Natural Resources Defense Council, Inc. v. United States Environmental Protection Agency

907 F.2d 1146, 285 U.S. App. D.C. 140

• Court: Court of Appeals for the D.C. Circuit
• Decided: June 29, 1990
• Docket: Nos. 88-1657, 88-1734, 88-1746, 88-1754, 88-1758
• Status: Published

Opinions

Opinion for the Court filed PER CURIAM.

Opinion dissenting in part filed by Chief Judge WALD.

PER CURIAM:¹

This case concerns the disposal of hazardous waste by “deep injection”; that is, the injection of hazardous waste into “wells” located thousands of feet beneath the surface of the earth. The Environmental Protection Agency (“EPA”) has issued regulations under the Resource Conservation and Recovery Act (“RCRA”) governing this method of hazardous waste disposal, and various petitioners challenge them. Industry petitioners claim the regulations are too stringent; environmental petitioners claim they are too lenient. We hold that the regulations are reasonable exercises of the EPA’s authority and discretion under RCRA except insofar as they relate to the disposal of hazardous waste in “geologic repositories,” that is, salt domes, salt beds, underground mines, and caves. We believe the EPA has ignored its statutory duty to promulgate standards for these repositories and has not adequately defended its permitting process against NRDC’s statutory challenges, and so we remand these issues for further agency consideration. In all other respects, we deny the petitions for review.

I. BACKGROUND

The Resource Conservation and Recovery Act of 1976, as amended, 42 U.S.C. §§ 6901 et seq. (1982 & Supp. Ill 1985), governs the land disposal of hazardous waste. As originally passed, RCRA dedared that “disposal of solid waste and hazardous waste in or on the land without careful planning and management can present a danger to human health and the environment,” and that “alternatives to existing methods of land disposal must be developed.” 90 Stat. 2795, 2797 (1976). RCRA directed the Administrator of the EPA to promulgate regulations establishing “such performance standards, applicable to owners and operators of facilities for the treatment, storage, or disposal of hazardous waste ... as may be necessary to protect human health and the environment.” 42 U.S.C. § 6924.

Eight years later, a frustrated Congress, irritated at the slow pace at which the EPA was achieving RCRA’s goals,² passed the Hazardous and Solid Waste Amendments of 1984 (“HSWA”), Pub.L. No. 98-616, 98 Stat. 3221 (codified at scattered sections of 42 U.S.C. (Supp. Ill 1985)). As amended, RCRA pronounced that “land disposal, particularly landfill and surface impoundment, should be the least favored method for managing hazardous wastes.” 42 U.S.C. § 6901(b)(7). HSWA also legislated new “land ban” provisions governing the land disposal of hazardous waste, § 6924(d)-(g) (set out in the margin).³ Subsection (d) governs all methods of land disposal, except disposal by deep injection, of certain, specified wastes (mostly toxic metals), popularly known as the “California list” wastes. Subsection (e) governs all methods of land disposal, except disposal by deep injection, of solvents and dioxins. Subsection (f) governs disposal by deep injection of the wastes specified in subsections (d) and (e). Finally, subsection (g) governs all methods of land disposal, including disposal by deep injection, of all hazardous wastes other than the “California list” or solvents and dioxins covered by subsections (d), (e), and (f). In toto, subsections (d) through (g) of § 6924 govern all methods of land disposal of all hazardous wastes.

In 1988, the EPA promulgated final rules pursuant to subsections (f) and (g) governing the disposal of hazardous wastes by deep injection. 53 Fed.Reg. 28,118 (1988). These rules supplemented the EPA’s Underground Injection Control (“UIC”) program, which governed underground injection of solid waste, to take into account injection of hazardous waste. The general scheme adopted by the EPA provided that underground injection of hazardous wastes would be prohibited unless the would-be injector obtained a permit from the EPA for a particular underground injection well. New rules located at 40 C.F.R. Part 148 identified the wastes otherwise prohibited from underground injection and specified the procedures for obtaining a permit to allow their injection. New Subpart G of 40 C.F.R. Part 146 set out technical criteria that underground injection wells must satisfy before they could be approved for hazardous waste disposal. In sum, the EPA’s regulatory scheme for deep well injection of hazardous waste contained two important components: (1) the substantive standard that a hazardous waste injection well must meet, and (2) the permit procedures by which an injector must demonstrate that a well meets that standard.

The new rules also presented the EPA's interpretations of several key statutory terms. The permissibility of a land disposal method under subsections (d) through (g) of § 6924 turns on whether the method will be “protective of human health and the environment for as long as the waste remains hazardous.” Subsections (d), (e), and (g), but not (f), state that to meet this standard, an applicant must demonstrate to the Administrator, to a reasonable degree of certainty, that “there will be no migration of hazardous constituents from the disposal unit or injection zone for as long as the wastes remain hazardous.” With regard to these safety standards, the EPA made the following interpretations: First, the EPA decided to apply the “no migration” standard to waste disposal governed by subsection (f) (deep injection of the “California list” wastes and solvents and dioxins) as well as to disposal governed by the other subsections.

Second, it decided that the term “the wastes” in the statutory no migration standard refers to the wastes that migrate out of the injection zone, and that the no migration standard is therefore satisfied if the injector demonstrates that no hazardous waste will migrate out of the injection zone.

Third, it decided that the term “migration” encompasses not only fluid migrations, but also migrations by molecular diffusion.

Fourth, it decided that in demonstrating that there will be no improper migration, an injector must show that there will be no migration for as long as the wastes remain hazardous, or for 10,000 years, whichever period is shorter.

Finally, the EPA decided that the term “injection zone,” which is not defined in the statute, means any geological formation, group of formations, or part of a formation, that can meet RCRA’s safety standards for containing waste.

In the preamble to its regulations, the EPA stated that the regulations might apply to the disposal of waste in geologic repositories. The EPA would, however, use individualized permit proceedings to determine whether the new regulations could appropriately govern disposal of hazardous waste in geologic repositories.

The Chemical Manufacturers Association and other industry groups (collectively referred to as “CMA” or “industry petitioners”) claim that the EPA’s rules impose unreasonably strict requirements that are unrelated to protection of human health and the environment. The Natural Resources Defense Council and other environmental groups (collectively referred to as “NRDC” or “environmental petitioners”) claim the rules are not as protective of human health and the environment as the statute requires. The environmental petitioners also claim that the EPA cannot, consistent with the statutory directives of § 6924(b),⁴ apply the regulations to the injection of waste into geologic repositories.

In passing on the claims of the petitioners, we are guided, as always, by Chevron U.S.A., Inc. v. NRDC, 467 U.S. 837, 104 S.Ct. 2778, 81 L.Ed.2d 694 (1984). With respect to each claim, we inquire “whether Congress has directly spoken to the precise question at issue.” Id. at 842, 104 S.Ct. at 2781. In determining the intent of Congress, we must look to “the particular statutory language at issue, as well as the language and design of the statute as a whole,” K Mart Corp. v. Cartier, Inc., 486 U.S. 281, 108 S.Ct. 1811, 1817, 100 L.Ed.2d 313 (1988), and we must employ traditional tools of statutory construction, including, where appropriate, legislative history. Ohio v. United States Department of the Interior, 880 F.2d 432, 441 (D.C.Cir.1989). If the intent of Congress is clear, we must give it effect. 467 U.S. at 842-43, 104 S.Ct. at 2781. If, however, the statute is silent or ambiguous on a particular issue, we must defer to the agency’s interpretation of the statute so long as it is reasonable and consistent with the statutory purpose. Id. at 844-45, 104 S.Ct. at 2782-83.

