ENTERGY CORP. v. RIVERKEEPER, INC., et al.

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208 OCTOBER TERM, 2008
Syllabus
ENTERGY CORP. v. RIVERKEEPER, INC., et al.
certiorari to the united states court of appeals for
the second circuit
No. 07–588. Argued December 2, 2008—Decided April 1, 2009*
Petitioners’ powerplants have “cooling water intake structures” that
threaten the environment by squashing against intake screens (“im
pingement”) or suctioning into the cooling system (“entrainment”)
aquatic organisms from the water sources tapped to cool the plants.
Thus, the facilities are subject to regulation under the Clean Water Act,
which mandates that “[a]ny standard established pursuant to section
1311 . . . or section 1316 . . . and applicable to a point source shall require
that the location, design, construction, and capacity of cooling water
intake structures reflect the best technology available for minimizing
adverse environmental impact.” 33 U. S. C. § 1326(b). Sections 1311
and 1316, in turn, employ a variety of “best technology” standards to
regulate effluent discharge into the Nation’s waters. The Environmen
tal Protection Agency (EPA) promulgated the § 1326(b) regulations at
issue after nearly three decades of making the “best technology avail
able” determination on a case-by-case basis. Its “Phase I” regulations
govern new cooling water intake structures, while the “Phase II” rules
at issue apply to certain large existing facilities. In the latter rules,
the EPA set “national performance standards,” requiring most Phase II
facilities to reduce “impingement mortality for [aquatic organisms] by
80 to 95 percent from the calculation baseline,” and requiring a subset
of facilities to reduce entrainment of such organisms by “60 to 90 percent
from [that] baseline.” 40 CFR § 125.94(b)(1), (2). However, the EPA
expressly declined to mandate closed-cycle cooling systems, or equiva
lent reductions in impingement and entrainment, as it had done in its
Phase I rules, in part because the cost of rendering existing facilities
closed-cycle compliant would be nine times the estimated cost of compli
ance with the Phase II performance standards, and because other tech
nologies could approach the performance of closed-cycle operation. The
Phase II rules also permit site-specific variances from the national per
formance standards, provided that the permit-issuing authority imposes
remedial measures that yield results “as close as practicable to the appli
*Together with No. 07–589, PSEG Fossil LLC et al. v. Riverkeeper, Inc.,
et al., and No. 07–597, Utility Water Act Group v. Riverkeeper, Inc., et al.,
also on certiorari to the same court.

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Syllabus
cable performance standards.” § 125.94(a)(5)(i), (ii). Respondents—
environmental groups and various States—challenged the Phase II reg
ulations. Concluding that cost-benefit analysis is impermissible under
33 U. S. C. § 1326(b), the Second Circuit found the site-specific cost
benefit variance provision unlawful and remanded the regulations to the
EPA for it to clarify whether it had relied on cost-benefit analysis in
setting the national performance standards.
Held: The EPA permissibly relied on cost-benefit analysis in setting the
national performance standards and in providing for cost-benefit vari
ances from those standards as part of the Phase II regulations.
Pp. 217–227.
(a) The EPA’s view that § 1326(b)’s “best technology available for min
imizing adverse environmental impact” standard permits consideration
of the technology’s costs and of the relationship between those costs
and the environmental benefits produced governs if it is a reasonable
interpretation of the statute—not necessarily the only possible interpre
tation, nor even the interpretation deemed most reasonable by the
courts. Chevron U. S. A. Inc. v. Natural Resources Defense Council,
Inc., 467 U. S. 837, 843–844. The Second Circuit took “best technology”
to mean the technology that achieves the greatest reduction in adverse
environmental impacts at a reasonable cost to the industry, but it may
also describe the technology that most efficiently produces a good, even
if it produces a lesser quantity of that good than other available technol
ogies. This reading is not precluded by the phrase “for minimizing ad
verse environmental impact.” Minimizing admits of degree and is not
necessarily used to refer exclusively to the “greatest possible reduc
tion.” Other Clean Water Act provisions show that when Congress
wished to mandate the greatest feasible reduction in water pollution, it
used plain language, e. g., “elimination of discharges of all pollutants,”
§ 1311(b)(2)(A). Thus, § 1326(b)’s use of the less ambitious goal of “mini
mizing adverse environmental impact” suggests that the EPA has some
discretion to determine the extent of reduction warranted under the
circumstances, plausibly involving a consideration of the benefits de
rived from reductions and the costs of achieving them. Pp. 217–220.
(b) Considering § 1326(b)’s text, and comparing it with the text and
statutory factors applicable to parallel Clean Water Act provisions,
prompts the conclusion that it was well within the bounds of reasonable
interpretation for the EPA to conclude that cost-benefit analysis is not
categorically forbidden. In the Phase II rules the EPA sought only to
avoid extreme disparities between costs and benefits, limiting variances
from Phase II’s “national performance standards” to circumstances
where the costs are “significantly greater than the benefits” of compli

