will of Congress. Chevron, supra, at 843, n. 9; SEC v. Sloan,436 U.S. 103, 117-118 (1978).
NRDC insists that the language of § 301(l) is itself enough to require affirmance of the Court of Appeals, since on its face it forbids any modifications of the effluent limitations that EPA must promulgate for toxic pollutants. If the word "modify" in § 301(l) is read in its broadest sense, that is, to encompass any change or alteration in the standards, NRDC is correct. But it makes little sense to construe the section to forbid EPA to amend its own standards, even to correct an error or to impose stricter requirements. Furthermore, reading § 301(l) in this manner would forbid what § 307(b)(2) expressly directs: EPA is there required to "revise" its pretreatment standards "from time to time, as control technology, processes, operating methods, or other alternatives change." As NRDC does and must concede, Tr. of Oral Arg. 25-26, § 301(l) cannot be read to forbid every change in the toxic waste standards. The word "modify" thus has no plain meaning as used in § 301(l), and is the proper subject of construction by EPA and the courts. NRDC would construe it to forbid the kind of alteration involved in an FDF variance, while the Agency would confine the section to prohibiting the partial modifications that § 301(c) would otherwise permit. Since EPA asserts that the FDF variance is more like a revision permitted by § 307 than it is like a § 301(c) or (g) modification, and since, as will become evident, we think there is a reasonable basis for such a position, we conclude that the statutory language does not foreclose the Agency's view of the statute. We should defer to that view unless the legislative history or the purpose and structure of the statute clearly reveal a contrary intent on the part of Congress. NRDC submits that the legislative materials evince such a contrary intent. We disagree.
The legislative history of § 301(l) is best understood in light of its evolution. The 1972 amendments to the Act added § 301(c), which allowed EPA to waive BAT and pretreatment requirements on a case-by-case basis when economic circumstances justified such a waiver. Pub. L. 92-500, 86 Stat. 845. In 1977, the Senate proposed amending § 301(c) by prohibiting such waivers for toxic pollutants. See S. 1952, 92d Cong., 2d Sess., 30 (1977), Leg. Hist. 584,14 and S. Rep. No. 95-370, p. 44 (1977), Leg. Hist. 677. At the same time, the Senate bill added what became § 301(g), which allowed waivers from BAT and pretreatment standards where such waivers would not impair water quality, but which, like § 301(c), prohibited waivers for toxic pollutants. S. 1952, at 28-29, Leg. Hist. 582-583.15 The bill did not contain § 301(l). That section was proposed by the Conference Committee, which also deleted the toxic pollutant prohibition in § 301(c) and redrafted § 301(g) to prohibit water-quality waivers for conventional pollutants and thermal discharges as well as for toxic pollutants.16 While the Conference Committee Report did not explain the reason for proposing § 301(l), Representative Roberts, the House floor manager, stated:
Another indication that Congress did not intend to forbid FDF waivers as well as § 301(c) and (g) modifications is its silence on the issue. Under NRDC's theory, the Conference Committee did not merely tinker with the wording of the Senate bill, but boldly moved to eliminate FDF variances. But if that was the Committee's intention, it is odd that the Committee did not communicate it to either House, for only a few months before we had construed the Act to permit the very FDF variance NRDC insists the Conference Committee was silently proposing to abolish. In E. I. du Pont de Nemours & Co. v. Train,430 U.S. 112 (1977), we upheld EPA's class and category effluent limitations, relying on the availability of FDF waivers. Id., at 128. Congress was undoubtedly aware of Du Pont,17 and absent an expression of legislative will, we are reluctant to infer an intent to amend the Act so as to ignore the thrust of an important decision. Edmonds v. Compagnie Generale Transatlantique,443 U.S. 256, 266-267 (1979).18
NRDC argues that Congress' discussion of the Act's provisions supports its position. Several legislators' comments seemed to equate "modifications" with "waivers" or "variances."19 Many of these statements, however, came in the specific context of discussing the "waiver" provisions of § 301(c) and (g), not the prohibition in § 301(l). See, e. g., 123 Cong. Rec. 39183-39184 (1977), Leg. Hist. 458, 461 (Sen. Muskie); 123 Cong. Rec. 38961 (1977), Leg. Hist. 331 (Rep. Roberts); S. Rep. No. 95-370, pp. 40-44, Leg. Hist. 673-677 (discussing water-quality based modifications). Simply because Members of Congress or Committees referred to modifications authorized by § 301(c) and (g) as "variance" provisions, does not mean that FDF variances are also modifications barred by § 301(l).
After examining the wording and legislative history of the statute, we agree with EPA and CMA that the legislative history itself does not evince an unambiguous congressional intention to forbid all FDF waivers with respect to toxic materials. Chevron, 467 U. S., at 842-843, and n. 9.