II. INDUSTRY CLAIMS

A. The Permit Petition Process

CMA claims that the EPA cannot generally ban the disposal of hazardous waste by deep injection, and require site-specific deep injection permits under subsection (f) or (g), because it has failed to make the determination, required by subsections (f)(2) and (g)(5), that absent a site-specific permitting process, deep injection of hazardous waste may not be protective of human health and the environment for as long as the wastes remain hazardous. We disagree.

1. Subsection (g)

The text of subsection (g) makes clear that the EPA may ban any method of land disposal of wastes governed by subsection (g) without first determining that the method is not protective of human health and the environment. Subsection (g)(1) provides that the Administrator must submit to Congress a schedule of the dates by which the agency plans to regulate the hazardous wastes not covered by subsections (d), (e) and (f). The EPA submitted this schedule to Congress in 1986. See 51 Fed.Reg. 19,300 (1986). Subsection (g) then goes on to state (and this is the language upon which CMA relies) that by the dates specified in the schedule, the Administrator “shall promulgate final regulations prohibiting one or more methods of land disposal of the hazardous wastes [on the schedule] except for methods of land disposal which the Administrator determines will be protective of human health and the environment for as long as the waste remains hazardous.” Subsection (g)(5).

Nothing in this language requires the Administrator to determine that a method of land disposal is not safe before prohibiting it. Rather, the statute commands the Administrator to promulgate prohibitory regulations unless he has made an affirmative determination of safety. If by the dates specified in the schedule the Administrator has made no determinations at all (because, for example, he finds the evidence concerning safety to be in equipoise), he must promulgate regulations prohibiting one or more methods of land disposal. It seems indisputable to us that Congress did not require the EPA, before banning methods of land disposal, to determine affirmatively that they were not safe. Congress had already legislatively assumed that fact, subject only to the Administrator’s decision otherwise.

Subsection (g) states further that the schedule sent to Congress “shall require . that the Administrator shall promulgate regulations in accordance with paragraph (5) or make a determination under paragraph (5)” by dates certain. Subsection (g)(4) (emphasis added). The disjunctive “or” reinforces the interpretation that the EPA may promulgate the prohibitory regulations referred to in subsection (g)(5) without making any formal determinations. That is, although subsection (g) requires the Administrator to make a determination of safety before permitting a method of land disposal, it clearly does not require the Administrator to make any determination before prohibiting a method of land disposal.

CMA, however, points to the last sentence of subsection (g)(4), which states that whenever the EPA adds a new waste to its list of hazardous wastes, the agency must “determine” within six months of the new listing whether the waste shall be prohibited from disposal. CMA argues that this sentence states explicitly that the EPA must make determinations regarding the appropriate scope of prohibitions before promulgating prohibitions. This sentence, in our view, provides scant support for CMA’s argument. It requires the EPA to determine only whether disposal of the newly listed waste is to be prohibited, not whether disposal is protective of human health and the environment. Since, as we have seen, the EPA can prohibit a disposal method under subsection (g) without making a safety determination, this sentence would seem to have no bearing on the issue of whether a safety determination is required. It requires only that a decision to prohibit or not to prohibit be made in the six month timeframe.

CMA also notes that the “hammer” provision of subsection (g)(6)(C), which prohibits the land disposal of any scheduled hazardous waste if the EPA fails to promulgate regulations or make determinations regarding that waste within five and a half years, provides the EPA with an unusually long leadtime if its only duty is to determine whether or not to do anything to prevent automatic prohibition. This argument misses the purpose of the hammer provision. Obviously, it would not have taken the EPA much time to issue a blanket ban on land disposal of all wastes on the schedule. But the EPA would need time to decide which, if any, waste disposal methods not to ban. The statute requires the EPA to prohibit one or more methods of land disposal except those it positively determines are safe; the 66 months were designed to afford the EPA sufficient time to affirmatively declare a safe harbor for those methods of land disposal it found worthy of rescue from the statutory hammer.

2. Subsection (j)

a. The EPA’s Determination.

Subsection (f), which deals with deep injection of the “California list” wastes and solvents and dioxins, is worded differently from subsection (g). It says that the Administrator

shall make a determination regarding the disposal by underground injection into deep injection wells of the hazardous wastes referred to in [subsections (d) and (e)]. The Administrator shall promulgate final regulations prohibiting the disposal of such wastes into such wells if it may reasonably be determined that such disposal may not be protective of human health and the environment for as long as the waste remains hazardous[.]

Subsection (f)(2). We find that the Administrator determined that deep well injection might not protect human health and the environment without a permit petition process.

In the preamble to the regulations, the EPA stated its belief that the regulations’ new substantive standards for hazardous waste injection wells provided an appropriate level of protection for the injection of hazardous waste. 53 Fed.Reg. at 28,131 (col. 1). The EPA noted that it had identified several important ways to improve the protectiveness of the original UIC regulations. Id. For instance, the new regulations tightened the requirements concerning the choice of the well site, see id. at 28,132-33, widened the “area of review” (the geographical area within which a permit applicant must identify all existing wells and determine whether they have been properly completed or plugged and abandoned), see id. at 28,134-35, and imposed new requirements concerning the construction and operation of the well, see id. at 28,136-39. Despite these new safeguards, however, the EPA concluded that it could determine whether hazardous waste injection would protect human health and the environment only in the context of site-specific permit proceedings. Id. at 28,121 (col. 1). The agency specifically rejected the feasibility of any substantive standards that would permit a generic finding that hazardous waste injection would protect human health and the 'environment so long as injectors certified to the agency that their wells met the substantive standards. Id. It decided that individualized determinations, made in site-specific proceedings, would be more reliable. Id.

We think it reasonable to infer from the preamble that the EPA made the following determinations: (1) that deep well injection could, under appropriate circumstances, be protective of human health and the environment, and (2) that deep well injection might not be protective of human health and the environment unless it were governed by the agency’s regulatiqns, and particularly by the requirement that injectors obtain a site-specific permit from the agency before injecting any hazardous waste. The first determination is reflected in the EPA’s decision to set up procedures for the issuance of deep injection permits; it would hardly have set up permitting procedures unless it believed that permits could be issued. As to the second determination, the agency’s statement was clarion: it could not decide generically that deep injection protected human health and the environment; only a site-specific permit procedure would produce the necessary information for such a conclusion.

These determinations easily satisfy subsection (f). Indeed, insofar as CMA’s claim that the EPA’s determinations do not justify prohibitive regulations is concerned, the second determination, that the EPA could not generically find deep well injection to be safe, is all that is required in refutation. CMA claims that the agency failed to determine that deep injection could not be safe without a permit petition procedure. But the second sentence of subsection (f)(2) (assuming that it requires any determination at all) requires only that the EPA determine that deep well injection “may not be” protective of human health and the environment before prohibiting it. The agency’s explicit rejection of the concept of deep well injection by injector certification certainly reveals an agency determination that such a process might not be protective of human health and the environment.⁵

b. Support for the EPA’s Determination to Require Site-Specific Permits.