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210 ENTERGY CORP. v. RIVERKEEPER, INC.
Opinion of the Court
ance. 40 CFR § 125.94(a)(5)(ii). In defining “national performance
standards” the EPA assumed the application of technologies whose ben
efits approach those estimated for closed-cycle cooling systems at a
fraction of the cost. That the EPA has for over 30 years interpreted
§ 1326(b) to permit a comparison of costs and benefits, while not conclu
sive, also tends to show that its interpretation is reasonable and hence a
legitimate exercise of its discretion. Even respondents and the Second
Circuit ultimately recognize that some comparison of costs and benefits
is permitted. The Second Circuit held that § 1326(b) mandates only
those technologies whose costs can be reasonably borne by the industry.
But whether it is reasonable to bear a particular cost can very well
depend on the resulting benefits. Likewise, respondents concede that
the EPA need not require that industry spend billions to save one
more fish. This concedes the principle, and there is no statutory basis
for limiting the comparison of costs and benefits to situations where
the benefits are de minimis rather than significantly disproportionate.
Pp. 220–226.
475 F. 3d 83, reversed and remanded.
Scalia, J., delivered the opinion of the Court, in which Roberts, C. J.,
and Kennedy, Thomas, and Alito, JJ., joined. Breyer, J., filed an opin
ion concurring in part and dissenting in part, post, p. 230. Stevens, J.,
filed a dissenting opinion, in which Souter and Ginsburg, JJ., joined,
post, p. 236.
Maureen E. Mahoney argued the cause for petitioners.
With her on the briefs for petitioners Entergy Corp. et al.
were J. Scott Ballenger, Cassandra S. Bernstein, Elise N.
Zoli, Kevin P. Martin, Abigail Hemani, Chuck D. Barlow,
and John G. Valeri, Jr. Kristy A. N. Bulleit filed briefs for
petitioner Utility Water Act Group.
Deputy Solicitor General Joseffer argued the cause for
the federal parties as respondents under this Court’s Rule
12.6 in support of petitioners. With him on the briefs were
former Solicitor General Garre, Assistant Attorney General
Tenpas, Deputy Solicitor General Kneedler, Cynthia J. Mor
ris, and Jessica O’Donnell.
Richard J. Lazarus argued the cause for respondents.
With him on the brief for respondents Riverkeeper, Inc.,
et al. were Reed W. Super, Edward Lloyd, and P. Kent Cor

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211 Cite as: 556 U. S. 208 (2009)
Counsel
rell. A brief for respondents State of Rhode Island et al.
was filed by Patrick C. Lynch, Attorney General of Rhode
Island, and Tricia O’Hare Jedele, Special Assistant Attorney
General, Richard Blumenthal, Attorney General of Connect
icut, and Kimberly Massicotte and Matthew Levine, Assist
ant Attorneys General, Martha Coakley, Attorney General
of Massachusetts, and Andrew Goldberg, Assistant Attorney
General, Andrew M. Cuomo, Attorney General of New York,
Barbara D. Underwood, Solicitor General, Andy D. Bing,
Deputy Solicitor General, Denise A. Hartman, Assistant So
licitor General, and Maureen F. Leary, Assistant Attorney
General, Joseph R. Biden III, Attorney General of Delaware,
and Kevin Maloney, Deputy Attorney General, Anne Mil
gram, Attorney General of New Jersey, and Ellen Barney
Balint, Deputy Attorney General.†
†Briefs of amici curiae urging reversal were filed for the State of Ne
braska et al. by Jon Bruning, Attorney General of Nebraska, and David
D. Cookson, Chief Deputy Attorney General, and by the Attorneys Gen
eral and other officials for their respective States as follows: Troy King,
Attorney General of Alabama, Dustin McDaniel, Attorney General of Ar
kansas, John W. Suthers, Attorney General of Colorado, Bill McCollum,
Attorney General of Florida, Steve Carter, Attorney General of Indiana,
Stephen N. Six, Attorney General of Kansas, and Jared S. Maag, Deputy
Solicitor General, Jack Conway, Attorney General of Kentucky, James D.
Caldwell, Attorney General of Louisiana, Michael A. Cox, Attorney Gen
eral of Michigan, Jeremiah W. (Jay) Nixon, Attorney General of Missouri,
Gary K. King, Attorney General of New Mexico, Wayne Stenehjem, Attor
ney General of North Dakota, Henry McMaster, Attorney General of
South Carolina, Robert E. Cooper, Jr., Attorney General of Tennessee,
Greg Abbott, Attorney General of Texas, and Robert F. McDonnell, Attor
ney General of Virginia, and William E. Thro, State Solicitor General; for
the American Chemistry Council et al. by Russell S. Frye, Leslie A.
Hulse, Richard S. Wasserstrom, Robin S. Conrad, Amar D. Sarwal, Jan
S. Amundson, and Quentin Riegel; for the American Petroleum Institute
by Daniel P. Albers, David T. Ballard, Harry Ng, and Michael See; for
the California Council for Environmental and Economic Balance by Kevin
M. Fong; for the National Association of Home Builders by Messrs.
Albers and Ballard, Duane J. Desiderio, and Thomas J. Ward; for the
Nuclear Energy Institute by Seth P. Waxman, Edward C. DuMont, Brian

