dissenting.
In these cases, the Environmental Protection Agency (EPA) maintains that it may issue, on a case-by-case basis, individualized variances from the national standards that limit the discharge of toxic water pollutants. EPA asserts *135this power in the face of a provision of the Clean Water Act that expressly withdraws from the agency the authority to “modify” the national standards for such pollutants. The Court today defers to EPA’s interpretation of the Clean Water Act even though that interpretation is inconsistent with the clear intent of Congress, as evidenced by the statutory language, history, structure, and purpose. I had not read our cases to permit judicial deference to an agency’s construction of a statute when that construction is inconsistent with the clear intent of Congress.
I
The Clean Water Act requires the EPA Administrator to regulate two types of industrial facilities: (1) “direct” dischargers, i. e., facilities that discharge waste water directly into navigable waters; and (2) “indirect” dischargers, i. e., facilities that discharge waste water into publicly owned treatment works prior to discharge into navigable waters. For both types of requirements, EPA conducts rulemaking proceedings and promulgates nationwide, categorical limitations, that is, limitations applicable to categories of discharg-ers (e. g., iron and steel plants).
The Act provides for the phased implementation of progressively more stringent requirements for direct discharg-ers. By July 1, 1977, existing direct dischargers were required to meet effluent limitations based on the “best practicable control technology currently available” (BPT). § 301(b)(1)(A), 86 Stat. 844, 33 U. S. C. § 1311(b)(1)(A). By July 1, 1984, such dischargers were obligated to meet limitations based on the “best available technology economically achievable” (BAT). § 301(b)(2)(A).1
Indirect dischargers are subject to “pretreatment” standards applicable to pollutants, including toxic pollutants, that *136are not susceptible to treatment by or would interfere with the operation of public treatment facilities. § 307(b). Pursuant to a consent decree, EPA has set limitations on existing indirect dischargers using the same two-phase scheme used for direct dischargers. See ante, at 119. Thus, pretreatment standards for existing indirect dischargers are set by reference to BPT and BAT levels.
In 1978, EPA issued pretreatment regulations that contained a variance provision for “fundamentally different factors” (FDF). See 43 Fed. Reg. 27757 (1978). An FDF variance is a case-by-case adjustment of the relevant nationwide standard. See 40 CFR §403.13(b)(1) (1984). A discharger may obtain such a variance if the factors relating to its discharges are fundamentally different from those taken into account by EPA in setting the nationwide standard. § 403.13(c)(ii).
In a petition for review filed in the Court of Appeals for the Third Circuit, respondent NRDC challenged the FDF variance provision on two grounds. First, it argued that EPA lacked the inherent authority to issue such variances. Second, it argued that even if, in general, EPA had the authority to grant such variances, it could not do so in the case of toxic pollutants, because § 301(1), which was enacted as part of the 1977 amendments to the Act, bans all “modifications” from the toxic standards. The Third Circuit agreed with the latter argument, holding that § 301(1) prohibits FDF variances in the case of toxic pollutants. National Assn. of Metal Finishers v. EPA, 719 F. 2d 624, 644-646 (1983).2 The *137court remanded the variance provision back to EPA without considering the question of EPA’s inherent authority to grant such variances.3
EPA advances — and the Court defers to — two independent statutory constructions in support of its position that § 301(1) does not ban FDF variances from the toxic standards. First, EPA argues that § 301(1) prohibits only modifications otherwise expressly allowed by two other statutory provisions — §§ 301(c) and (g) — and thus does not apply to FDF variances, which are nonstatutory. The plain meaning of §301(1), the changes made prior to enactment to the bill containing this provision, and the clearly expressed congressional objectives in enacting §301(1) — to deal vigorously and comprehensively with the extremely serious environmental problem caused by toxic pollutants — establish that this provision’s scope was meant to be considerably broader than that attributed to it by EPA. As part of its effort to strengthen the control of toxic pollutants, Congress clearly intended to prohibit all exceptions to the nationwide, categorical standards.
Second, in a strained attempt to characterize the challenged variances in a way that would bring them outside the scope of the § 301(1) prohibition, EPA contends that the case-by-case FDF variance procedure provides a permissible alternative to the statutory mechanism for “revising” standards. The Court defers to this argument, and in so doing, it ignores the relevance of the central feature of the 1972 amendments to the Act — that Congress pointedly determined that water pollution control standards should take the form of general rules, to apply uniformly to categories of dischargers. As a result, the Court validates outcomes substantially less protective of the environment than those *138mandated by Congress. The only view of FDF variances consistent with the scheme of the Clean Water Act is that they are individual exceptions that soften the hardship of general rules. As such, they are undoubtedly disallowed by §301(1).
These cases are not about whether exceptions are useful adjuncts to regulatory schemes of general applicability. That is a policy choice on which courts should defer to Congress in the first instance, and to the administrative agency in the absence of a clear congressional mandate. Here, Congress has made the policy choice. It has weighed competing goals and determined that, whatever the general merits of exceptions schemes, they are simply inappropriate in the context of the control of toxic water pollution. As a result, an exceptions scheme such as the one challenged here simply cannot stand.
II
I first consider EPA’s argument that §301(1) proscribes only those modifications otherwise authorized by §§ 301(c) and (g). Under these provisions, EPA can “modify” the categorical standard if a discharger makes an adequate showing that such a standard is not within the discharger’s economic capability and that a less stringent standard would nonetheless result in reasonable environmental progress, § 301(c),4 or that a less stringent standard adequately pro*139tects water quality, § 301(g).5 This limited view of §301(l)’s scope is clearly inconsistent with congressional intent; the plain meaning of the statute and its legislative history show a clear congressional intent to ban all “modifications.”
A
Section 301(1) 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, 33 U. S. C. § 1311(Z).
The statute does not define either “modify” or “modification.” The phrase “may not modify any requirement,” however, expressly proscribes all “modifications” of the standards for toxics. Nothing on the face of the statute suggests that Congress intended that qualifying language be read into this prohibition. On the contrary, the prohibition is unqualified.
EPA’s argument that § 301(1) bans only those modifications otherwise authorized by §§ 301(c) and (g) is therefore incon*140sistent with the plain meaning of the statute. By its terms, the statutory prohibition has universal scope, not the limited scope attributed to it by EPA.