CMA further claims that the EPA’s decision to require individual applications for site-specific permits under subsection (f) was arbitrary and capricious. CMA asserts that the EPA’s own reports show that deep well injection could safely be regulated by a rule permitting its use whenever it satisfied the substantive standards of the UIC regulations. On the basis of the record before us, we uphold the EPA’s determination. That record contains ample evidence from which the EPA could rationally conclude that only site-specific inquiries by the agency into the geology of particular deep injection sites could adequately insure that deep injection would protect human health and the environment. The EPA reports cited by CMA establish, at most, that the EPA found the UIC regulations, as amended, to be acceptable substantive standards by which to judge the safety of a particular deep injection well; they do not support a conclusion that the EPA had determined that injectors could be trusted to decide for themselves whether a particular well met the standards.⁶

B. Application of the “No Migration” Standard to Subsection (f)

Subsections (d), (e), and (g) all state that the Administrator may not determine a method of land disposal to be protective of human health and the environment unless an interested person demonstrates that there will be no migration of hazardous constituents from the injection zone for as long as the wastes remain hazardous. Subsection (f) does not explicitly require that deep injection satisfy this “no migration” standard. However, the EPA, as a matter of policy, decided to apply the no migration standard to disposal under subsection (f) anyway. 53 Fed.Reg. at 28,120 (col. 3). The industry petitioners claim that the EPA lacked authority to inject that requirement into subsection (f).

Although subsection (f) does not contain the no migration standard, it does require the EPA to prohibit deep well injection if it determines that this method of land disposal may not be protective of human health and the environment. The phrase “protective of human health and the environment” is not defined by the statute. But surely it was within the Administrator’s discretion to decide that a disposal method must meet the congressionally sanctioned no migration standard in order to protect human health and the environment. Indeed, it would have been downright surprising for him to do otherwise. Subsection (g) of the statute, which governs deep injection of all wastes not specified in subsections (d) and (e), explicitly contains the no migration standard. Had the Administrator decided that the no migration standard did not apply to subsection (f), he would have created the anomaly of legalizing injection of the wastes specified in subsections (d) and (e) without meeting the statutory standard laid down for the wastes included in subsection (g), even though there is no reason to think that the subsections (d) and (e) wastes are less hazardous than any others.⁷ The Administrator’s decision to apply the no migration standard to subsection (f) seems almost foreordained.

CMA, however, claims that the EPA acted arbitrarily because the no migration standard, which focuses on whether any hazardous constituents cross the injection zone boundary, is totally disconnected from the protection of human health and the environment. Human health and the environment, CMA contends, are not affected until and unless hazardous waste migrates to a point of contact with something that humans actually use, such as drinking water. We find this argument unpersuasive. Congress itself decided that the no migration standard is an essential part of protecting human health and the environment so far as injection of subsection (g) wastes is concerned. How then could it be unreasonable for the EPA to apply the same standard regarding injection of subsections (d) and (e) wastes? ⁸

C. The Content of the “No Migration" Standard — Industry Challenges

1. RCRA and SDWA

The industry petitioners claim that as a matter of statutory interpretation the no migration standard of RCRA is identical to the safety standard of the Safe Drinking Water Act (“SDWA”), 42 U.S.C. § 300h(b)(1).⁹ We disagree.

The texts of RCRA and SDWA provide no support for the CMA’s identity argument. SDWA protects sources of drinking water; RCRA protects human health and the environment. SDWA states that underground injection must not endanger drinking water sources; RCRA states that there must be no migration of hazardous constituents from the injection zone for as long as the wastes remain hazardous; it makes no reference to anything outside the injection zone that might be threatened by such a migration. The statutory texts provide no evidence whatsoever that Congress intended that the RCRA and SDWA standards be identical.'

CMA’s argument to the contrary rests almost entirely on some remarks made by Senator Bentsen on the Senate floor during the debates on HSWA. See Brief for Petitioner CMA at 40-41.¹⁰ These remarks do support the CMA’s interpretation, but they are the remarks of a single Senator, made on the Senate floor, and without echo in the remarks of other Senators, or in the House or Senate reports. We have often noted that the comments of a single legislator cannot carry the day for a statutory interpretation contrary to that indicated by the statutory text. See, e.g., United States v. McGoff, 831 F.2d 1071, 1090 (D.C.Cir.1987); International Brotherhood of Electrical Workers v. NLRB (St. Francis Hospital), 814 F.2d 697, 713 (D.C.Cir.1987).

The only question remaining is whether the EPA’s interpretation of RCRA as imposing a stricter standard on deep well injection of hazardous waste than SDWA is reasonable and consistent with RCRA’s purposes. We believe that it is. RCRA’s facially stronger text, requiring “no migration of hazardous constituents from the ... injection zone for as long as the wastes remain hazardous,” subsection (g)(5), makes the EPA’s tougher stance entirely reasonable. Its unstinting interpretation is consistent with RCRA’s expressly stated purpose of minimizing the land disposal of hazardous waste.

2. Molecular Diffusion

The EPA is also accused of acting unreasonably in requiring injectors to show under subsections (f) and (g) that there would be no migration of hazardous constituents outside the injection zone via molecular diffusion.¹¹ CMA argues that the EPA’s own studies show that molecular diffusion causes hazardous constituents to migrate only a small distance from the injection zone, and poses no significant threat to underground sources of drinking water. However, CMA’s characterization of the studies, even assuming it to be correct, is irrelevant. The question is not whether molecular diffusion causes hazardous waste to migrate far from the injection zone, or to migrate in a way that threatens drinking water. The EPA properly recognized the correct question to be whether molecular diffusion can cause hazardous waste to migrate outside the injection zone in violation of RCRA’s “no migration” standard.

On the record before us, we uphold the EPA’s finding that molecular diffusion may result in the migration of hazardous levels of hazardous constituents outside the injection zone in some cases. The EPA points to a study that shows that molecular diffusion can cause hazardous constituents to migrate dozens of feet outside the injection zone. D. Morganwalp & R. Smith, Modeling of Representative Injection Sites (undated), reprinted in J.A. 1232. Of itself this may not seem a large migration, but it must be remembered that the statute requires that there be no migration of hazardous constituents outside the injection zone. The EPA could conclude that the statute forbids even a migration of only a few feet.

The EPA therefore properly concluded from the record that molecular diffusion was a physical mechanism that could cause a forbidden migration of hazardous constituents. In designing its permitting process, the EPA certainly had the power to require that applicants, when making their “no migration” demonstrations, take into account all known mechanisms that can cause forbidden migrations. The EPA therefore properly required applicants to demonstrate that there would be no migration of hazardous constituents via molecular diffusion.

3. Ten Thousand Years

Did the EPA abuse its discretion in requiring injectors to show that there will be no migration of hazardous constituents from the injection zone for 10,000 years? CMA claims both that Congress intended a shorter period, and that 10,000 years is longer than the hazardous life of the wastes.