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212 ENTERGY CORP. v. RIVERKEEPER, INC.
Opinion of the Court
Justice Scalia delivered the opinion of the Court.
These cases concern a set of regulations adopted by the
Environmental Protection Agency (EPA or agency) under
§ 316(b) of the Clean Water Act, 33 U. S. C. § 1326(b).
69 Fed. Reg. 41576 (2004). Respondents—environmental
groups and various States 1—challenged those regulations,
and the Second Circuit set them aside. Riverkeeper, Inc. v.
EPA, 475 F. 3d 83, 99–100 (2007). The issue for our decision
is whether, as the Second Circuit held, the EPA is not per
mitted to use cost-benefit analysis in determining the content
of regulations promulgated under § 1326(b).
I
Petitioners operate—or represent those who operate—
large powerplants. In the course of generating power, those
M. Boynton, and Ellen C. Ginsberg; and for the Pacific Legal Foundation
by M. Reed Hopper and Steven Geoffrey Gieseler.
Briefs of amici curiae urging affirmance were filed for the State of
Illinois et al. by Lisa Madigan, Attorney General of Illinois, Michael A.
Scodro, Solicitor General, and Jane Elinor Notz, Deputy Solicitor General,
by Roberto J. Sa´ nchez-Ramos, Secretary of Justice of Puerto Rico, by
Susan Shinkman and Richard P. Mather, and by the Attorneys General
for their respective States as follows: Tom Miller of Iowa, Douglas F.
Gansler of Maryland, Mike McGrath of Montana, Nancy H. Rogers of
Ohio, and W. A. Drew Edmondson of Oklahoma; for Commercial Fisher
men of America, et al., by Elizabeth J. Hubertz and Stephanie Tai; for
Environment America et al. by Christopher J. Wright and Timothy J.
Simeone; for Environmental Law Professors by Jared A. Goldstein; for
the National Wildlife Federation et al. by David K. Mears; for OMB Watch
by Amy Sinden; and for Voices of the Wetlands et al. by Deborah A. Sivas
and Leah J. Russin.
Briefs of amici curiae were filed for the AEI Center for Regulatory
and Market Studies et al. by Robert E. Litan; for the Clean Air Task Force
et al. by Ann Brewster Weeks; and for Frank Ackerman et al. by David
M. Driesen and Douglas A. Kysar.
1 The EPA and its Administrator appeared as respondents in support
of petitioners. See Brief for Federal Parties as Respondents Supporting
Petitioners. References to “respondents” throughout the opinion refer
only to those parties challenging the EPA rules at issue in these cases.

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Opinion of the Court
plants also generate large amounts of heat. To cool their
facilities, petitioners employ “cooling water intake struc
tures” that extract water from nearby water sources.
These structures pose various threats to the environment,
chief among them the squashing against intake screens (ele
gantly called “impingement”) or suction into the cooling
system (“entrainment”) of aquatic organisms that live in the
affected water sources. See 69 Fed. Reg. 41586. Accord
ingly, the facilities are subject to regulation under the Clean
Water Act, 33 U. S. C. § 1251 et seq., which mandates:
“Any standard established pursuant to section 1311 of
this title or section 1316 of this title and applicable to a
point source shall require that the location, design, con
struction, and capacity of cooling water intake struc
tures reflect the best technology available for minimiz
ing adverse environmental impact.” § 1326(b).
Sections 1311 and 1316, in turn, employ a variety of “best
technology” standards to regulate the discharge of effluents
into the Nation’s waters.
The § 1326(b) regulations at issue here were promulgated
by the EPA after nearly three decades in which the deter
mination of the “best technology available for minimizing
[cooling water intake structures’] adverse environmental im
pact” was made by permit-issuing authorities on a case-by
case basis, without benefit of a governing regulation. The
EPA’s initial attempt at such a regulation came to nought
when the Fourth Circuit determined that the agency had
failed to adhere to the procedural requirements of the Ad
ministrative Procedure Act. Appalachian Power Co. v.
Train, 566 F. 2d 451, 457 (1977). The EPA withdrew the
regulation, 44 Fed. Reg. 32956 (1979), and instead published
“draft guidance” for use in implementing § 1326(b)’s require
ments via site-specific permit decisions under § 1342. See
EPA, Office of Water Enforcement Permits Div., {Draft}
Guidance for Evaluating the Adverse Impact of Cooling

