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.
I
As part of a consolidated lawsuit, respondent Natural Resources Defense Council (NRDC) sought a declaration that §301(1) of the Clean Water Act, 33 U. S. C. §1311(0, prohibited EPA from issuing “fundamentally different factor” (FDF) variances for pollutants listed as toxic under the Act. 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).
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, 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 factors 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. 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. 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.
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). Section 301(1), however, added by the 1977 amendments, provides:
“The Administrator may not modify any requirement of this section as it applies to any specific pollutant which is on the toxic pollutant list under section 307(a)(1) of this Act.” 91 Stat. 1590.
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 standards and BAT requirements. Under these regulations, EPA granted FDF variances, but infrequently.
As part of its consolidated lawsuit, respondent NRDC here challenged pretreatment standards for indirect dischargers and sought a declaration that § 301(1) barred any FDF variance with respect to toxic pollutants. In an earlier case, the Fourth Circuit had rejected a similar argument, finding that § 301(1) 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(1) forbids the issuance of FDF variances for toxic pollutants. National Assn. of Metal Finish ers 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.
II
Section 301(1) states that EPA may not “modify any requirement of § 301 insofar as toxic materials are concerned. EPA insists that §301(1) 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(1), 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).
A
NRDC insists that the language of § 301(1) 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(1) 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(1) 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(1) cannot be read to forbid every change in the toxic waste standards. The word “modify” thus has no plain meaning as used in §301(1), 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.
B
The legislative history of § 301(1) 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, 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. The bill did not contain § 301(1). 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. While the Conference Committee Report did not explain the reason for proposing §301(1), Representative Roberts, the House floor manager, stated:
“Due to the nature of toxic pollutants, those identified for regulation will not be subject to waivers from or modification of the requirements prescribed under this section, specifically, neither section 301(c) waivers based on the economic capability of the discharger nor 301(g) waivers based on water quality considerations shall be available.” Leg. Hist. 328-329 (emphasis added).
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, 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 Generate Transatlantique, 443 U. S. 256, 266-267 (1979).
NRDC argues that Congress’ discussion of the Act’s provisions supports its position. Several legislators’ comments seemed to equate “modifications” with “waivers” or “variances.” 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(1). 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(1).
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.
C
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.
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 Act 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. 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(1) 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, EPA must collect and analyze large amounts of technical information concerning complex industrial categories. 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.
HH HH HH
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(1) does not prohibit FDF variances. That interpretation gives the term “modify” a consistent meaning in §§ 301(c), (g), and (1), and draws support from the legislative evolution of §301(1) 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. 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.
The judgment of the Court of Appeals is reversed.
It is so ordered.
Hereinafter, the Clean Water Act will be referred to, interchangeably, by its entire name or simply as the Act.
EPA is required, under § 307(a)(1) of the Act, 33 U. S. C. § 1317(a)(1), to publish a list of toxic pollutants. Upon designation of a pollutant as toxic, § 307(a)(2), 33 U. S. C. § 1317(a)(2), requires EPA to set standards for its discharge.
See E. I. du Pont de Nemours & Co. v. Train, 430 U. S. 112, 121 (1977). BAT standards are set on the basis of categories and classes of sources under rules promulgated by the EPA under § 304(b), 33 U. S. C. § 1314(b). Although the statute indicated that BPT standards be established for point sources, rather than categories of sources, we held in Du Pont that the EPA could also set BPT limitations on the basis of classes and categories, as long as allowance was made for variations in individual plants through a variance procedure. 430 U. S., at 128.
Lawsuits by NRDC resulted in a consent decree placing EPA under deadlines for promulgating categorical pretreatment standards based on BPT and BAT criteria. NRDC v. Train, 8 ERC 2120, 6 Env. L. Rep. 20588 (DC 1976), modified sub nom. NRDC v. Costle, 12 ERC 1833, 9 Env. L. Rep. 20176 (DC 1979), modified sub nom. NRDC v. Gorsuch, No. 72-2153 (Oct. 26, 1982), modified sub nom. NRDC v. Ruckelshaus, No. 73-2153 (Aug. 2,1983), and 14 Env. L. Rep. 20185 (1984). In the 1977 amendments to the Act, Congress sanctioned this approach to establishing pretreatment standards for indirect dischargers. Environmental Defense Fund, Inc. v. Costle, 205 U. S. App. D. C. 101, 115-116, 636 F. 2d 1229, 1243-1244 (1980).