First, Congress never decided for itself that a shorter period than 10,000 years was adequate for keeping hazardous wastes in the injection zone. CMA can point to no decisive support for its less than 10,000 year claim in the statutory text or history. It is true that the legislative reports speak of the decomposition time of hazardous waste in terms of centuries rather than millennia. See S.Rep. No. 284, 98th Cong., 1st Sess. 15 (1983) (“Wastes chemically decompose in a land disposal facility, although often this decomposition occurs very slowly stretching over centuries.”); H.R.Rep. No. 198, 98th Cong., 1st Sess. 33, reprinted in 1984 U.S.Code Cong. & Ad. News 5592 (“land disposal of wastes ... might be appropriate if there is a reasonable certainty that wastes will be contained in the very long-term (i.e., at least several hundred years).”). However, the statute mandates containment “for as long as the wastes remain hazardous.” Subsection (g)(5). Congress could hardly have desired the EPA to regulate waste disposal only for centuries even if scientific predictions project that the waste could remain hazardous for a much longer period. The EPA chose 10,000 years as a time limit because it would be long enough to insure that the “no migration” standard would be met, and yet short enough to come within the limitations of predictability. See 53 Fed.Reg. at 28,126. The EPA’s choice was reasonable and consistent with the statutory purpose of preventing migration of hazardous constituents for as long as the wastes remain hazardous.

CMA may be correct that some injected wastes will cease to be hazardous in less than 10,000 years; in such a case the regulations specifically allow the injector to seek relief on the basis of such a showing. The regulations require the injector to show that there will be no migration so long as the wastes remain hazardous, or for 10,000 years, whichever period is shorter. The EPA’s regulation properly places its emphasis on the statutory requirement of preventing migration for as long as the wastes remain hazardous. It does not require injectors to control the wastes for a longer period.

D. Conclusion

For the foregoing reasons, we deny the industry’s petitions for review.

III. NRDC CLAIMS

A. EPA's Interpretation of the “No Migration” Standard.

Subsections (d), (e) and (g) of § 6924 ban the land disposal of hazardous wastes unless, for each of three specified methods, it has

been demonstrated to the Administrator, to a reasonable degree of certainty, that there will be no migration of hazardous constituents from the disposal unit or injection zone for as long as the wastes remain hazardous.

42 U.S.C. § 6924(d), (e), (g) (emphasis added). As we noted above, EPA read this “no migration” standard into subsection (f) as well, in the interests of uniform treatment. See 53 Fed.Reg. at 28,120 (col. 3). The issue here is whether the “no migration” standard would be violated if hazardous constituents seeped out of the storage area at a time where some of the stored wastes were still hazardous, but not those actually leaving.

“Hazardous constituent” is a term of art referring to a list of chemical compounds compiled at 40 C.F.R. Part 261, Appendix VIII. Since these are defined by molecular formulae without reference to concentrations, a single molecule of such a chemical is a “hazardous constituent.” A hazardous waste, by contrast, is such only if various factors, including the concentration of hazardous constituents,¹² actually make it hazardous to human health or the environment. 40 C.F.R. § 261.11 (a)(3)(ii); 42 U.S.C. § 6921(b) (requiring the Administrator to list wastes as hazardous if they contain “certain constituents (such as identified carcinogens, mutagens, or teratagens) at levels in excess of levels which endanger human health.”). Thus, read literally, the “no migration” standard would seem to prohibit the migration of even a single molecule (or perhaps an appropriate de minimis amount) for the statutory time period, even though the migrating waste is itself not hazardous at all.

There is, however, an ambiguity in the statutory definition of the period — “as long as the wastes remain hazardous.” To which “wastes” does the clause refer? Since the consistency of wastes may vary throughout an injection zone (or a disposal unit), wastes in one part, particularly near the perimeter, may no longer be hazardous while wastes in another part still are. Can hazardous constituents in such non-hazardous wastes migrate without violating the “no migration” standard? Or must the operator assure no migration of constituents until all wastes throughout the injection zone are no longer hazardous? Noting this textual ambiguity, the EPA interpreted “the wastes” to refer to the wastes containing the hazardous constituents that are leaving the injection zone.¹³ 52 Fed.Reg. at 32,453 (col. 3); 53 Fed.Reg. at 28,122 (col. 2). Under this reading, hazardous constituents may migrate so long as “the wastes” immediately surrounding them at the border are no longer hazardous, or, putting it slightly differently, so long as they do not migrate in high enough concentrations to be hazardous wastes.

NRDC’s most powerful argument is then that EPA has artfully transformed the standard “no migration of hazardous constituents ” into a quite different standard —“no migration of hazardous wastes.” We find this challenge quite close, but the ambiguity is serious enough to allow EPA under Chevron to resolve the issue in the way it has.

The EPA rests in part on a structural contention that “[o]rdinarily the term ‘hazardous constituents’ has no regulatory effect unless concentrations are also considered.” 53 Fed.Reg. at 28,122 (col. 3). See also Brief for Respondent at 49-51; 55. This is true in the sense that RCRA “ordinarily” regulates “hazardous wastes,” which of course are defined (in part) in terms of concentrations of hazardous constituents, but it is obviously not a basis for reading “constituents” as “wastes” when Congress chooses the former. Nonetheless the exceptional character of the usage may suggest a need to be on guard against facile literalism.

The parties agree that we can learn something from § 6925(j), one of the rare other places in RCRA where Congress used the term “constituents” with a material regulatory effect. Section 6925(j) conditions a variance from certain technological control requirements for certain surface impoundments on the operator’s proving that there will be “no migration of any hazardous constitutent [sic] into ground water or surface water at any future time.” 42 U.S.C. § 6925(j)(4). Even the EPA assumes that this other “no migration” standard prohibits migration of any hazardous constituent (down to a single molecule or an appropriate de minimis level). See Brief for Respondent at 54-55. Nonetheless, § 6925(j) does not contain the ambiguous “for so long as the wastes remain hazardous” clause. Furthermore, it contains an additional, if not earth-shattering, textual difference. While § 6924 prohibits “migration of hazardous constituents,” § 6925(j) prohibits “migration of any hazardous constituent.” If Congress had wanted § 6924’s “no migration” standards to cover a single molecule, it could have expressed the point as aggressively as in § 6925. And of course the context suggests a reason for a difference: § 6925(j)(4) protects water resources, which are not directly implicated under the controlling language of § 6924, and its “no migration” requirement works only as a condition for a variance. Overall, however, we find the statutory language and structure ambiguous and so turn to the legislative history.

Going into the Joint Conference, the House bill’s precursor of § 6924 did not insist on anything like “no migration.” It required the EPA to ban disposal of hazardous waste into deep injection wells if it “may not be protective of human health and the environment for as long as the waste remains hazardous.” H.R. 2867, 98th Cong., 1st Sess. § 5(a) (“Land Disposal of Certain Hazardous Wastes”) at 21 (1983). Other sections contained similar standards for prohibiting various methods of land disposal. See id. at 18-19, 23-24. The bill’s only “no migration” standard was in its version of what became § 6925(j). See id. § 5(b) (“Interim Status Surface Impoundments”) at 29. But for purposes of § 6925(j) the House bill specially redefined the term “hazardous constituent” to encompass only such constituents as would migrate “in concentrations which may adversely affect human health or the environment.” Id. § 18 (“Hazardous Constituents”) at 76. See also H.R. Conf.Rep. No. 1133, 98th Cong., 2d Sess. 96-97, reprinted in 1984 U.S.Code Cong. & Admin. News 5649, 5667-68 (noting House’s restrictive definition). The House specifically added this because it was concerned that “there may be no facilities that meet that kind of a rigorous test,” i.e., the strict “no migration” test. 129 Cong.Rec. 27,673; see also 129 Cong.Rec. 30,827-28 (adopting definition to clarify House’s intention).