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214 ENTERGY CORP. v. RIVERKEEPER, INC.
Opinion of the Court
Water Intake Structures on the Aquatic Environment: Sec
tion 316(b) P. L. 92–500 (May 1, 1977), online at http://
www.epa.gov/waterscience/316b/files/1977AEIguid.pdf (all
Internet materials as visited Mar. 30, 2009, and available in
Clerk of Court’s case file); 69 Fed. Reg. 41584 (describing
system of case-by-case permits under the draft guidance).
In 1995, the EPA entered into a consent decree which, as
subsequently amended, set a multiphase timetable for the
EPA to promulgate regulations under § 1326(b). See River
keeper, Inc. v. Whitman, No. 93 Civ. 0314 (AGS), 2001 WL
1505497, *1 (SDNY, Nov. 27, 2001). In the first phase the
EPA adopted regulations governing certain new, large cool
ing water intake structures. 66 Fed. Reg. 65256 (2001)
(Phase I rules); see 40 CFR §§ 125.80(a), 125.81(a) (2008).
Those rules require new facilities with water-intake flow
greater than 10 million gallons per day to, among other
things, restrict their inflow “to a level commensurate with
that which can be attained by a closed-cycle recirculating
cooling water system.” 2 § 125.84(b)(1). New facilities with
water-intake flow between 2 million and 10 million gallons
per day may alternatively comply by, among other things,
reducing the volume and velocity of water removal to cer
tain levels. § 125.84(c). And all facilities may alternatively
comply by demonstrating, among other things, “that the
technologies employed will reduce the level of adverse envi
ronmental impact . . . to a comparable level” to what would
be achieved by using a closed-cycle cooling system.
§ 125.84(d). These regulations were upheld in large part by
the Second Circuit in Riverkeeper, Inc. v. EPA, 358 F. 3d
174 (2004).
2 Closed-cycle cooling systems recirculate the water used to cool the fa
cility, and consequently extract less water from the adjacent waterway,
proportionately reducing impingement and entrainment. Riverkeeper,
Inc. v. EPA, 358 F. 3d 174, 182, n. 5 (CA2 2004); 69 Fed. Reg. 41601, and
n. 44 (2004).

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Opinion of the Court
The EPA then adopted the so-called “Phase II” rules at
issue here.3 69 Fed. Reg. 41576. They apply to existing
facilities that are point sources, whose primary activity is
the generation and transmission (or sale for transmission) of
electricity, and whose water-intake flow is more than 50 mil
lion gallons of water per day, at least 25 percent of which is
used for cooling purposes. Ibid. Over 500 facilities, ac
counting for approximately 53 percent of the Nation’s
electric-power generating capacity, fall within Phase II’s
ambit. See EPA, Economic and Benefits Analysis for
the Final Section 316(b) Phase II Existing Facilities Rule,
p. A3–13 (Table A3–4, Feb. 2004), online at http://www.
epa.gov/waterscience/316b/phase2/econbenefits/final/a3.
pdf. Those facilities remove on average more than 214 bil
lion gallons of water per day, causing impingement and en
trainment of over 3.4 billion aquatic organisms per year. 69
Fed. Reg. 41586.
To address those environmental impacts, the EPA set “na
tional performance standards,” requiring Phase II facilities
(with some exceptions) to reduce “impingement mortality for
all life stages of fish and shellfish by 80 to 95 percent from the
calculation baseline”; a subset of facilities must also reduce
entrainment of such aquatic organisms by “60 to 90 percent
from the calculation baseline.” 40 CFR § 125.94(b)(1), (2);
see § 125.93 (defining “calculation baseline”). Those targets
are based on the environmental improvements achievable
through deployment of a mix of remedial technologies, 69
Fed. Reg. 41599, which the EPA determined were “commer
cially available and economically practicable,” id., at 41602.
In its Phase II rules, however, the EPA expressly declined
to mandate adoption of closed-cycle cooling systems or equiv
3 The EPA has also adopted Phase III rules for facilities not subject
to the Phase I and Phase II regulations. 71 Fed. Reg. 35006 (2006). A
challenge to those regulations is currently before the Fifth Circuit, where
proceedings have been stayed pending disposition of these cases. See
ConocoPhillips Co. v. EPA, No. 06–60662.

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216 ENTERGY CORP. v. RIVERKEEPER, INC.
Opinion of the Court
alent reductions in impingement and entrainment, as it had
done for new facilities subject to the Phase I rules. Id.,
at 41601. It refused to take that step in part because of
the “generally high costs” of converting existing facilities
to closed-cycle operation, and because “other technologies
approach the performance of this option.” Id., at 41605.
Thus, while closed-cycle cooling systems could reduce im
pingement and entrainment mortality by up to 98 percent,
id., at 41601 (compared to the Phase II targets of 80 to 95
percent impingement reduction), the cost of rendering all
Phase II facilities closed-cycle-compliant would be approxi
mately $3.5 billion per year, id., at 41605, nine times the esti
mated cost of compliance with the Phase II performance
standards, id., at 41666. Moreover, Phase II facilities com
pelled to convert to closed-cycle cooling systems “would
produce 2.4 percent to 4.0 percent less electricity even while
burning the same amount of coal,” possibly requiring the
construction of “20 additional 400–MW plants . . . to replace
the generating capacity lost.” Id., at 41605. The EPA thus
concluded that “[a]lthough not identical, the ranges of im
pingement and entrainment reduction are similar under both
options . . . . [Benefits of compliance with the Phase II
rules] can approach those of closed-cycle recirculating sys
tems at less cost with fewer implementation problems.” Id.,
at 41606.
The regulations permit the issuance of site-specific vari
ances from the national performance standards if a facility
can demonstrate either that the costs of compliance are “sig
nificantly greater than” the costs considered by the agency
in setting the standards, 40 CFR § 125.94(a)(5)(i), or that the
costs of compliance “would be significantly greater than the
benefits of complying with the applicable performance stand
ards,” § 125.94(a)(5)(ii). Where a variance is warranted, the
permit-issuing authority must impose remedial measures
that yield results “as close as practicable to the applicable
performance standards.” § 125.94(a)(5)(i), (ii).