The factors relating to the assessment of BAT standards, set out in § 304(b)(2)(B) of the Act, include the age of equipment and facilities involved, the process employed, the engineering aspects of the application of various types of control techniques, the cost of achieving effluent reduction, and nonwater-quality environmental impacts. 33 U. S. C. § 1314(b)(2)(B).
See n. 4, supra.
The challenged FDF variance regulation with respect to indirect dischargers, 40 CFR §403.13 (1984), provides in relevant part:
“§403.13 Variances from categorical pretreatment standards for fundamentally different factors.
“(a) Definition. The term ‘Requester’ means an Industrial User or a [publicly owned treatment work] or other interested person seeking a variance from the limits specified in a categorical Pretreatment Standard.
“(b) Purpose and scope. (1) In establishing categorical Pretreatment Standards for existing sources, the EPA will take into account all the information it can collect, develop and solicit regarding the factors relevant to pretreatment standards under section 307(b). In some cases, information which may affect these Pretreatment Standards will not be available or, for other reasons, will not be considered during their development. As a result, it may be necessary on a case-by-case basis to adjust the limits in categorical Pretreatment Standards, making them either more or less stringent, as they apply to a certain Industrial User within an industrial category or subcategory. This will only be done if data specific to that Industrial User indicates it presents factors fundamentally different from those considered by EPA in developing the limit at issue. Any interested person believing that factors relating to an Industrial User are fundamentally different from the factors considered during development of a categorical Pretreatment Standard applicable to that User and further, that the existence of those factors justifies a different discharge limit from that specified in the applicable categorical Pretreatment Standard, may request a fundamentally different factors variance under this section or such a variance request may be initiated by the EPA.
“(c) Criteria — (1) General Criteria. A request for a variance based upon fundamentally different factors shall be approved only if:
“(i) There is an applicable categorical Pretreatment Standard which specifically controls the pollutant for which alternative limits have been requested; and
“(ii) Factors relating to the discharge controlled by the categorical Pretreatment Standard are fundamentally different from the factors considered by EPA in establishing the Standards; and
“(iii) The request for a variance is made in accordance with [applicable procedural requirements].
“(2) Criteria applicable to less stringent limits. A variance request for the establishment of limits less stringent than required by the Standard shall be approved only if:
“(i) The alternative limit requested is no less stringent than justified by the fundamental difference;
“(ii) The alternative limit will not result in a violation of prohibitive discharge standards prescribed by or established under § 403.5;
“(iii) The alternative limit will not result in a non-water quality environmental impact (including energy requirements) fundamentally more adverse than the impact considered during development of the Pretreatment Standards; and
“(iv) Compliance with the Standards (either by using the technologies upon which the Standards are based or by using other control alternatives) would result in either:
“(A) A removal cost (adjusted for inflation) wholly out of proportion to the removal cost considered during development of the Standards; or
“(B) A non-water quality environmental impact (including energy requirements) fundamentally more adverse than the impact considered during development of the Standards.
“(3) Criteria applicable to more stringent limits. A variance request for the establishment of limits more stringent than required by the Standards shall be approved only if:
“(i) The alternative limit request is no more stringent than justified by the fundamental difference; and
“(ii) Compliance with the alternative limit would not result in either:
“(A) A removal cost (adjusted for inflation) wholly out of proportion to the removal cost considered during development of the Standards; or
“(B) A non-water quality environmental impact (including energy requirements) fundamentally more adverse than the impact considered during development of the Standards.
ed) Factors considered fundamentally different. Factors which may be considered fundamentally different are:
“(1) The nature or quality of pollutants contained in the raw waste load of the User’s process wastewater:
“(2) The volume of the User’s process wastewater and effluent discharged;
“(3) Non-water quality environmental impact of control and treatment of the User’s raw waste load;
“(4) Energy requirements of the application of control and treatment technology;
“(5) Age, size, land availability, and configuration as they relate to the User’s equipment or facilities; processes employed; process changes; and engineering aspects of the application of control technology;
“(6) Cost of compliance with required control technology.