The Senate bill provided a “no migration” standard in its prototype of § 6924 but, as even NRDC recognizes, it was clearly not as stringent as NRDC’s interpretation of § 6924 as enacted. See Brief for Petitioners NRDC at 22-23. It provided that

if a specified waste contains significant concentrations of one or more hazardous constituents that is highly toxic, highly mobile, or has a strong propensity to bioaccumulate, a method of land disposal may not be determined to be protective of human health and the environment for such specified hazardous waste, unless ... there will be no migration of such constituents from the disposal unit or injection zone for as long as the wastes remain hazardous.

130 Cong.Rec. 20,867-68 (setting forth final Senate Bill) (emphasis added). See also S.Rep. No. 284 at 84 (identical standard in earlier Senate draft). First, as NRDC concedes, this proposal required a showing of “no migration” only for those wastes that contained significant concentrations of certain hazardous constituents (those that are highly toxic, highly mobile, or have a strong propensity to bioaccumulate). Brief for Petitioner NRDC at 22. Moreover, although there is some ambiguity in the proposal’s “no migration” standard too, the best interpretation seems to be that “such constituents” refers back to “significant concentrations” of highly toxic, etc. “hazardous constituents.” That reading explains why the Senate Report uses the terms constituents and wastes interchangeably in describing the standard. See S.Rep. No. 284 at 14 (describing the standard as “no migration of [certain] constituents ”) (emphasis added); id. at 15 (describing the standard as prohibiting the “migration of the wastes ”) (emphasis added).

To complete the picture on the eve of conference, we note the Senate’s version of § 6925(j)’s “no migration” standard. This used virtually the same language as Congress enacted in § 6925(j), see H.R. 2867 as amended in Senate, 98th Cong., 2d Sess. § 6(c), 130 Cong.Rec. 20,866, 20,869 (col. 3),¹⁴ with no limiting definition of “hazardous constituent” equivalent to the House’s. The Senate expressly noted the special ground for concern that we have already mentioned — § 6925(j)’s application to migration into ground water. See 130 Cong. Rec. 20,847 (statement accompanying amendment) (expressing grave concern that hazardous contents in surface impoundments can migrate into “the surrounding soils where even dilute concentrations of toxic substances can, over time, pollute the ground water”). Thus, of the four clauses heading into conference (one from each house for each of §§ 6924 and 6925(j)), only one, the Senate version of § 6925(j), had a clause barring migration of hazardous constituents regardless of concentration.

The Conference’s discussion of the various clauses gives no hint of any intention to go beyond the two houses’ standards for § 6924. As to § 6925(j), there was a clear conflict between the House and Senate versions, because the House’s “no migration” standard included a limiting definition of “hazardous constituent” and the Senate’s did not. The Conference Report explicitly opted for the more stringent Senate version:

[T]he owner or operator must demonstrate no migration of any hazardous constituent into groundwater or surface water at any future time. In this formulation, the Conferees explicitly reject the provision contained in section 18 of the original House bill modifying the definition of hazardous constituent.

H.R.Conf.Rep. No. 1133 at 98, reprinted in 1984 U.S.Code Cong. & Admin.News at 5669. By contrast, the Conference Report is silent on § 6924’s “no migration” standard, even though a conflict existed here as well, between the Senate’s “no migration” standard and the House’s general requirement of protecting human health and the environment. Obviously the Conference followed the Senate, changing its language in the course of doing so. In place of “no migration of such constituents” (“such” referring back to “significant concentrations” of certain types of hazardous constituents), it substituted “no migration of hazardous constituents.” Textually this minor change was necessary because another substantive change in the section¹⁵ had made it grammatically impossible for “such” to refer to its previous antecedent. If NRDC’s interpretation were adopted, however, this small textual change would have radically altered the Senate’s “no migration” standard and imposed a standard significantly more stringent than either house had passed.

Of course nothing prevents a conference from adopting a position quite different from both houses. There can be compromises like the British naval rearmament decision of 1909, when, as Churchill put it, “The Admiralty had demanded six ships; the economists offered four; and we finally compromised on eight.” See Ted Morgan, Churchill: Young Man in a Hurry, 1874-1915 249 (1982). But one would expect at least a word of explanation in the Conference Report. In fact all it said was that “[t]he Conference substitute adopts provisions from both the House bill and the Senate amendment.” H.R.Conf.Rep. No. 1133 at 86; reprinted in 1984 U.S.Code Cong. & Admin.News at 5657. Moreover, the participants seem most often to have either ignored or misunderstood the distinction between hazardous constituents and hazardous wastes, though it is central to NRDC’s interpretation. See S.Rep. No. 284 at 14-15 (conflating its no migration of “such constituents" standard with a no migration of “wastes” standard); 129 Cong. Rec. 27,673 (col. 2) — 74 (col. 1) (Rep. Breaux repeatedly assuring other Congressmen that “hazardous constituent” would be defined in terms of concentration levels, even before the House amended the definition of hazardous constituent). As to § 6925(j), of course, Congress ultimately did focus on the distinction, made a choice, and explained it, and clearly expressed it in the statute. As to § 6924, however, there is neither focus nor choice nor explanation nor clarity.

Finding no clear congressional determination in § 6924 that hazardous constitu-. ents must not migrate regardless of hazard, and the reasonableness of the EPA’s policy choice being unquestioned, we affirm its decision.

B. The Definition of “Injection Zone. ”

Though nowhere in RCRA is there a statutory definition of the term “injection zone,” both EPA and NRDC agree that Congress intended to incorporate the preexisting regulatory definition from EPA’s Safe Drinking Water Act regulations, 40 C.F.R. § 146.3:

Injection zone means a geological “formation”, group of formations, or part of a formation receiving fluids through a well.

See S.Rep. No. 284 at 16 (citing regulations); 53 Fed.Reg. at 28,121 (col. 3); Brief for Respondent at 59; Brief for Petitioners NRDC at 27.

The EPA adopted this definition for the regulations, but it added a new spin, at least as NRDC views it. Under the new interpretation, the injection zone could extend across both the porous formations into which fluid is actually injected and the confining material, which is composed of less permeable layers of rock and clay. 53 Fed.Reg. at 28,121 (col. 3) — 22 (col. 1). As a result of this view, seepage of hazardous constituents into confining material will not qualify as forbidden migration where the particular confining material has been included as part of the injection zone. NRDC argues that confining material cannot be included in an injection zone because it does not “receiv[e] fluids through a well.”