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Opinion of the Court
Respondents challenged the EPA’s Phase II regulations,
and the Second Circuit granted their petition for review and
remanded the regulations to the EPA. The Second Circuit
identified two ways in which the EPA could permissibly con
sider costs under 33 U. S. C. § 1326(b): (1) in determining
whether the costs of remediation “can be ‘reasonably borne’
by the industry,” and (2) in determining which remedial tech
nologies are the most cost effective, that is, the technologies
that reach a specified level of benefit at the lowest cost. 475
F. 3d, at 99–100. See also id., at 98, and n. 10. It concluded,
however, that cost-benefit analysis, which “compares the
costs and benefits of various ends, and chooses the end with
the best net benefits,” id., at 98, is impermissible under
§ 1326(b), id., at 100.
The Court of Appeals held the site-specific cost-benefit
variance provision to be unlawful. Id., at 114. Finding it
unclear whether the EPA had relied on cost-benefit analysis
in setting the national performance standards, or had only
used cost-effectiveness analysis, it remanded to the agency
for clarification of that point. Id., at 104–105. (The remand
was also based on other grounds which are not at issue here.)
The EPA suspended operation of the Phase II rules pend
ing further rulemaking. 72 Fed. Reg. 37107 (2007). We
then granted certiorari limited to the following question:
“Whether [§ 1326(b)] . . . authorizes the [EPA] to compare
costs with benefits in determining ‘the best technology avail
able for minimizing adverse environmental impact’ at cooling
water intake structures.” 552 U. S. 1309 (2008).
II
In setting the Phase II national performance standards
and providing for site-specific cost-benefit variances, the
EPA relied on its view that § 1326(b)’s “best technology
available” standard permits consideration of the technology’s
costs, 69 Fed. Reg. 41626, and of the relationship between
those costs and the environmental benefits produced, id., at

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Opinion of the Court
41603. That view governs if it is a reasonable interpretation
of the statute—not necessarily the only possible interpreta
tion, nor even the interpretation deemed most reasonable
by the courts. Chevron U. S. A. Inc. v. Natural Resources
Defense Council, Inc., 467 U. S. 837, 843–844 (1984).4
As we have described, § 1326(b) instructs the EPA to set
standards for cooling water intake structures that reflect
“the best technology available for minimizing adverse envi
ronmental impact.” The Second Circuit took that language
to mean the technology that achieves the greatest reduction
in adverse environmental impacts at a cost that can reason
ably be borne by the industry. 475 F. 3d, at 99–100. That
is certainly a plausible interpretation of the statute. The
“best” technology—that which is “most advantageous,” Web
ster’s New International Dictionary 258 (2d ed. 1953)—may
well be the one that produces the most of some good, here a
reduction in adverse environmental impact. But “best tech
nology” may also describe the technology that most effi
ciently produces some good. In common parlance one could
certainly use the phrase “best technology” to refer to that
which produces a good at the lowest per-unit cost, even if it
produces a lesser quantity of that good than other available
technologies.
Respondents contend that this latter reading is precluded
by the statute’s use of the phrase “for minimizing adverse
4 The dissent finds it “puzzling” that we invoke this proposition (that a
reasonable agency interpretation prevails) at the “outset,” omitting the
supposedly prior inquiry of “ ‘whether Congress has directly spoken to the
precise question at issue.’ ” Post, at 241, n. 5 (opinion of Stevens, J.)
(quoting Chevron, 467 U. S., at 842). But surely if Congress has directly
spoken to an issue then any agency interpretation contradicting what Con
gress has said would be unreasonable.
What is truly “puzzling” is the dissent’s accompanying charge that the
Court’s failure to conduct the Chevron step-one inquiry at the outset “re
flects [its] reluctance to consider the possibility . . . that Congress’ silence
may have meant to foreclose cost-benefit analysis.” Post, at 241, n. 5.
Our discussion of that issue, infra, at 222–223, speaks for itself.