B
Moreover, the legislative history demonstrates that Congress meant what it said, and it evidences a clear congressional intent to ban all “modifications.” First, the legislative history firmly establishes that § 301(1) was enacted as part of a program to deal effectively and comprehensively with the problem of toxic pollutants, and that its prohibition was an integral part of this program. Under any canon of statutory construction, the congressional purposes in enacting a provision would be deemed relevant to the question of the scope of that provision, but the Court simply fails to discuss this issue.
In 1977, when it enacted the amendments to the Clean Water Act containing §301(1), Congress regarded the problem of toxic pollution as a very serious one. For example, Senator Muskie, the major drafter and Senate manager of the bill containing § 301(1), remarked:
“The seriousness of the toxics problem is just beginning to be understood. New cases are reported each day of unacceptable concentrations of materials in the aquatic environment, in fish and shellfish, and even in mother’s milk. Empirical evidence has shown a statistical correlation between materials in New Orleans’ drinking water and cancer mortality rates; Repone has destroyed the James River, one of America’s most productive, ánd most historic rivers; PCB’s are pervasive and have ruined the fishing in the Hudson River and the Great Lakes; carbon tetrachloride is only the most recent material to contaminate the Ohio River; the *141pesticide endrin has been found in Mississippi; perhaps worst of all, are the ones we do not know yet.
“The more we find out, the more cause there is for concern. It is imperative that these materials be controlled.” 123 Cong. Rec. 39181 (1977), Legislative History of the Clean Water Act of 1977, p. 454 (1978) (1977 Leg. Hist.).6
Similarly, Representative Roberts, the House manager of the bill, stated:
“[Toxics] have not only polluted drinking water and destroyed both commercial and sport fishing, but in many major water bodies they also constitute a hazard to aquatic environment and public health that has yet to be fully recognized.” 123 Cong. Rec. 38960 (1977), 1977 Leg. Hist. 327.
See also 1977 Leg. Hist. 334 (House Subcommittee memorandum).
The primary purpose of the 1977 amendments was to strengthen the regulation “of the increasingly evident toxic hazard.” 123 Cong. Rec. 38960 (1977), 1977 Leg. Hist. 326 (Rep. Roberts). See also 123 Cong. Rec. 39219 (1977), 1977 Leg. Hist. 549 (Sen. Moynihan) (“There is no room for compromise here: toxics must be controlled”). The §301(1) ban on “modifications” was an integral part of this effort to make the environment safe from toxics, and through it, Congress sought to prevent any weakening of the categorical standards for the control of toxic pollutants. It is clear that Congress knew full well what effects the rule might have on industry, and that it went forward nonetheless. For example, the legislators were aware that the prohibition against *142“modifications” of the standards for toxic pollutants could lead to “new regulations more restrictive than any previously contemplated.” 123 Cong. Rec. 38993 (1977), 1977 Leg. Hist. 411 (Rep. Buchanan). Congress also realized that such regulations would cost industry “millions of dollars and result only in a little more clean-up of our waters.” 123 Cong. Rec. 38952 (1977), 1977 Leg. Hist. 305 (Rep. Roberts). But Congress found that for toxics, unlike for other pollutants, ibid., such high costs of pollution control were justified in view of the serious environmental dangers at stake. Cf. §502(13) (defining “toxic” pollutants as pollutants that “cause death, disease, behavioral abnormalities, cancer, genetic mutations, physiological malfunctions (including malfunctions in reproduction) or physical deformations”).
It is readily apparent that a complete ban on modifications would most directly and completely accomplish the congressional goal. EPA offers no evidence in the legislative history to explain why this goal would be promoted by banning the statutory modifications of §§ 301(c) and (g), but would not more effectively be advanced by banning other modifications as well. It points to no evidence that Congress singled out the §§ 301(c) and (g) modifications as more pernicious from the standpoint of an effective toxic control program than modifications based on other factors. In fact, the statutory scheme suggests that the converse is true, as Congress specifically provided for statutory exemptions in these areas but not in other areas.
In the case of § 301(c), Congress was aware that certain firms would be driven to bankruptcy if they were required to comply strictly with the categorical standards. Congress determined that avoiding bankruptcies was an important social goal, and one that was not automatically outweighed by the goal of protecting the environment. Section 301(c) reflects the tension between these two goals: As long as a firm can make reasonable pollution control progress, it will not be driven to bankruptcy by its inability to meet higher pollution control standards.
*143Similarly, in the case of § 301(g) water-quality modifications, Congress decided not to force dischargers to meet standards higher than those that could be justified by legitimate environmental considerations. Thus, as long as a discharge did not interfere with the attainment of adequate water quality, a discharger would not be forced to expend additional resources in pollution control merely because a higher standard was “economically achievable.” Cf. 123 Cong. Rec. 38960 (1977), 1977 Leg. Hist. 326 (Rep. Roberts).
If these two modifications are the only ones now prohibited, the result is wholly counterintuitive. EPA is in effect contending that economic and water-quality factors present the most compelling case for modification of the standard in the nontoxic context — as they are explicitly authorized by statute — but the least compelling case for modification in the toxic context — as they are the only modifications prohibited by §301(1). As might be expected, EPA does not present any theory, much less a logical argument, or evidence in the legislative history, to support this extremely inconsistent result.
Moreover, if Congress had not intended to prohibit all modifications, it would almost certainly either have defined explicitly the scope of permissible modifications, or given the agency some guidance on how to go about doing so. Only in this way would Congress have had any assurance that modifications would be allowed only when they promoted interests of sufficient importance to outweigh Congress’ foremost goal of protecting the environment against toxic pollution.
C
The changes made in conference to the 1977 amendments, which ultimately included §301(1), provide further support for the proposition that Congress did not intend to limit §301(1) in the way suggested by EPA. Of the three provisions that undergird EPA’s theory — subsections (c), (g), and (1) of § 301 — only subsection (c) was adopted before the 1977 *144amendments, as part of the 1972 amendments. See 33 U. S. C. § 1311(c). The 1977 Senate bill contained two provisions of interest here. First, the bill proposed amending subsection (c) to prohibit, in the case of toxic pollutants, variances based on economic factors. S. 1952, 95th Cong., 1st Sess., §26(c) (1977), 1977 Leg. Hist. 584. Second, the Senate proposed what ultimately became subsection (g), which authorized modifications that did not interfere with water-quality goals. Like the proposed amendment to subsection (c), subsection (g) prohibited modifications in the case of toxic pollutants. The Senate bill did not contain subsection (1).