First, though both parties agree that the Senate Report’s reference to the SDWA definition binds EPA, we think the proper standard of review here is highly deferential. When Congress refers to a pre-existing regulatory definition, it may intend that the then-current interpretation be frozen as part of the definition or that the agency retain at least an approximation of its ordinarily large discretion to interpret (and reinterpret) its own regulations. See United States v. Larionoff, 431 U.S. 864, 872-73, 97 S.Ct. 2150, 2155-56, 53 L.Ed.2d 48 (1977) (agency’s interpretation of its own regulations will be accepted unless “plainly inconsistent with the wording of the regulations”); General Carbon Co. v. Occupational Safety and Health Review Comm’n, 860 F.2d 479, 483 (D.C.Cir.1988) (same). Of course the interpretive discretion may not be quite as broad as where the regulation is exclusively of the agency’s own choice; here (on the parties’ and our assumption) the agency lacks the power to amend the regulation, and that power is the main rationale for the agency’s special discretion in reading its own regulations. In this case, in any event, the Senate Report suggests the dynamic view — an expectation that as the EPA implemented the 1984 Amendments to RCRA, it would apply its experience to the definitional process:

In determining appropriate confinement from which migration shall not be allowed to occur, the terms disposal unit or . injection zones should be construed ... in terms of the overall environmental integrity of the disposal practice, keeping in mind, in particular, the potential for contamination of groundwater or surface water resources.

S.Rep. No. 284 at 15.

But even if we were to ignore this passage and to treat the injection zone definition in § 146.3 as if it were part of the statute, Chevron would require us to accept the agency’s interpretation.

NRDC suggests that EPA’s construction has created an improper overlap between “injection zone” and “confining zone,” another term defined in the pre-existing SDWA regulations:

a geological formation, group of formations, or part of a formation that is capable of limiting fluid movement above an injection zone.

40 C.F.R. § 146.3. NRDC argues that EPA’s new reading allows the injection zone to extend into what should properly be considered only the confining zone. But the language of the definitions does not explicitly indicate that they are to be mutually exclusive. Nor does their logic or sense.

NRDC also urges that the EPA so distorted the term injection zone that it needed to create a new term, the “injection interval,” defined as “that part of the injection zone ... in which the waste is ... directly emplaced,” 53 Fed.Reg. at 28,148 (col. 2), codified at 40 C.F.R. § 146.61(b), to serve the function Congress contemplated for the injection zone when it defined that as formations “receiving fluids through a well.”

But in fact fluids from an injection well can seep into the confining material, and when they do that material will “receiv[e] fluids through a well.” Moreover, because some such seeping is inevitable, see 53 Fed.Reg. at 28,122 (col. 1), NRDC’s interpretation of injection zone, coupled with the “no migration” standards of § 6924, would result in an absolute ban on deep well injection of wastes — a result Congress plainly did not intend. Furthermore, as EPA has argued (and NRDC offered no evidence to dispute), “there is often not a line where a large permeable strata meets relatively less permeable strata.” 53 Fed. Reg. at 28,122 (col. 1). The definitional issue here is highly technical, and we can find no legal fault in EPA’s solution.

C. Geological Repositories or Salt Domes.

Section 6924(b)(1) prohibits the disposal of certain wastes in salt dome formations, salt bed formations, underground mines, or caves (collectively referred to here as salt domes) until three conditions have been satisfied. Two are relevant to this case:

(B) the Administrator has promulgated performance and permitting standards for such facilities under this subchapter, and;

(C) a permit has been issued under section 6925(c) of this title for the facility concerned.

§ 6924(b)(l)(BMC).

As to (B)’s requirement, EPA announced that the new regulations governing underground injection of hazardous wastes (the “amended Part 146 regulations”) “could constitute performance and permitting standards for [salt domes].” 53 Fed.Reg. at 28,131 (col. 2) (emphasis added). See also Brief for Respondent at 66. But the agency hedged on the point, adding:

It is premature to discuss specific features which would make the application of Part 146 standards inappropriate. Such determinations will be made in the context of a facility’s permit application.

53 Fed.Reg. at 28,131 (col. 3). NRDC charges that as caves and mines are different from the sort of porous strata for which the regulations are explicitly designed, EPA never established any basis for believing that the regulations could accomplish Congress’s purpose. Comments of the Natural Resources Defense Council, Inc., et al., on EPA’s Proposed Land Disposal Ban Rule for Underground Injection Wells at 66-67 (FRL-3220-3). See also 129 Cong.Rec. 27,664 (col. 3) (Rep. Breaux, sponsor of the amendment covering salt domes, stating that it banned disposal “until such time as we have a study to show that it can be done safely”). Instead of addressing this challenge, EPA said that the permit process would allow the public to challenge the applicability of Part 146 in specific geological settings. Brief for Respondent at 74; 53 Fed.Reg. at 28,131 (col. 3).

EPA says the issue is not yet ripe because it has made no final decision on when the Part 146 standards would be appropriate for salt domes; it will do so case by case as it reviews permit applications. 53 Fed.Reg. at 28,131 (col. 3); Brief for Respondent at 66. But this fails to respond to NRDC’s ancillary procedural point: that subsection (B) requires advance promulgation of regulations suitable for salt domes. The point may sound persnickety, but isn’t. If these regulations may be applied to salt domes, then this rulemaking is the only rulemaking format within which NRDC (or others) could challenge their suitability. NRDC naturally wants a chance to challenge generic standards rather than being reduced to prolonged guerilla warfare in the permit process. EPA’s statements in both the proposed and final rules leave no doubt that it has reached the conclusion that the Part 146 regulations can (in appropriate circumstances) be used for geological formations. See 52 Fed.Reg. at 32,449 (col. 2) (“The Agency is proposing that the [amended Part 146] regulations ... constitute performance and permitting standards for such facilities as required by section [6924(b)(1)(B)].”); 53 Fed.Reg. at 28,131 (cols. 2-3). Thus the procedural issue — must EPA have regulations in place, which it has established, in a rulemaking, as at least contingently applicable to salt domes, before it issues any permits? — is clearly ripe. Cf. Air Transport Ass’n of America v. Dep’t of Transportation, 900 F.2d 369, 374 (D.C.Cir.1990).

Despite the usual broad discretion of agencies to select as between rulemaking and adjudication, SEC v. Chenery Corp., 332 U.S. 194, 203, 67 S.Ct. 1575, 1580, 91 L.Ed. 1995 (1947), NLRB v. Bell Aerospace Co., 416 U.S. 267, 294, 94 S.Ct. 1757, 1771, 40 L.Ed.2d 134 (1974), we agree with NRDC. Section 6924(b)(1)(B) requires prior promulgation of standards “under this sub-chapter,” surely incorporating § 6924(a)’s procedural requirement that § 6924 regulations be promulgated “after opportunity for public hearing.” See § 6924(a) and § 6924(b)(1)(B) (referencing the procedures of § 6924(a)). EPA has, to be sure, promulgated standards, but it has not explained their application to salt domes (which would be required substantively if they are to apply) nor defined in any way the class of salt domes to which they may apply.

We assume great flexibility for EPA in this. For example, it could presumably develop one or more sets of permit requirements for salt domes, with some sort of gateway indicating — perhaps in quite general terms¹⁶ — which types fall under which set. And there is no procedural obstacle to its identifying the Part 146 regulations as such a set. But for it to do so without giving a clue as to when those regulations will be applicable (and defending the lawfulness of their application to such cases) deprives NRDC of the procedural opportunity that § 6924 provided. Accordingly, we remand for the EPA either to drop the assertion of potential applicability, or to state the class of salt domes covered and justify the sense of doing so.