“(e) Factors which will not be considered fundamentally different. A variance request or portion of such a request under this section may not be granted on any of the following grounds:
“(1) The feasibility of installing the required waste treatment equipment within the time the Act allows;
“(2) The assertion that the Standards cannot be achieved with the appropriate waste treatment facilities installed, if such assertion is not based on factors listed in paragraph (d) of this section;
“(3) The User’s ability to pay for the required waste treatment; or
“(4) The impact of a Discharge on the quality of the [publicly owned treatment works’] receiving waters.”
The regulation also provides for public notice of the FDF application and opportunity for public comments and a public hearing. EPA has promulgated an analogous provision for direct dischargers, 40 CFR § 125.30 (1984).
Sources subject to new source performance standards (NSPS) under the Act are those who begin construction after the publication of proposed new source standards, 33 U. S. C. § 1316, and they are ineligible for FDF variances. See 40 CFR § 403.13(b) (1984).
33U. S. C. §§ 1311(c) and (g). Those provisions explain in relevant part: “(c) The Administrator may modify the requirements of [§ 301’s effluent limitations] with respect to any point source for which a permit application is filed after July 1, 1977, upon a showing by the owner or operator of such point source satisfactory to the Administrator that such modified requirements (1) will represent the maximum use of technology within the economic capability of the owner or operator; and (2) will result in reasonable further progress toward the elimination of the discharge of pollutants.
“(g)(1) The Administrator, with the concurrence of the State, shall modify the requirements of [§ 301’s effluent limitations] with respect to the discharge of any pollutant (other than pollutants identified pursuant to section 1314(a)(4) of this title, toxic pollutants subject to section 1317(a) of this title, and the thermal component of discharges) from any point soured upon a showing by the owner or operator of such a point source satisfactory to the Administrator that—
“(C) such modification will not interfere with the attainment or maintenance of that water quality which shall assure protection of public water supplies, and the protection and propagation of a balanced population of shellfish, fish, and wildlife, and allow recreational activities, in and on the water and such modification will not result in the discharge of pollutants in quantities which may reasonably be anticipated to pose an unacceptable risk to human health or the environment. . . .”
EPA and NRDC appear to be at odds as to whether § 301(c) and § 301(g) modifications are available to indirect dischargers as well as direct dis-chargers. Compare Brief for EPA 33, n. 23, and Reply Brief for EPA 2-3, with Brief for NRDC 29, and n. 41. Resolution of the seeming disagreement is not necessary to adjudicate these cases.
40 CFR § 403.13 (1984). This variance regulation was issued on June 26, 1978, 43 Fed. Reg. 27736-27773, and amended on January 28, 1981, 46 Fed. Reg. 9404-9460. The 1978 regulation differed in respects not relevant here.
See 44 Fed. Reg. 32854, 32893-32894 (1979).
NRDC áeknowledges the limited availability of FDF variances. Brief for NRDC in Opposition 7-8. By 1977, only 50 of 4,000 major industrial dischargers covered by BPT limits had applied for FDF variances, and only two variances had been granted. Id., at 12. By 1984, a total of four FDF variances had been granted to direct dischargers, and none had been granted to an indirect discharger. EPA estimates that in the entire country approximately 40 FDF variance requests filed by indirect dischargers are still pending. Brief for EPA 36, n. 28.