The Conference Committee changed the Senate bill in three relevant ways. First, it took out of subsection (c) the ban against modifications for toxics. Second, it reworded subsection (g) to prohibit water-quality modifications for conventional pollutants and for all thermal discharges, but it left unaffected the Senate bill’s prohibition against modifications for toxic pollutants. Third, it added subsection (1), which creates a ban of general applicability on modifications for toxic pollutants.
In explaining these changes, petitioner CMA contends that during the Conference Committee deliberations, “it was decided that, rather than repeating the identical limiting clause [for toxic pollutants] at the end of § 301(c) and what had become § 301(g) of the Act, the limitation would be put into a separate §301(1).” Brief for Petitioners in No. 83-1013, pp. 29-30. The debates of the Conference bill do not suggest that such a thing was “decided”; in fact, the reasons for the changes are not discussed at all. Moreover, if cleaning up the statutory language was in fact the objective of the changes, the Conference Committee was remarkably unsuccessful at doing so. Indeed, while the Committee took the prohibition against toxic modifications out of subsection (c), it left this prohibition undisturbed in subsection (g). Thus, the language of the Act simply belies CMA’s explanation.
*145More importantly, the wording of §301(1) strongly suggests that the purpose of the change was not to improve the style of the statute, but to expand the scope of the prohibition against “modifications.” Indeed, there is an important difference in the wording of subsections (c), (g), and (1). Subsections (c) and (g), which authorize exceptions, apply by their terms only to modifications of “the requirements of subsection (b)(2)(A).”7 If the Conference Committee was attempting merely to consolidate the bans on modifications of toxic standards, then it would similarly have limited the applicability of subsection (1) to subsection (b)(2)(A) requirements. Instead, subsection (1) applies to “any requirement of this section,” which includes numerous standards in addition to those of subsection (b)(2)(A).8
In fact, it appears that EPA once agreed that the changes made in conference expanded the scope of the ban on “modifications.” In the past, EPA construed §301(1) to prohibit, in the case of toxics, not only subsection (c) and (g) modifications, but also modifications from secondary treatment standards otherwise authorized by subsection (h), Brief for EPA on Petition to Enforce Mandate and Petitions for Review 24 in Appalachian Power Co. v. Train, 620 F. 2d 1040 (CA4 1980). Cf. FMC v. Seatrain Lines, Inc., 411 U. S. 726, 745 *146(1973) (administrative interpretation entitled to additional deference if “longstanding”).
In summary, the Conference changes provide further support for a broad reading of §301(1). See FTC v. Raladam Co., 283 U. S. 643, 648 (1931). The Court, however, appears to draw the opposite conclusion. But in doing so, it completely ignores the difference in the scope of §§ 301(c) and (g) on the one hand, and §301(1) on the other, and instead rests on an explanation of congressional activity that in fact explains almost nothing. See ante, at 126-127.
D
The Court and EPA both attach great importance to the congressional silence regarding FDF variances. EPA argues that E. I. du Pont de Nemours & Co. v. Train, 430 U. S. 112 (1977), held that FDF variances are “appropriate.” According to EPA, if Congress had intended to reverse this result it would have made its intention clear. See Brief for EPA 28-29. This contention, which the Court finds persuasive, see ante, at 127-128, is based on a misunderstanding of what was at stake in Du Pont. That case did not authorize the issuance of variances in any context that is relevant here.
Du Pont involved a challenge to EPA’s authority to issue, to direct dischargers, categorical effluent limitations for BPT and BAT. The Court had little difficulty in upholding such categorical limitations in the BAT context, as the statute provided that the limitations be set for “categories and classes” of dischargers, § 301(b)(1)(B). See Du Pont, supra, at 127. In contrast, the statute provided that BPT limitations be set for “point sources.” § 301(b)(1)(A). Several chemical manufacturers argued that, given this language, individualized BPT limitations were necessary, and that regulation by categories and classes of dischargers was inappropriate. This Court rejected the industry’s challenge, holding that BPT *147limitations could be set by industrywide regulation, so long as some allowance — such as FDF variances — was made for variations in individual plants. 430 U. S., at 128.
In support of its position that the Court broadly endorsed the issuance of FDF variances and that the congressional silence is noteworthy, EPA cites as dispositive one sentence in the opinion, which reads:
“We conclude that the statute authorizes the 1977 limitations [BPT] as well as the 1983 limitations [BAT] to be set by regulation, so long as some allowance is made for variations in individual plants, as EPA has done by including a variance clause in its 1977 limitations.” Ibid.
Only by taking this sentence out of context can one find support for the proposition that Du Pont requires FDF variances from BAT limitations, just as it does in the case of BPT limitations.9 When read in context, the sentence cited by EPA clearly means that BPT standards, like BAT standards, can be set by regulation, but if EPA does so in the BPT context, it must allow for variances. Indeed, the Court had earlier concluded that “§301 unambiguously provides for the use of regulations to establish the [BAT] effluent limitations.” Du Pont, supra, at 127. The Court did not qualify this conclusion in any way, but instead went on to discuss the BPT problem. The sentence that EPA refers to comes at the end of the discussion of BPT limitations, and is thus logically related to that discussion.
*148Furthermore, the Court upheld the regulations challenged in Du Pont even though they did not contain an FDF variance clause for BAT limitations. See 430 U. S., at 123,127.10 If the sentence in question has the meaning that EPA now ascribes to it, the Court would presumably have had to reverse on that point.
In summary, the portion of Du Pont on which EPA relies, has absolutely no bearing on the question of whether FDF variances are “appropriate” — to use the language employed by EPA, see n. 9, supra — when the statute calls for limitations for categories or classes of dischargers. See EPA v. National Crushed Stone Assn., 449 U. S. 64, 72 (1980) (“[Du Pont] indicated that a variance provision was a necessary aspect of BPT limitations applicable by regulation to classes and categories of point sources”); id., at 73, n. 12 {“[DuPont] held that a uniform BPT limitation must contain a variance provision, if it is to be valid”). Both the facts and the rationale of this portion of Du Pont are of relevance only to cases in which EPA issues categorical standards in the face of a statutory scheme that calls for regulation of “point sources.”
This distinction is of crucial significance because the standards for toxic pollutants, like all BAT and pretreatment standards, are to be set not for “point sources,” but instead “for the applicable category or class of point sources.” § 307(a)(2) (emphasis added) (toxics); see also § 301(b)(2)(A) (BAT); § 307(b)(3) (pretreatment). Du Pont did not consider whether such standards are necessary, or even appropriate, in this context.11 We should scarcely attribute any signifi-*149canee to the legislative failure to discuss Du Pont because Du Pont considered a fundamentally different scheme of regulation. It may be that one day the Clean Water Act will be read to permit, for nontoxic pollutants, FDF variances from BAT and pretreatment standards; however, there is no reason why Congress should have said anything in 1977, when it enacted § 301(1), about a legal development that has not yet taken place, eight years later.