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Opinion of the Court
environmental impact.” Minimizing, they argue, means re
ducing to the smallest amount possible, and the “best tech
nology available for minimizing adverse environmental im
pacts” must be the economically feasible technology that
achieves the greatest possible reduction in environmental
harm. Brief for Respondent Riverkeeper, Inc., et al. 25–
26. But “minimize” is a term that admits of degree and is
not necessarily used to refer exclusively to the “greatest pos
sible reduction.” For example, elsewhere in the Clean
Water Act, Congress declared that the procedures imple
menting the Act “shall encourage the drastic minimization
of paperwork and interagency decision procedures.” 33
U. S. C. § 1251(f). If respondents’ definition of the term
“minimize” is correct, the statute’s use of the modifier “dras
tic” is superfluous.
Other provisions in the Clean Water Act also suggest the
agency’s interpretation. When Congress wished to mandate
the greatest feasible reduction in water pollution, it did so in
plain language: The provision governing the discharge of
toxic pollutants into the Nation’s waters requires the EPA
to set “effluent limitations [which] shall require the elimina
tion of discharges of all pollutants if the Administrator
finds . . . that such elimination is technologically and economi
cally achievable,” § 1311(b)(2)(A) (emphasis added). See also
§ 1316(a)(1) (mandating “where practicable, a standard [for
new point sources] permitting no discharge of pollutants”
(emphasis added)). Section 1326(b)’s use of the less ambi
tious goal of “minimizing adverse environmental impact”
suggests, we think, that the agency retains some discretion
to determine the extent of reduction that is warranted under
the circumstances. That determination could plausibly in
volve a consideration of the benefits derived from reductions
and the costs of achieving them. Cf. 40 CFR § 125.83 (defin
ing “minimize” for purposes of the Phase I regulations as
“reduc[ing] to the smallest amount, extent, or degree reason
ably possible”). It seems to us, therefore, that the phrase

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220 ENTERGY CORP. v. RIVERKEEPER, INC.
Opinion of the Court
“best technology available,” even with the added specifica
tion “for minimizing adverse environmental impact,” does
not unambiguously preclude cost-benefit analysis.5
Respondents’ alternative (and, alas, also more complex) ar
gument rests upon the structure of the Clean Water Act.
The Act provided that during its initial implementation pe
riod existing “point sources”—discrete conveyances from
which pollutants are or may be discharged, 33 U. S. C.
§ 1362(14)—were subject to “effluent limitations . . . which
shall require the application of the best practicable control
technology currently available.” § 1311(b)(1)(A) (emphasis
added). (We shall call this the “BPT” test.) Following that
transition period, the Act initially mandated adoption, by
July 1, 1983 (later extended to March 31, 1989), of stricter
effluent limitations requiring “application of the best avail
able technology economically achievable for such category
or class, which will result in reasonable further progress to
ward the national goal of eliminating the discharge of all pol
lutants.” § 1311(b)(2)(A) (emphasis added); see EPA v. Na
tional Crushed Stone Assn., 449 U. S. 64, 69–70 (1980). (We
shall call this the “BATEA” test.) Subsequent amendment
limited application of this standard to toxic and nonconven
tional pollutants, and for the remainder established a (pre
sumably laxer) test of “best conventional-pollutant control
technology.” § 1311(b)(2)(E).6 (We shall call this “BCT.”)
5 Respondents concede that the term “available” is ambiguous, as it
could mean either technologically feasible or economically feasible. But
any ambiguity in the term “available” is largely irrelevant. Regardless
of the criteria that render a technology “available,” the EPA would still
have to determine which available technology is the “best” one. And as
discussed above, that determination may well involve consideration of the
technology’s relative costs and benefits.
6 The statute does not contain a hyphen between the words “conven
tional” and “pollutant.” “Conventional pollutant” is a statutory term,
however, see 33 U. S. C. § 1314(a)(4), and it is clear that in § 1311(b)(2)(E)
the adjective modifies “pollutant” rather than “control technology.” The
hyphen makes that clear.

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Opinion of the Court
Finally, § 1316 subjected certain categories of new point
sources to “the greatest degree of effluent reduction which
the Administrator determines to be achievable through ap
plication of the best available demonstrated control tech
nology.” § 1316(a)(1) (emphasis added); § 1316(b)(1)(B). (We
shall call this the “BADT” test.) The provision at issue
here, applicable not to effluents but to cooling water intake
structures, requires, as we have described, “the best tech
nology available for minimizing adverse environmental
impact,” § 1326(b) (emphasis added). (We shall call this the
“BTA” test.)
The first four of these tests are elucidated by statutory
factor lists that guide their implementation. To take the
standards in (presumed) order of increasing stringency, see
Crushed Stone, supra, at 69–70: In applying the BPT test
the EPA is instructed to consider, among other factors, “the
total cost of application of technology in relation to the efflu
ent reduction benefits to be achieved.” § 1314(b)(1)(B). In
applying the BCT test it is instructed to consider “the rea
sonableness of the relationship between the costs of attain
ing a reduction in effluents and the effluent reduction ben
efits derived.” § 1314(b)(4)(B) (emphasis added). And in
applying the BATEA and BADT tests the EPA is instructed
to consider the “cost of achieving such effluent reduction.”
§§ 1314(b)(2)(B), 1316(b)(1)(B). There is no such elucidating
language applicable to the BTA test at issue here. To facili
tate comparison, the texts of these five tests, the clarifying
factors applicable to them, and the entities to which they
apply are set forth in the Appendix, infra.
The Second Circuit, in rejecting the EPA’s use of cost
benefit analysis, relied in part on the propositions that
(1) cost-benefit analysis is precluded under the BATEA and
BADT tests; and (2) that, insofar as the permissibility of
cost-benefit analysis is concerned, the BTA test (the one at
issue here) is to be treated the same as those two. See 475
F. 3d, at 98. It is not obvious to us that the first of these