In the Court of Appeals, NRDC also argued that EPA had neither statutory nor inherent authority to issue FDF variances from either BAT or pretreatment standards even when toxic pollutants were not involved. The court below did not reach this argument, National Assn, of Metal Finishers v. EPA, 719 F. 2d 624, 643-645 (1983), and we need not address it. For present purposes, we assume, without deciding, that EPA would have authority under the Act to issue the FDF variances in question here absent the provisions of § 301(1).
Citations to the legislative history (Leg. Hist.) are to Senate Committee on Environment and Public Works, A Legislative History of the Clean Water Act of 1977, prepared by the Environmental Policy Division of the Congressional Research Service of the Library of Congress (Comm. Print 1978).
The 1977 House bill to amend the Clean Water Act contained no comparable water quality waiver provision. H. R. 3199, 95th Cong., 1st Sess. (1977), Leg. Hist. Il67.
In view of §301(1), the ban on toxic waste waivers in § 301(g) was unnecessary. But there can be no doubt that § 301(1) forbade §§ 301(c) and (g) modifications for toxic materials, and the presence of a similar ban in § 301(g) lends little support for the notion that § 301(1) forbids FDF variances.
A representative of NRDC testified before Congress that a “fundamental variance provision” was integral to the Act’s system of “national, uniform, minimum effluent limitations.” See Federal Water Pollution Control Act Amendments of 1977, Hearings before the Subcommittee on Environmental Pollution, Senate Committee on Environment and Public Works, 95th Cong., 1st Sess., Ser. No. 95-H25, pt. 9, p. 37 (1977).
There is other evidence that both this Court’s decision in Du Pont and an earlier decision of the Fourth Circuit approving variances that took all statutory factors into account in Appalachian Power Co. v. Train, 545 F. 2d 1351 (1976), were brought to the attention of Congress during the debates on the 1977 amendments. Referring to a Library of Congress report, Representative Clausen, ranking minority member of the Subcommittee on Water Resources, stated during the House debate on the Conference Report to the final 1977 amendments that “full understanding of [the 1972 Clean Water Act amendments] can only be achieved by having an understanding of the case law interpreting the public law.” 123 Cong. Rec. 38976 (1977), Leg. Hist. 374. The Library of Congress report Senator Clausen referred to specifically discussed both Du Pont and Appalachian Power. See Case Law Under the Federal Water Pollution Control Act Amendments of 1972 (Committee Print compiled for the House Committee on Public Works and Transportation by the Library of Congress), Ser. No. 95-35, pp. 20, 28 (1977).
NRDC suggests that eliminating FDF variances would not overrule Dú Pont, because the rationale for Du Pont’s holding applied only to BPT standards. Since BPT standards were due to be phased out, NRDC suggests, Congress had no reason to address Du Pont’s requirements of FDF waivers. Even if we were to accept NRDC’s narrow reading of Du Pont —and we recognize that the Du Pont opinion arguably applies to BAT standards as well, 430 U. S., at 128; Brief for EPA 20-21 — this argument ignores that the BPT regulations at issue in Du Pont contained a variance clause, and applied to pollutants that Congress declared toxic in the 1977 amendments. See, e. g., 40 CFR §§416.62 and 415.172 (1976). Moreover, BPT standards remain in effect even today. For many industries— as a result of a consent decree authored in relevant part by NRDC — EPA is required to promulgate BPT level pretreatment standards as an interim measure pending development of potentially more technology-forcing BAT standards. See NRDC v. Train, 8 ERC, at 2128, 6 Env. L. Rep., at 20688. The electroplating pretreatment standards unsuccessfully challenged in the consolidated lawsuit below were one such regulation.
See, e.g., S. Rep. No. 96-370, p. 44 (1978), Leg. Hist. 677; Sen. Muskie, 123 Cong. Rec. 39183 (1977), Leg. Hist. 468; Rep. Roberts, 123 Cong. Rec. 38959-38961 (1977), Leg. Hist. 305.
As EPA itself has explained:
“No discharger . . . may be excused from the Act’s requirement to meet ... a pretreatment standard through this variance clause. A discharger may instead receive an individualized definition of such a . . . standard where the nationally prescribed limit is shown to be more or less stringent than appropriate for the discharger under the Act.” 44 Fed. Reg. 32854, 32893 (1979).