There is, moreover, another reason for the legislative silence on FDF variances. The legislative history of the 1977 amendments shows that Congress believed — correctly, as it turns out — that the courts had not yet determined whether FDF variances were permissible in the BAT context. See S. Rep. No. 95-370 (1977), 1977 Leg. Hist. 674.12 Only by misreading Du Pont and ignoring the relevant legislative history can the Court say that Du Pont “construed the Act to permit the very FDF variance NRDC insists the Conference Committee was silently proposing to abolish.” Ante, at 128.13
*150E
EPA also relies heavily on a statement by Representative Roberts:
“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.” 123 Cong. Rec. 38960 (1977), 1977 Leg. Hist. 328-329 (emphasis added).
However, other statements in the debates fail similarly to restrict the scope of the provision. For example, Senator Muskie stated:
“Like toxic pollutants for which there are no waivers or modifications, there are no potential waivers or modifications for conventional pollutants.” 123 Cong. Rec. 39183 (1977), 1977 Leg. Hist. 458 (emphasis added).
See also 123 Cong; Rec. 38952 (1977), 1977 Leg. Hist. 305 (“Strict requirements are still in effect for damaging pollutants, such as toxics. However, for certain other pollutants, industry may get a waiver”) (Rep. Roberts); 123 Cong. Rec. 38993 (1977), 1977 Leg. Hist. 411 (referring to “denial of any waiver” with respect to toxics) (Rep. Buchanan) (emphasis added).
Taken as a whole, the legislative history firmly supports the plain meaning of the statute, namely, that § 301(1) bans all *151“modifications,” and not just those otherwise permitted by §§ 301(c) and (g). EPA’s strongest argument in support of its position on this score is that, during the course of debates, one of the bill’s managers used the word “specifically” instead of “for example. ” Under any accepted canon of construction, this choice of words is insufficient to overcome the other, more probative indications of congressional intent that emerge from an analysis of the legislative history. And, with the language and the legislative history pointing so definitely in the same direction, there can be no doubt that congressional intent was clear.
F
The determination that Congress clearly intended that § 301(1) do more than just ban modifications otherwise permitted by §§ 301(c) and (g) compels the conclusion that EPA’s construction to the contrary cannot stand. As this Court has repeatedly stated:
“The interpretation put on the statute by the agency charged with administering it is entitled to deference, but the courts are the final authorities on issues of statutory construction. They must reject administrative constructions of the statute, whether reached by adjudication or by rulemaking, that are inconsistent with the statutory mandate or that frustrate the policy that Congress sought to implement.” FEC v. Democratic Senatorial Campaign Committee, 454 U. S. 27, 31-32 (1981) (citations omitted).
See also SEC v. Sloan, 436 U. S. 103, 117-118 (1978); FMC v. Seatrain Lines, Inc., 411 U. S., at 745-746; Volkswagenwerk v. FMC, 390 U. S. 261, 272 (1968); NLRB v. Brown, 380 U. S. 278, 291 (1965); Social Security Board v. Nierotko, 327 U. S. 358, 369 (1946); Burnet v. Chicago Portrait Co., 285 U. S. 1, 16 (1932); Webster v. Luther, 163 U. S. 331, 342 (1896).
*152This case is thus unlike Chevron U. S. A. Inc. v. NRDC, 467 U. S. 837 (1984), on which the Court and EPA rely. In Chevron, the Court reviewed an EPA regulation that treated all pollution-emitting devices within the same industrial grouping as though they were encased within a single “bubble.” This regulation was challenged on the ground that it was not based on a proper construction of the statutory term “stationary source.” Analyzing the statutory language, the Court concluded that “parsing of general terms in the text of the statute” would not reveal the actual intent of Congress. Id., at 861. Similarly, it found the legislative history “unilluminating.” Id., at 862. Given these two conclusions, the Court determined that deference to the Agency’s reasonable interpretation was appropriate.
Chevron’s deference requirement, however, was explicitly limited to cases in which congressional intent cannot be discerned through the use of the traditional techniques of statutory interpretation. Indeed, Chevron reaffirmed the principle that “[t]he judiciary is the final authority on issues of statutory construction and must reject administrative constructions which are contrary to clear congressional intent.” Id., at 843, n. 9.14
My disagreement with the Court does not center on its reading of Chevron, but instead on its analysis of the congressional purposes behind §301(1). If I agreed with the Court’s analysis of the statute and the legislative history, I too would conclude that Chevron commands deference to the administrative construction.
EPA’s second construction of the statutory scheme is, on the surface, a more plausible one. EPA argues that FDF *153variances do not excuse compliance with the correct standards, but instead provide a means for setting more appropriate standards. It is clear that, pursuant to § 307(b)(2), EPA can “revise” the pretreatment standards, as long as it does so “following the procedure established ... for the promulgation of such standards.” The statute contemplates that the standards will be set and revised through notice-and-comment rulemaking and will be applicable to categories of sources. See §§ 307(b)(2), (3); see also Brief for EPA 9. EPA argues that such a “revision,” which is clearly not proscribed by § 301(1), would be substantively indistinguishable from an FDF variance. Thus, according to the Agency, NRDC’s concern stems not from the result achieved when an FDF variance is granted, but rather from the procedure employed in reaching that result. EPA relies on Vermont Yankee Nuclear Power Corp. v. NRDC, 435 U. S. 519 (1978), for the proposition that an agency is free to choose between two procedures for reaching the same substantive ends. See Brief for EPA 11, 36.
To support its argument, EPA points out that the factors that may justify an FDF variance are the same factors that may be taken into account in setting and revising the national pretreatment standards. Compare § 304(b)(2) (statutory standard) with 40 CFR §403.13(d) (1984) (FDF variance provision). EPA also points out that, in considering whether an FDF variance will be granted, it cannot take into account factors that could not have justified a change in the national standards. See Brief for EPA 31; 40 CFR §403.13(e)(1984). EPA acknowledges that the statute requires that the national pretreatment standards be established — and therefore revised — for “categories” of dis-chargers, see § 307(b)(3) (pretreatment standards); Brief for EPA 11; see also § 307(a)(2) (toxic standards), and not on a case-by-case basis. It argues, however, that nothing in the Clean Water Act precludes EPA from defining a subcategory that has only one discharger. See Brief for EPA 31.