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222 ENTERGY CORP. v. RIVERKEEPER, INC.
Opinion of the Court
propositions is correct, but we need not pursue that point,
since we assuredly do not agree with the second. It is cer
tainly reasonable for the agency to conclude that the BTA
test need not be interpreted to permit only what those other
two tests permit. Its text is not identical to theirs. It has
the relatively modest goal of “minimizing adverse environ
mental impact” as compared with the BATEA’s goal of “elim
inating the discharge of all pollutants.” And it is unencum
bered by specified statutory factors of the sort provided for
those other two tests, which omission can reasonably be
interpreted to suggest that the EPA is accorded greater
discretion in determining its precise content.
Respondents and the dissent argue that the mere fact that
§ 1326(b) does not expressly authorize cost-benefit analysis
for the BTA test, though it does so for two of the other tests,
displays an intent to forbid its use. This surely proves too
much. For while it is true that two of the other tests au
thorize cost-benefit analysis, it is also true that all four of
the other tests expressly authorize some consideration of
costs. Thus, if respondents’ and the dissent’s conclusion re
garding the import of § 1326(b)’s silence is correct, it is
a fortiori true that the BTA test permits no consideration
of cost whatsoever, not even the “cost-effectiveness” and
“feasibility” analysis that the Second Circuit approved, see
supra, at 217, that the dissent would approve, post, at 237,
and that respondents acknowledge. The inference that re
spondents and the dissent would draw from the silence is, in
any event, implausible, as § 1326(b) is silent not only with
respect to cost-benefit analysis but with respect to all poten
tially relevant factors. If silence here implies prohibition,
then the EPA could not consider any factors in implementing
§ 1326(b)—an obvious logical impossibility. It is eminently
reasonable to conclude that § 1326(b)’s silence is meant to
convey nothing more than a refusal to tie the agency’s hands
as to whether cost-benefit analysis should be used, and if so
to what degree.

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Opinion of the Court
Contrary to the dissent’s suggestion, see post, at 238–240,
our decisions in Whitman v. American Trucking Assns.,
Inc., 531 U. S. 457 (2001), and American Textile Mfrs. Insti
tute, Inc. v. Donovan, 452 U. S. 490 (1981), do not undermine
this conclusion. In American Trucking, we held that the
text of § 109 of the Clean Air Act, “interpreted in its statu
tory and historical context . . . , unambiguously bars cost
considerations” in setting air quality standards under that
provision. 531 U. S., at 471. The relevant “statutory con
text” included other provisions in the Clean Air Act that
expressly authorized consideration of costs, whereas § 109
did not. Id., at 467–468. American Trucking thus stands
for the rather unremarkable proposition that sometimes stat
utory silence, when viewed in context, is best interpreted as
limiting agency discretion. For the reasons discussed ear
lier, § 1326(b)’s silence cannot bear that interpretation.
In American Textile, the Court relied in part on a stat
ute’s failure to mention cost-benefit analysis in holding that
the relevant agency was not required to engage in cost
benefit analysis in setting certain health and safety stand
ards. 452 U. S., at 510–512. But under Chevron, that an
agency is not required to do so does not mean that an agency
is not permitted to do so.
This extended consideration of the text of § 1326(b), and
comparison of that with the text and statutory factors appli
cable to four parallel provisions of the Clean Water Act, lead
us to the conclusion that it was well within the bounds of
reasonable interpretation for the EPA to conclude that cost
benefit analysis is not categorically forbidden. Other argu
ments may be available to preclude such a rigorous form of
cost-benefit analysis as that which was prescribed under the
statute’s former BPT standard, which required weighing
“the total cost of application of technology” against “the . . .
benefits to be achieved.” See supra, at 221. But that ques
tion is not before us.

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224 ENTERGY CORP. v. RIVERKEEPER, INC.
Opinion of the Court
In the Phase II requirements challenged here the EPA
sought only to avoid extreme disparities between costs and
benefits. The agency limited variances from the Phase II
“national performance standards” to circumstances where
the costs are “significantly greater than the benefits” of com
pliance. 40 CFR § 125.94(a)(5)(ii). In defining the “national
performance standards” themselves the EPA assumed the
application of technologies whose benefits “approach those
estimated” for closed-cycle cooling systems at a fraction of
the cost: $389 million per year, 69 Fed. Reg. 41666, as com
pared with (1) at least $3.5 billion per year to operate compli
ant closed-cycle cooling systems, id., at 41605 (or $1 billion
per year to impose similar requirements on a subset of Phase
II facilities, id., at 41606), and (2) significant reduction in the
energy output of the altered facilities, id., at 41605. And
finally, the EPA’s assessment of the relatively meager finan
cial benefits of the Phase II regulations that it adopted—
reduced impingement and entrainment of 1.4 billion aquatic
organisms, id., at 41661, Exh. XII–6, with annualized use
benefits of $83 million, id., at 41662, and nonuse benefits of
indeterminate value, id., at 41660–41661—when compared to
annual costs of $389 million, demonstrates quite clearly that
the agency did not select the Phase II regulatory require
ments because their benefits equaled their costs.
While not conclusive, it surely tends to show that the
EPA’s current practice is a reasonable and hence legitimate
exercise of its discretion to weigh benefits against costs that
the agency has been proceeding in essentially this fashion
for over 30 years. See Alaska Dept. of Environmental
Conservation v. EPA, 540 U. S. 461, 487 (2004); Barnhart v.
Walton, 535 U. S. 212, 219–220 (2002). As early as 1977, the
agency determined that, while § 1326(b) does not require
cost-benefit analysis, it is also not reasonable to “interpret
Section [1326(b)] as requiring use of technology whose cost
is wholly disproportionate to the environmental benefit to be
gained.” In re Public Service Co. of New Hampshire, 1