33 U. S. C. § 1317(b)(2).
In the aftermath of the decision by the Court of Appeals below, EPA announced that it would entertain petitions for amended rulemaking by certain indirect dischargers previously eligible for FDF variances, explaining that in such cases “it may be appropriate to issue specific categorical standards for such facilities, treating them as a separate subcategory with more, or less, stringent standards as appropriate.” 48 Fed. Reg. 52396 (1983).
EPA was directed by § 304(g) of the Act, 33 U. S. C. § 1314(g), to publish promptly guidelines for the establishment of pretreatment standards and categories. As with the statutory deadlines for the setting of guidelines for direct dischargers, 33 U. S. C. § 1314(c), the time limits proved beyond the Agency’s capability. As a result of lawsuits brought by NRDC, EPA has been placed under court-ordered deadlines for promulgating effluent limitations. See n. 4, supra.
Typically, EPA must engage in an extensive data-collection effort, compiling information on the pollutants discharged by an industry, the process employed, the treatment technologies used by the industry or available for use, the treatability of the pollutants, and the economics of the industry. Often, the data indicate differences among segments of the industry, and EPA will establish subcategories to reflect those differences in the effluent limitations and standards that are promulgated.
The scope of the task of formulating national categorical standards is illustrated by the procedures followed by EPA in developing the BPT-level pretreatment standards for electroplating, which were unsuccessfully challenged in the consolidated lawsuit below. Of the 500 plants identified as potentially within the category of sources and sent questionnaires by EPA, approximately 200 provided some of the requested information. EPA conducted on-site visits of 82 of these in order to take samples of raw and treated waste water over several days, inspect treatment technology already in place, and collect other firsthand information. From these visits, EPA determined that 25 of the plants were representative in treatment technology, character of raw waste water, and other factors. The data from these plants were then used to derive achievable effluent limitations, using a combination of statistical methodologies and engineering judgments. Brief for EPA 5, n. 3.
■ The FDF variances at issue here are available only for sources fundamentally different in a way which would have required EPA to place them initially in a separate category had their situation been considered. EPA v. National Crushed Stone Assn., 449 U. S. 64, 77-78 (1980). Particularly in light of the limited availability of FDF variances, see n. 12, supra, and the requirement that such variances are permissible only when standards were originally set after Considering a range of facilities which did not include those similar to the source covered by the requested variance, we harbor no fear that the variance scheme will lead to the breakdown of the categorical approach taken by Congress, so long as the EPA, as it is required, grants variances only for sources fundamentally different. 40 CFR § 403.13(b) (1984). This does not allow EPA to single out for different treatment the least or most efficient plants legitimately within a category that was drawn after considering the relevant range of factors.
In the aftermath of Du Pont, Congress well may have chosen to allow the FDF variance procedure in order to ensure that the Act’s pretreatment standards were not overturned. This Court has previously upheld regulations in part because of a provision for an exception or variance helped assure the parties of due process. See United States v. Allegheny-Ludlum Steel Corp., 406 U. S. 742, 755 (1972); FPC v. Texaco, Inc., 377 U. S. 33, 40-41 (1964); United States v. Storer Broadcasting Co., 351 U. S. 192, 205 (1956). Other courts have found that the FDF variance procedure is critical to EPA’s promulgation of treatment requirements of existing sources. See, e. g., Kennecott Copper Corp. v. EPA, 612 F. 2d 1232, 1243-1244 (CA10 1979) (upholding regulations challenged for failure to take the statutory factors into account across the industry, since FDF variance procedures were available to apply those factors to fundamentally different plants); Weyerhaeuser Co. v. Costle, 191 U. S. App. D. C. 309, 338-339, 590 F. 2d 1011, 1040-1041 (1978) (upholding the promulgation of industrywide effluent limitations because the “crucial” variance mechanism provided the necessary flexibility).
See n. 10, supra.
See n. 12, supra.
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