*154The logic of EPA’s position is superficially powerful. If EPA can, through rulemaking, define a subcategory that includes only one discharger, why should it not be able to do so through a variance procedure? In fact, if rulemaking and the variance procedure were alternative means to the same end, I might have no quarrel with EPA’s position, which the Court has accepted. Ante, at 132-133. Indeed, “[ajbsent constitutional constraints or extremely compelling circumstances the administrative agencies should be free to fashion their own rules of procedure and to pursue methods of inquiry capable of permitting them to discharge their multitudinous duties.” Vermont Yankee, supra, at 543 (citations omitted); see also SEC v. Chenery Corp., 332 U. S. 194, 202-203 (1947).
However, the Agency’s position does not withstand more than superficial analysis. An examination of the legislative history of the 1972 amendments to the Clean Water Act — the relevance of which both the Court and EPA ignore — reveals that Congress attached great substantive significance to the method used for establishing pollution control requirements.
The Conference Committee Report directed EPA to “make the determination of the economic impact of an effluent limitation on the basis of classes and categories of point sources, as distinguished from a plant by plant determination.” 1972 Leg. Hist. 304 (emphasis added).15 Representative Dingell, one of the House conferees, described this principle as “very important” and stated that “a plant-by-plant determination of the economic impact of an effluent limitation is neither expected, nor desired, and, in fact, it should be avoided.” 118 Cong. Rec. 33758 (1972), 1972 Leg. Hist. 254-255.
*155Similarly, Senator Muskie stated:
“The Conferees intend that the factors described in section 304(b) be considered only within categories and classes of point sources and that such factors not be considered at the time of the application of an effluent limitation to an individual point source within such a category or class.” 118 Cong. Rec. 33697 (1972), 1972 Leg. Hist. 172 (emphasis added).
See also Du Pont, 430 U. S., at 130; American Iron & Steel Institute v. EPA, 526 F. 2d 1027, 1051 (CA3 1975) (“Congress clearly intended that the Administrator consider costs on a class or category basis, rather than on a plant-by-plant basis”) (emphasis added). Moreover, in a letter urging the President to approve the 1972 amendments, William Ruckelshaus, EPA’s Administrator, observed that the Act’s standards should be set “for industrial categories, taking into account processes involved, age of equipment, and cost, considered on a national, industry-unde basis.” 118 Cong. Rec. 36775 (1972), 1972 Leg. Hist. 145 (emphasis added). It is difficult to imagine a legislative history that would make more clear that standards should not be set — and therefore should not be revised — on an individual basis.
The legislative history also makes clear why Congress found it so important that the standards be set for “categories” of dischargers, and not for individual dischargers. Congress intended to use the standards as a means to “force” the introduction of more effective pollution control technology. Thus, Congress directed EPA to establish BPT levels by reference to “the average of the best existing performance by plants of various sizes, ages, and unit processes within each industrial category.” 118 Cong. Rec. 33696 (1972), 1972 Leg. Hist. 169 (Sen. Muskie). In establishing BAT levels, it directed EPA to look at “the best performer in an industrial category.” 118 Cong. Rec. 33696 (1972), 1972 Leg. Hist. 170. By requiring that the standards be set by reference to *156either the “average of the best” or very “best” technology, the Act seeks to foster technological innovation. 118 Cong. Rec. 33696 (1972), 1972 Leg. Hist. 170. See generally La Pierre, Technology-Forcing and Federal Environmental Protection Statutes, 62 Iowa L. Rev. 771, 805-829 (1977); Note, Forcing-Technology: The Clean Air Act Experience, 88 Yale L. J. 1713 (1979).
Unlike the statutory revision mechanism of § 307(b), FDF variances are set not by reference to a category of dis-chargers, but instead by reference to a single discharger. In evaluating an application for a variance, EPA does not look at the group of dischargers in the same position as the applicant, but instead focuses solely on the characteristics of the applicant itself. Under the FDF program, there is no mechanism for EPA to ascertain whether there are any other dis-chargers in that position. Moreover, there is no mechanism for EPA to group together similarly situated dischargers. Quite to the contrary, a scheme in which the initial screening may be done by the individual States, at times determined by when the variance application is filed, is unlikely to lead to the identification of new subcategories. See 40 CFR §403.13(k) (1984).
The FDF variance procedure leads to substantive results that are different in two fundamental ways from those attained through the rulemaking for categories of dischargers contemplated in § 307(b). First, it is less protective of the environment. If, for example, a discharger shows that its production processes — and, as a result, its costs of compliance — are significantly different from those taken into account in setting the categorical standards, that discharger would be eligible for an FDF variance, and EPA could set a new requirement based on the applicant’s peculiar situation. See 40 CFR §§403.13(d)(5), (6) (1984); Tr. of Oral Arg. 14. It may turn out, however, that there are many other discharg-ers in the same situation, and that all of these dischargers use production processes that make pollution control possible *157át a much lower cost. If EPA took into account the production processes of these more efficient dischargers — as it presumably would have to do if it proceeded through rule-making on a categorical scale — it would set a requirement far more stringent than that adopted as part of the FDF variance mechanism.
In the aggregate, if EPA defines a new pretreatment subcategory through rulemaking, the BAT-level pollution control requirement of each discharger would be determined by reference to the capability of the “best” performer. In contrast, if EPA provides individual variances to each plant in this group, only one discharger would have a requirement based on the capability of the best performer — the best performer itself. The others would necessarily be subject to less stringent standards.16
The second important difference is that FDF variances do not spur technological innovation to the same extent as § 307(b) revisions. In the preceding example, the discharger with environmentally unsound production processes would probably be compelled to purchase new technology if it were subjected to a pollution control requirement set by reference to the characteristics of the “best” discharger. Under the less stringent requirement adopted through the FDF variance procedure, it might not need to do so. The additional demand for new technology that results from the § 307(b) procedures creates incentives for technological innovation. In the long run, such innovation would lead to even better technology and to the possibility of further tightening of the pollution control requirements, as such technology became cheaper. In fact, Congress envisioned that this iterative procedure would ultimately lead to an elimination of harmful discharges. See 118 Cong. Rec. 33696 (1972), 1972 Leg. Hist. 170 (Sen. Muskie).