EPA’s second contested decision was that a permit issued under the underground injection well program of the Safe Drinking Water Act (40 C.F.R. parts 144 and 145) would satisfy subsection (C)’s RCRA permit requirement (“a permit ... issued under section 6925(c)”) because such a permit qualifies as a RCRA permit under EPA’s permit-by-rule regulations, 40 C.F.R. § 270.60. NRDC argues that because the injection well permit process does not include some of the procedural protections required by statute for RCRA permits, the regulation transforming those permits into RCRA permits is invalid. See Brief for Petitioner NRDC at 37-39, citing §§ 6974(b) (public notice requirements), 6925(a) (requirement that permit issue before construction of facility begins), 6925(c)(3) (five-year review requirement and agency’s omnibus power to add conditions to permit).

EPA’s sole defense is that NRDC’s statutory challenges are untimely; the original permit-by-rule regulation was promulgated in May 1980, see Consolidated Permit Regulations, 45 Fed.Reg. 33,290, 33,325-26, 33,435 (May 19, 1980), and the 90-day period for filing petitions for review, see 42 U.S.C. § 6976(a), has long since expired. Brief for Respondent at 64-65. See also 53 Fed.Reg. at 28,131 (col. 2) (justifying application of permit-by-rule by reference to pre-existing regulation).

EPA’s timeliness defense is dead wrong. All of NRDC’s objections are based on statutory provisions added after the 90-day period for review of the 1980 permit-by-rule decision had run.¹⁷ The statutory time bar is expressly qualified to allow challenges after 90 days if “based solely on grounds arising after such ninetieth day.” 42 U.S.C. § 6976(a). ,

As EPA has offered no other defenses, we must reverse and remand this issue. On remand, EPA will, in the first instance, be able to explain more fully the procedural requirements of the various permit processes and to interpret the statutory requirements for § 6925(c) permits. Nothing prevents the agency from integrating the underground injection well and RCRA procedures so as to avoid needless duplication (see, e.g., 45 Fed.Reg. at 33,325 (col. 3)), or from using other permissible techniques to alleviate administrative burdens in permitting (see NRDC v. Costle, 568 F.2d 1369, 1380-82 (D.C.Cir.1977) (Leventhal, J.) (discussing possible permitting procedures)). But before EPA can use its permit-by-rule regulations to satisfy § 6924(b)(1)(C)’s requirement that a permit be “issued under section 6925(c),” it needs to demonstrate that the process meets the statutory requirements for such a permit.

D. Conclusion.

We reverse the EPA’s actions on salt domes and remand for further proceedings consistent with this opinion. As to all other challenges, we deny NRDC’s petition for review.

So ordered.

1 . Parts I and II were written by Chief Judge Wald; Part III by Judge Williams.

2 . See H.R.Rep. No. 198, 98th Cong., 1st Sess. 20, reprinted in 1984 U.S.Code Cong. & Ad.News 5576, 5578-79.

3 . Section 6924 now provides:

(d) Prohibitions on land disposal of specified wastes

(1) Effective 32 months after November 8, 1984 (except as provided in subsection (f) of this section with respect to underground injection into deep injection wells), the land disposal of the hazardous wastes referred to in paragraph (2) is prohibited unless the Administrator determines the prohibition on one or more methods of land disposal of such waste is not required in order to protect human health and the environment for as long as the waste remains hazardous, taking into account—

(A) the long-term uncertainties associated with land disposal,

(B) the goal of managing hazardous waste in an appropriate manner in the first instance, and

(C) the persistence, toxicity, mobility, and propensity to bioaccumulate of such hazardous wastes and their hazardous constituents. For the purposes of this paragraph, a method of land disposal may not be determined to be protective of human health and the environment for a hazardous waste referred to in paragraph (2) (other than a hazardous waste which has complied with the pretreatment regulations promulgated under subsection (m) of this section), unless, upon application by an interested person, it has been demonstrated to the Administrator, to a reasonable degree of certainty, that there will be no migration of hazardous constituents from the disposal unit or injection zone for as long as the wastes remain hazardous.

(2) Paragraph (1) applies to the following hazardous wastes listed or identified under section 6921 of this title:

(e) Solvents and dioxins

(1) Effective twenty-four months after November 8, 1984 (except as provided in subsection (f) of this section with respect to underground injection into deep injection wells), the land disposal of the hazardous wastes referred to in paragraph (2) is prohibited unless the Administrator determines the prohibition of one or more methods of land disposal of such waste is not required in order to protect human health and the environment for as long as the waste remains hazardous, taking into account the factors referred to in subparagraph (A) through (C) of subsection (d)(1) of this section. For the purposes of this paragraph, a method of land disposal may not be determined to be protective of human health and the environment for a hazardous waste referred to in paragraph (2) (other than a hazardous waste which has complied with the pretreatment regulations promulgated under subsection (m) of this section), unless upon application by an interested person it has been demonstrated to the Administrator, to a reasonable degree of certainty, that there will be no migration of hazardous constituents from the disposal unit or injection zone for as long as the wastes remain hazardous. (2) The hazardous wastes to which the prohibition under paragraph (1) applies are as follows—

(f)Disposal into deep injection wells; specified subsection (d) wastes, solvents and dioxins

(1) Not later than forty-five months after November 8, 1984, the Administrator shall complete a review of the disposal of all hazardous wastes referred to in paragraph (2) of subsection (d) of this section and in paragraph (2) of subsection (e) of this section by underground injection into deep injection wells.

(2) Within forty-five months after November 8, 1984, the Administrator shall make a determination regarding the disposal by underground injection into deep injection wells of the hazardous wastes referred to in paragraph (2) of subsection (d) of this section and the hazardous wastes referred to in paragraph (2) of subsection (e) of this section. The Administrator shall promulgate final regulations prohibiting the disposal of such wastes into such wells if it may reasonably be determined that such disposal may not be protective of human health and the environment for as long as the waste remains hazardous, taking into account the factors referred to in subparagraphs (A) through (C) of subsection (d)(1) of this section. In promulgating such regulations, the Administrator shall consider each hazardous waste referred to in paragraph (2) of subsection (d) of this section or in paragraph (2) of subsection (e) of this section which is prohibited from disposal into such wells by any State.

(3) If the Administrator fails to make a determination under paragraph (2) for any hazardous waste referred to in paragraph (2) of subsection (d) of this section or in paragraph (2) of subsection (e) of this section within forty-five months after November 8, 1984, such hazardous waste shall be prohibited from disposal into any deep injection well.

(4) As used in this subsection, the term "deep injection well" means a well used for the underground injection of hazardous waste other than a well to which section 6979a(a) of this title applies.

(g) Additional land disposal prohibition determination

(1) Not later than twenty-four months after November 8, 1984, the Administrator shall submit a schedule to Congress for—

(A) reviewing all hazardous wastes listed (as of November 8, 1984) under section 6921 of this title other than those wastes which are referred to in subsection (d) or (e) of this section; and

(B) taking action under paragraph (5) of this subsection with respect to each such hazardous waste.

(4) The schedule under this subsection shall require that the Administrator shall promulgate regulations in accordance with paragraph (5) or make a determination under paragraph (5)—

(A) for at least one-third of all hazardous wastes referred to in paragraph (1) by the date forty-five months after November 8, 1984;

(B) for at least two-thirds of all such listed wastes by the date fifty-five months after November 8, 1984; and

(C) for all such listed wastes and for all hazardous wastes identified under 6921 of this title by the date sixty-six months after November 8, 1984.

In the case of any hazardous waste identified or listed under section 6921 of this title after November 8, 1984, the Administrator shall determine whether such waste shall be prohibited from one or more methods of land disposal in accordance with paragraph (5) within six months after the date of such identification or listing.