Neither are we convinced that FDF variances threaten to frustrate the goals and operation of the statutory scheme set up by Congress. The nature of FDF variances has been spelled out both by this Court and by the Agency itself. The regulation explains that its purpose is to remedy categories which were not accurately drawn because information was either not available to or not considered by the Administrator in setting the original categories and limitations. 40 CFR § 403.13(b) (1984). An FDF variance does not excuse compliance with a correct requirement, but instead represents an acknowledgment that not all relevant factors were taken sufficiently into account in framing that requirement originally, and that those relevant factors, properly considered, would have justified — indeed, required — the creation of a subcategory for the discharger in question. As we have recognized, the FDF variance is a laudable corrective mechanism, "an acknowledgment that the uniform . . . limitation was set without reference to the full range of current practices, to which the Administrator was to refer." EPA v. National Crushed Stone Assn.,449 U.S. 64, 77-78 (1980). It is, essentially, not an exception to the standard-setting process, but rather a more fine-tuned application of it.20
We are not persuaded by NRDC's argument that granting FDF variances is inconsistent with the goal of uniform effluent limitations under the Act. Congress did intend uniformity among sources in the same category, demanding that "similar point sources with similar characteristics . . . meet similar effluent limitations," S. Rep. No. 92-1236, p. 126 (1972). EPA, however, was admonished to take into account the diversity within each industry by establishing appropriate subcategories. Leg. Hist. 455.
NRDC concedes that EPA could promulgate rules under § 307 of the Act21 creating a subcategory for each source which is fundamentally different from the rest of the class under the factors the EPA must consider in drawing categories. The same result is produced by the issuance of an FDF variance for the same failure properly to subdivide a broad category.22 Since the dispute is therefore reduced to an argument over the means used by EPA to define subcategories of indirect dischargers in order to achieve the goals of the Act, these are particularly persuasive cases for deference to the Agency's interpretation. Cf. Vermont Yankee Nuclear Power Corp. v. NRDC,435 U.S. 519, 543 (1978); NLRB v. Bell Aerospace Co.,416 U.S. 267, 293 (1974).
NRDC argues, echoing the concern of the Court of Appeals below, that allowing FDF variances will render meaningless the § 301(l) prohibition against modifications on the basis of economic and water-quality factors. That argument ignores the clear difference between the purpose of FDF waivers and that of § 301(c) and (g) modifications, a difference we explained in National Crushed Stone. A discharger that satisfies the requirements of § 301(c) qualifies for a variance "simply because [it] could not afford a compliance cost that is not fundamentally different from those the Administrator has already considered" in creating a category and setting an effluent limitation. 449 U. S., at 78. A § 301(c) modification forces "a displacement of calculations already performed, not because those calculations were incomplete or had unexpected effects, but only because the costs happened to fall on one particular operator, rather than on another who might be economically better off." Ibid. FDF variances are specifically unavailable for the grounds that would justify the statutory modifications. 40 CFR § 403.13(e)(3) and (4) (1984). Both a source's inability to pay the foreseen costs, grounds for a § 301(c) modification, and the lack of a significant impact on water quality, grounds for a § 301(g) modification, are irrelevant under FDF variance procedures. Ibid.; see also Crown Simpson Pulp Co. v. Costle,642 F.2d 323 (CA9), cert. denied, 454 U.S. 1053 (1981).
EPA and CMA point out that the availability of FDF variances makes bearable the enormous burden faced by EPA in promulgating categories of sources and setting effluent limitations. Acting under stringent timetables,23 EPA must collect and analyze large amounts of technical information concerning complex industrial categories.24 Understandably, EPA may not be apprised of and will fail to consider unique factors applicable to atypical plants during the categorical rulemaking process, and it is thus important that EPA's nationally binding categorical pretreatment standards for indirect dischargers be tempered with the flexibility that the FDF variance mechanism offers, a mechanism repugnant to neither the goals nor the operation of the Act.25
Viewed in its entirety, neither the language nor the legislative history of the Act demonstrates a clear congressional intent to forbid EPA's sensible variance mechanism for tailoring the categories it promulgates. In the absence of a congressional directive to the contrary, we accept EPA's conclusion that § 301(l) does not prohibit FDF variances. That interpretation gives the term "modify" a consistent meaning in § 301(c), (g), and (l), and draws support from the legislative evolution of § 301(l) and from congressional silence on whether it intended to forbid FDF variances altogether and thus to obviate our decision in Du Pont.
Here we are not dealing with an agency's change of position with the advent of a different administration, but rather with EPA's consistent interpretation since the 1970's.26 NRDC argues that its construction of the statute is better supported by policy considerations. But we do not sit to judge the relative wisdom of competing statutory interpretations. Here EPA's construction, fairly understood, is not inconsistent with the language, goals, or operation of the Act. Nor does the administration of EPA's regulation undermine the will of Congress.27
The judgment of the Court of Appeals is reversed.
It is so ordered.