*158It is true, of course, that even the statutory revision procedure might identify a subcategory with only one discharger. That procedure, however, will have established that this discharger is indeed uniquely situated. In contrast, an FDF variance sets an individual requirement even where there may be similarly situated dischargers.
In summary, whatever else FDF variances might do, they do not further the same congressional goals as the notice- and-comment rulemaking required for § 307(b) revisions.17 Vermont Yankee is simply inapposite; Congress intended, for substantive reasons, that the pretreatment standards be set and revised through rulemaking for categories of dischargers.18 The Court’s conclusion to the contrary stems exclusively from its failure to consider why Congress chose to require categorical standards.
> HH
The analysis of Parts II and III compels the conclusion that neither of the alternative arguments advanced to support EPA’s construction of the statute can stand. That analysis *159also leads directly to the conclusion that § 301(1) in fact disallows FDF variances from the standards for toxic pollutants. Congress clearly intended that § 301(1) ban variances such as those at issue here, and the language and legislative history permit no other interpretation.
A
Part II shows that the language of § 301(1), the purposes that led to the adoption of the provision, and the changes made by the Conference Committee, indicate a clear congressional intent to ban all “modifications” to the standards for toxics, not merely those otherwise authorized by §§ 301(c) and (g). The legislative history also establishes that Congress banned “modifications” because it wanted to ensure that the serious problem of toxic pollution not be exacerbated by the granting of exceptions to the general rulemaking standards. See Part II-B, supra.
It is true, of course, that in many cases exceptions serve the important purpose of softening the impact of rules of general applicability. They mediate between demands for comprehensive solutions on the one hand, and individualized application of law on the other. See generally Diver, Policy-making Paradigms in Administrative Law, 95 Harv. L. Rev. 393 (1981).
Exceptions, however, are not without costs. For example, they are inappropriate where small errors could lead to irreversible or catastrophic results.19 In such cases, indi*160vidual equity should give way to comprehensive rationality. See id., at 431-432; Note, Regulatory Values and the Exceptions Process, 93 Yale L. J. 938, 955, and n. 85 (1984).
The decision of when exceptions are required, when they are permissible, and when they are prohibited is, in the first instance, one for Congress to make. It is an administrative decision only where Congress has left a gap for the agency to fill. See Chevron, 467 U. S., at 843-844. In this case, Congress determined that the flexibility resulting from exceptions would interfere with the furtherance of the more important goal of controlling toxic pollution. There is no question that courts should defer to this congressional judgment.
In fact, when Congress has attached great importance to certain environmental goals, we have disallowed exceptions even in the absence of an explicit statutory ban. For example, in TV A v. Hill, 437 U. S. 153 (1978), we reviewed a provision of the Endangered Species Act that required federal agencies “to insure that actions authorized, funded, or carried out by them do not jeopardize the continued existence” of an endangered species or “result in the destruction or modification of habitat of such species . . . .” 16 U. S. C. § 1536 (1976 ed.). Even though Congress had not expressly banned exceptions from the statutory requirements, the Court focused on the quoted language and found that it “admits of no exception.” Id., at 173. It further found that both the language and the legislative history “clearly” showed that Congress viewed the preservation of endangered species as a goal of great importance. Id., at 187-188. In light of this statutory construction, the Court concluded that any exemption from the statute’s requirements — other than exemptions specifically approved by Congress — would be inappropriate.20
*161Similarly, in Du Pont itself, the Court disallowed FDF variances from the Clean Water Act’s standards of performance for new sources, reasoning that such variances would be inconsistent with the environmental goals expressed in the statute and the legislative history. There, the Court stated that FDF variances “would be inappropriate in a standard that' was intended to insure national uniformity and ‘maximum feasible control of new sources.’” 430 U. S., at 138 (citation omitted). In this case, of course, Congress has not only indicated that the environmental goal at stake is extremely important, but it has also explicitly disallowed exceptions. Under such circumstances, it would be especially inappropriate to defer to the Agency’s decision to create exceptions.
B
Part III establishes that FDF variances are not an alternative way of complying with the statutory command to set rules of general applicability. They do not implement the Clean Water Act’s technology-based requirements; instead, like §§ 301(c) and (g) modifications, they are case-by-case departures from such requirements. In fact, in the past, EPA itself has referred to FDF variances as “exception[sj to [a] general rule of applicability.” Brief for EPA 47 in NRDC v. EPA, 537 F. 2d 642 (CA2 1976).
FDF variances not only take the same form as §§ 301(c) and (g) modifications, but they also serve closely analogous functions. As I have discussed, the purpose of exceptions is to soften the harshness of general rules. See supra, at 159. A § 301(c) modification, for example, relieves a firm of its obligation to meet an applicable rule when compliance with *162that rule would place the firm in a serious hardship. See EPA v. National Crushed Stone Assn., 449 U. S., at 78; S. Rep. No. 95-370, p. 41 (1977), 1977 Leg. Hist. 674 (Sen. Muskie); Brief for EPA 32-33. FDF variances also temper — albeit in a slightly different way — the effects of the nationwide, categorical standards. They relieve a firm of its obligation to comply with a rule that would impose on that firm a disproportionate share of the regulatory burden. See Tr. of Oral Arg. 14.21 In fact, EPA itself has characterized FDF variances as “ ‘safety valves’ in regulatory schemes of general applicability.” Brief for EPA 44 in NRDC v. EPA, 537 F. 2d 642 (CA2 1976); see also Hearings on Possible Amendments to the Federal Water Pollution Control Act *163before the Subcommittee on Water Resources of the House Committee on Public Works and Transportation, 98th Cong., 1st Sess., 2706, 2741 (1988) (EPA Administrator Ruckelshaus describing FDF variances as “safety valves”); NRDC v. EPA, 637 F. 2d, at 646 (“[T]he ‘variance’ clause was assertedly adopted as an administrative safety valve”). Thus, FDF variances are exceptions that provide the type of flexibility that § 301(1) sought to ban.22
The Court accepts EPA’s present characterization that FDF variances are a hybrid: “more like” a revision permitted *164by §307 than like a §§ 301(c) and (g) modification. Ante, at 126. But a requirement that, by definition, applies to only one discharger cannot be considered “more like” a rule of general applicability than like an exception to such a rule. Clearly, it is an exception.23
The Court’s error is to overlook the distinction between general rules and exceptions. Instead, it focuses on the differences between the grounds for exceptions provided by §§ 301(c) and (g) on the one hand, and by the FDF provisions on the other. Thus, the Court makes its cuts along an entirely different — and irrelevant — axis. For EPA to prevail, the Court must show that Congress found that exceptions based on economic capability or water-quality factors were especially undesirable. If this were true, then exceptions based on other factors would be less undesirable, and it would make sense to decide the cases on the basis of the extent to which the factors taken into account in granting FDF variances differ from §§ 301(c) and (g) factors. The Court’s position, however, is inconsistent with the clear purpose of §301(1). As I have shown, there is absolutely no reason to believe that this provision was designed to ban §§ 301(c) and (g) modifications because there was something particularly pernicious about such exceptions. See supra, at 143. Rather, the congressional concern was that exceptions would weaken the standards for the control of toxic pollutants. This concern defines the relevant criterion: whether something is a general rule or an exception to such a rule. Sections 301(c) and (g) modifications are at one end of the axis not because they are based on economic or water-*165quality factors, but because they are exceptions to general rules. Section 307(b) revisions are at the other end of the axis not because they are based on factors taken into account in setting the standards, but because they are rules of general applicability. Of course, FDF variances, which are nothing but exceptions to general rules, are at the same end of the axis as §§ 301(c) and (g) modifications.