(5) Not later than the date specified in the schedule published under this subsection, the Administrator shall promulgate final regulations prohibiting one or more methods of land disposal of the hazardous wastes listed on such schedule except for methods of land disposal which the Administrator determines will be protective of human health and the environment for as long as the waste remains hazardous, taking into account the factors referred to in subparagraph (A) through (C) of subsection (d)(1) of this section. For the purposes of this paragraph, a method of land disposal may not be determined to be protective of human health and the environment (except with respect to a hazardous waste which has complied with the pretreatment regulations promulgated under subsection (m) of this section) unless, upon application by an interested person, it has been demonstrated to the Administrator, to a reasonable degree of certainty, that there will be no migration of hazardous constituents from the disposal unit or injection zone for as long as the wastes remain hazardous.

(6) ...

(C)If the Administrator fails to promulgate regulations, or make a determination under paragraph (5) for any hazardous waste referred to in paragraph (1) within 66 months after November 8, 1984, such hazardous waste shall be prohibited from land disposal.

4 . 42 U.S.C. § 6924(b) provides, in pertinent part:

(b) Salt dome formations, salt bed formations, underground mines and caves

(1) Effective on November 8, 1984, the placement of any noncontainerized or bulk liquid hazardous waste in any salt dome formation, salt bed formation, underground mine, or cave is prohibited until such time as—

(A) the Administrator has determined, after notice and opportunity for hearing on the record in the affected areas, that such placement is protective of human health and the environment;

(B) the Administrator has promulgated performance and permitting standards for such facilities under this subchapter, and;

(C)a permit has been issued under section 6925(c) of this title for the facility concerned.

(3) No determination made by the Administrator under subsection (d), (e), or (g) of this section regarding any hazardous waste to which such subsection (d), (e), or (g) of this section applies shall affect the prohibition contained in paragraphs (1) or (2) of this subsection.

5 . The EPA claims that subsection (0, like subsection (g), does not require the agency to make any determination before prohibiting a method of waste disposal. Since we reject CMA’s statutory claim on the grounds that the EPA determined that deep injection might not be safe without a permit petition process, we need not decide whether subsection (f) positively requires this determination.

6 .The agency's Report to Congress on Injection of Hazardous Waste (1985), reprinted in Joint Appendix ("J.A.”) 34, for instance, stresses that the selection of an appropriate site is á critical step in deep well injection, and that knowledge of the regional and site-specific geological characteristics of the land involved is fundamental. Id. at III — 1, J.A. 92. The report details the numerous site-specific inquiries that must be made to determine whether injection will be safe. Id. at III — 2—III—19, J.A. 93-108. In view of the uncertainties involved in determining, for instance, whether a particular injection zone will adequately contain injected wastes, the EPA could rationally have decided that a government agent, and not merely the would-be injector, should pass on the acceptability of each site.

7 . The EPA specifically found that the subsections (d) and (e) wastes are just as hazardous as wastes covered by subsection (g). 53 Fed.Reg. at 28,120 (col. 3).

8 . We do not reach the NRDC claim that the statute required the Administrator to apply the no migration standard to subsection (f). We decide only that it was within the Administrator’s power to do so.

9 . Section 300h(b)(l) provides that the EPA’s regulations "shall contain minimum requirements for effective programs to prevent underground injection which endangers drinking water sources within the meaning of subsection (d)(2) of this section." Section 300h(d)(2) provides that underground injection endangers drinking water sources "if such injection may result in the presence in underground water which supplies or can reasonably be expected to supply any public water system of any contaminant, and if the presence of such contaminant may result in such system's not complying with any national primary drinking water regulation or may otherwise adversely affect the health of persons.”

10 . Senator Bentsen stated that:

Underground injection of hazardous wastes is regulated through the Safe Drinking Water Act. These regulations require [deep injection] wells to inject below drinking water sources and to assure that no migration will occur from the injection zone____ Thus, operators who can demonstrate that their wells meet the underground injection control regulations would be able to show that the waste that they dispose of would not migrate from the injection zone.

130 Cong.Rec. 20,817 (1984).

11 .Molecular diffusion is constituent movement caused not by the pressures built up during injection but by the physical tendency for molecules to move from an area of greater concentration to an area of lower concentration. See 52 Fed.Reg. 32,446, 32,452-53 (1987) (preamble to proposed rules).

12 . Wastes may be hazardous for other reasons, such as exhibiting a hazardous characteristic (ignitability, corrosivity, reactivity, toxicity). 40 C.F.R. §§ 261.11(a)(1), (a)(2), 261.20-261.24. The statute gives the Administrator broad discretion in determining the criteria for listing wastes and in listing specific wastes. See 42 U.S.C. § 6921(a), (b).

13 . The term "the wastes” also refers to each part of the larger body of wastes in the injection zone. It is not true then, as the dissent suggests, that under our interpretation "the wastes” are never hazardous. See Dissenting Op. at 161. Wastes at the very center of the injection zone that are still hazardous are "wastes” subject to the "no migration" prohibition, but the constituents in those wastes are not yet threatening to migrate. By the time those constituents have migrated to the boundary of the injection zone, "the wastes” surrounding them must have ceased to be hazardous. Thus the dissent is also wrong in asserting that our interpretation of the clause "for so long as the wastes remain hazardous” does not define a period of time. Id. If the "no migration” standard is obeyed, the time period before any migration beyond the border of the injection zone will be no less than the time it takes for wastes to reach that border in a non-hazardous condition. The wastes at the edge of the injection zone will never be hazardous, as the dissent correctly notes, id., but that only means that the time period has already expired for those particular wastes. EPA properly focused on "the wastes" that reach the injection zone boundary because those are the only wastes that threaten to violate the “no migration" prohibition.

14 . This standard was added on the Senate floor just before the bill was passed. See 130 Cong. Rec. 20,838 (cols. 1-3) (proposed amendment); id. at 20,847 (cols. 1-3) (accompanying statement).

15 . The other change was that under the conference proposal the “no migration” standard was to be triggered by the presence of any hazardous waste covered by the specific clause, rather than just those wastes that contained significant concentrations of certain hazardous constituents (those that are highly toxic, highly mobile, or have a strong propensity to bioaccumulate).

16 . The level of generality in which EPA could frame its regulations would turn on congressional intent, see 129 Cong.Rec. 27,664 (col. 3) (statements of Rep. Breaux), with the agency’s view entitled to great deference.

17 . Section 6974(b)’s public notice requirements were added in October of 1980. See Solid Waste Disposal Act Amendments of 1980, Pub.L. No. 96-482, § 26, 94 Stat. 2334, 2348-49 (1980). (The prior notice requirements vested great discretion in the Administrator. See Resource Conservation and Recovery Act of 1976, Pub.L. No. 94-580, § 7004(b), 90 Stat. 2795, 2826-27 (1976).) All the other statutory requirements that NRDC invokes were added in 1984. See Hazardous and Solid Waste Amendments of 1984, Pub.L. No. 98-616, § 211 (amending § 6925(a) to require a permit for construction of a facility), § 212 (amending § 6925(c) to require five-year review of permit and to give Administrator power to add conditions to permit), 98 Stat. 3221, 3240-41 (1984).