For the foregoing reasons, it is apparent that §301(1) prohibits FDF variances from the pretreatment standards for toxic pollutants. I therefore dissent.
New plants must meet new source performance standards (NSPS) based on the “best available demonstrated control technology.” §306.
Following the Third Circuit’s decision, EPA revised its FDF regulation to comply with that decision. See 49 Fed. Reg. 5132 (1984); 40 CFR § 403.13(b)(2) (1984) (“A fundamentally different factors variance is not available for any toxic pollutant controlled in a categorical Pretreatment Standard”). The Agency explicitly stated that it was adopting this change directly as a result of the Third Circuit’s decision. 49 Fed. Reg. 5132 (1984). No suggestion of mootness has been made by any of the parties, and EPA’s position before this Court is consistent with the view that it desires to reinstate its prior regulation. Given all of these circumstances, *137the revision of the regulation does not render this case moot. See Maher v. Roe, 432 U. S. 464, 468-469, n. 4 (1977).
Under the Court’s decision, the Third Circuit will now have to consider this question on remand.
Under § 301(c):
“The Administrator may modify the requirements of subsection (b)(2)(A) . . . 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.” 86 Stat. 845, 33 U. S. C. § 1311(c).
Section 301(g) provides, in pertinent part:
“(1) The Administrator, with the concurrence of the State, shall modify the requirements of subsection (b)(2)(A) of this section with respect to the discharge of any pollutant (other than pollutants identified pursuant to section 304(a)(4) of this Act, toxic pollutants subject to section 307(a) of this Act, and the thermal component of discharges) from any point source 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 *140risk to human health or the environment. . . .” 91 Stat. 1583, 33 U. S. C. § 1311(g).
Citations to the 1977 legislative history 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).
EPA argues that §§301(c) and (g) modifications are available only for BAT standards for direct dischargers. Brief for EPA 32, n. 23; Reply Brief for EPA 2-3, and n. 1. In contrast, NRDC argues that such modifications are available for pretreatment standards as well. See Brief for NRDC 29, and n. 41. That dispute is not central to these cases.
The argument that the Conference Committee was unaware of the effect of its changes is particularly unpersuasive in this context because many of the conferees were familiar with the intricacies of the Clean Water Act. Indeed, 7 of the 26 conferees had been members of the Conference Committee at the time of the 1972 amendments to the Clean Water Act; another 7 conferees had served on the Committees that considered the 1972 amendments.
In fact, EPA does not appear to argue that Du Pont requires FDF variances in the case of BAT standards for direct dischargers. Instead, it seems to say merely that Du Pont sanctioned such variances. See Brief for Petitioners in No. 83-1373, pp. 20-21. To the extent that the sentence in question is relevant to the BAT context, it would seem to support a requirement for FDF variance, rather than the more modest claim made by EPA. Such a requirement, however, is inconsistent with the result reached in Du Pont. See n. 10, infra.
Compare 40 CFR §§415.12, 415.22, 415.32, 415.42, 415.52, 415.62, 415.92, 415.112, 415.122, 415.132, 415.142, 415.162, 415.172, 415.202 (1977) (providing for FDF variances from BPT standards), with 40 CFR §§415.13, 415.23, 415.33, 415.43, 415.53, 415.113, 415.123, 415.133, 415.143, 415.163, 415.203 (1977) (not providing for FDF variances from BAT standards).
In fact, Du Pont dealt with one situation in which effluent standards were to be set for categories of dischargers: the new source standards of *149§ 306. There, the Court held not only that variances were not mandated, but that they would be impermissible. Du Pont, 430 U. S., at 138; see infra, at 160-161.
Also, the FDF variance provisions were probably not noteworthy enough to attract congressional attention. At the time Du Pont was decided, EPA had provided for FDF variances only in the case of BPT standards for direct dischargers, and only two out of the thousands of sources covered by BPT standards had actually received such a variance. See Hearings on Possible Amendments to the Federal Water Pollution Control Act before the Subcommittee on Water Resources of the House Committee on Public Works and Transportation, 98th Cong., 1st Sess., 2741 (1983) (EPA Administrator Ruekelshaus).
The Court also finds it noteworthy that, under the provisions of a consent decree, EPA is currently promulgating BPT-level standards that apply to toxics. The Court suggests that prohibiting FDF variances for those standards would be inconsistent with Du Pont. See ante, at 128-129, n. 18. What is relevant for the purposes of Du Pont, however, is not whether the standards in question are set by reference to BPT or BAT levels, but whether the statute calls for individualized or categorical standards. The pretreatment standards — for both toxics and nontoxics — are in *150the latter category, § 307(b), and the Du Pont variance requirement is therefore of no relevance to such standards.
Along similar lines, EPA points out that, prior to the 1977 amendments to the Act, it had granted an FDF variance for a toxic pollutant. At the time the variance was granted, however, that pollutant had not yet been designated as toxic. See Brief for EPA 12; Reply Brief for EPA 13.
The case explicitly acknowledged the continued validity of our long line of precedents holding that administrative constructions inconsistent with congressional intent cannot stand. 467 U. S., at 843, n. 9.
Citations to the 1972 legislative history are to Senate Committee on Public Works, A Legislative History of the Water Pollution Control Act Amendments of 1972, prepared by the Environmental Policy Division of the Congressional Research Service of the Library of Congress (Comm. Print 1973).
The same principle holds true — albeit to a lesser degree — for pretreatment standards set by reference to BPT levels.
Also, EPA’s argument on this score has no logical bounds. If FDF variances are a permissible alternative to the notice-and-comment procedure for “revisions” envisioned by § 307(b)(2), it must also be acceptable to set the standards in the first place through case-by-case determinations. See § 307(b)(2) (same procedures to be used in setting and revising standards). And, of course, there would be no reason for this theory to be confined solely to pretreatment standards. The argument that Congress was willing to tolerate case-by-case determinations of all of the water standards is so ludicrous as to hardly merit a reply. See supra, at 154-155.
In fact, following the decision of the Court of Appeals for the Third Circuit in this case, EPA indicated that it would entertain petitions for amended rulemaking by certain indirect dischargers who were previously eligible for FDF variances. The aim of such rulemaking would be to identify a new subcategory of dischargers and to set an appropriate standard for that subcategory. 48 Fed. Reg. 52396 (1983). By proceeding in this manner — consistent with the requirements of § 307(b)(2) — EPA promotes the environmental protection and technology-forcing goals that Congress found so important.
Environmental problems often present thresholds. For example, if the level of biochemical oxygen demand (BOD) in a river exceeds a certain level, fish life will become impossible. A slightly lower BOD level, however, would prevent this result. Thus, the cost of a relatively small mistake is very high. See B. Ackerman, S. Rose-Ackerman, J. Sawyer, & D. Henderson, The Uncertain Search for Environmental Quality 265-266 (1974). General rules, adopted after consideration of the comments of all interested parties, in a process fully open to public scrutiny, provide the best guarantee that such mistakes will not occur. See generally K. Davis, Discretionary Justice 65-66 (1969).
The fact that Congress amended the Endangered Species Act following the Court’s decision in TV A v. Hill is, of course, of no consequence to the analysis here. In these cases, however, Congress was asked to modify the *161decision of the Court of Appeals for the Third Circuit by authorizing PDF variances from toxic standards, but declined to do so. See H. R. 3282, 98th Cong., 2d Sess. (1984); Hearings on Possible Amendments to the Federal Water Pollution Control Act before the Subcommittee on Water Resources of the House Committee on Public Works and Transportation, 98th Cong., 1st Sess., 2705-2706, 2724-2726, 2740-2741, 2747-2748 (1983).
Commentators have identified two categories of exceptions that are relevant in these cases: hardship exceptions and fairness exceptions. See, e. g., Aman, Administrative Equity: An Analysis of Exceptions to Administrative Rules, 1982 Duke L. J. 277, 293-294; Shapiro, Administrative Discretion: The Next Stage, 92 Yale L. J. 1487,1504 (1983); Schuck, When the Exception Becomes the Rule: Regulatory Equity and the Formulation of Energy Policy Through an Exceptions Process, 1984 Duke L. J. 163, 283-289. Under this classification, a § 301(c) modification is a hardship exception and an FDF variance is a fairness exception. A § 301(g) modification is a different type of fairness exception. It seeks to ensure that a firm not be forced to comply with the categorical standards when no environmental benefit would accrue from such compliance. See Aman, supra, at 311-312.
This classification of exceptions is reflected in several statutes. For example, the Department of Energy Organization Act, 42 U. S. C. § 7194(a); the Natural Gas Policy Act, 15 U. S. C. § 3412(c); and the Energy Policy and Conservation Act, 42 U. S. C. § 6393(a)(4), all provide for exceptions based on “special hardship, inequity, or unfair distribution of burdens.” Of course, a “special hardship” exception is analogous to a § 301(c) modification; an “inequity or unfair distribution of burdens” exception is analogous to an FDF variance. Thus, the structure of these statutes supports the proposition that an FDF variance is an exception to a general rule. Cf. Overstreet v. North Shore Corp., 318 U. S. 125,128 (1943) (determining scope of phrase “engaged in interstate commerce” under the Fair Labor Standards Act by reference to use of that term in the Federal Employers’ Liability Act).
It is also relevant that, in the legislative history of § 301(1), the terms “modification,” “variance,” and “waiver” are often used interchangeably to describe exceptions to rules of general applicability. For example, during its Senate testimony, EPA used the term “variance” to describe statutory “modifications.” Hearings on Federal Water Pollution Control Act Amendments of 1977 before the Subcommittee on Environmental Pollution, 95th Cong., 1st Sess. (1977); see 1977 Leg. Hist. 1102, 1124, 1419. Similarly, both Senator Muskie and Representative Roberts equated the terms “modification” and “waiver.” 123 Cong. Rec. 39183 (1977), 1977 Leg. Hist. 458 (Sen. Muskie); 123 Cong. Rec. 38952 (1977), 1977 Leg. Hist. 305 (Rep. Roberts); see also S. Rep. No. 95-370, p. 44, 1977 Leg. Hist. 677.
Moreover, prior to the enactment of § 301(1), EPA repeatedly referred to the FDF variances as “modifications.” See 43 Fed. Reg. 27738 (1978) (“provision for case-by-case modifications of the categorical pretreatment standards)”; Brief for EPA 40-41, 44-45, in NRDC v. EPA, 537 F. 2d 642 (CA2 1976) (“a procedure for modification of the limits”; “a limited modification of the regulations”; a “modification procedure”).
In many other statutes, Congress has also used the terms “exceptions,” “variances,” “modifications,” “adjustments,” or “exemptions” interchangeably to refer to the identical concept: individual departures from general rules. See, e. g., Federal Trade Traffic Safety Act, 15 U. S. C. §§ 1410, 1417 (exemptions); Natural Gas Policy Act, 15 U. S. C. § 3412(c) (adjustments); Federal Mine Safety and Health Act, 30 U. S. C. § 811(e) (modifications, exceptions); Safe Drinking Water Act, 42 U. S. C. § 300g-4 (variances); Clean Air Act, 42 U. S. C. § 7410(i) (modifications); Department of Energy Organization Act, 42 U. S. C. § 7194(a) (modifications, exceptions, exemptions); Federal Aviation Act, 49 U. S. C. §§ 1386(b)(1), 1421(c) (exemptions).
EPA argues that an FDF variance is equivalent to a subcategory containing only one discharger, and that the Act does not proscribe such subcategories. There is no merit to this argument. FDF requirements are set individually not because the applicant is in a unique position, but because the FDF procedures provide no mechanism for EPA to ascertain whether there are other dischargers in the same position as the applicant. See supra, at 156-158.