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Opinion of the Court
E. A. D. 332, 340 (1977). See also In re Central Hudson Gas
and Electric Corp., EPA General Counsel Opinions, NPDES
Permits, No. 63, pp. 371, 381 (July 29, 1977) (“EPA ultimately
must demonstrate that the present value of the cumulative
annual cost of modifications to cooling water intake struc
tures is not wholly out of proportion to the magnitude of the
estimated environmental gains”); Seacoast Anti-Pollution
League v. Costle, 597 F. 2d 306, 311 (CA1 1979) (rejecting
challenge to an EPA permit decision that was based in part
on the agency’s determination that further restrictions
would be “ ‘wholly disproportionate to any environmental
benefit’ ”). While the EPA’s prior “wholly disproportionate”
standard may be somewhat different from its current “sig
nificantly greater than” standard, there is nothing in the
statute that would indicate that the former is a permissible
interpretation while the latter is not.
Indeed, in its review of the EPA’s Phase I regulations, the
Second Circuit seemed to recognize that § 1326(b) permits
some form of cost-benefit analysis. In considering a chal
lenge to the EPA’s rejection of dry cooling systems 7 as the
“best technology available” for Phase I facilities, the Second
Circuit noted that “while it certainly sounds substantial that
dry cooling is 95 percent more effective than closed-cycle
cooling, it is undeniably relevant that that difference repre
sents a relatively small improvement over closed-cycle cool
ing at a very significant cost.” Riverkeeper, 358 F. 3d, at
194, n. 22. And in the decision below rejecting the use of
cost-benefit analysis in the Phase II regulations, the Second
Circuit nonetheless interpreted “best technology available”
as mandating only those technologies that can “be reasonably
borne by the industry.” 475 F. 3d, at 99. But whether it is
“reasonable” to bear a particular cost may well depend on
7 Dry cooling systems use air drafts to remove heat, and accordingly
remove little or no water from surrounding water sources. See 66 Fed.
Reg. 65282 (2001).

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226 ENTERGY CORP. v. RIVERKEEPER, INC.
Opinion of the Court
the resulting benefits; if the only relevant factor was the fea
sibility of the costs, their reasonableness would be irrelevant.
In the last analysis, even respondents ultimately recognize
that some form of cost-benefit analysis is permissible. They
acknowledge that the statute’s language is “plainly not so
constricted as to require EPA to require industry petitioners
to spend billions to save one more fish or plankton.” Brief
for Respondent Riverkeeper, Inc., et al. 29. This concedes
the principle—the permissibility of at least some cost-benefit
analysis—and we see no statutory basis for limiting its use
to situations where the benefits are de minimis rather than
significantly disproportionate.
* * *
We conclude that the EPA permissibly relied on cost
benefit analysis in setting the national performance stand
ards and in providing for cost-benefit variances from those
standards as part of the Phase II regulations. The Court of
Appeals’ reliance in part on the agency’s use of cost-benefit
analysis in invalidating the site-specific cost-benefit variance
provision, 475 F. 3d, at 114, was therefore in error, as was
its remand of the national performance standards for clarifi
cation of whether cost-benefit analysis was impermissibly
used, id., at 104–105. We of course express no view on the
remaining bases for the Second Circuit’s remand which did
not depend on the permissibility of cost-benefit analysis.
See id., at 108, 110, 113, 115, 117, 120.8 The judgment of the
8 Justice Breyer would remand for the additional reason of what he
regards as the agency’s inadequate explanation of the change in its crite
rion for variances—from a relationship of costs to benefits that is “ ‘wholly
disproportionate’ ” to one that is “ ‘significantly greater.’ ” Post, at 236
(opinion concurring in part and dissenting in part). That question can
have no bearing upon whether the EPA can use cost-benefit analysis,
which is the only question presented here. It seems to us, in any case,
that the EPA’s explanation was ample. It explained that the “wholly out
of proportion” standard was inappropriate for the existing facilities sub
ject to the Phase II rules because those facilities lack “the greater flexibil

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227 Cite as: 556 U. S. 208 (2009)
Appendix to opinion of the Court
Court of Appeals is reversed, and the cases are remanded
for further proceedings consistent with this opinion.
It is so ordered.
APPENDIX
Statutory Standard Statutorily Mandated
Factors
Entities
Subject to
Regulation
BPT: “Factors relating to the Existing point
“[E]ffluent limita assessment of best sources during
tions . . . which practicable control the Clean
shall require the technology currently Water Act’s
application of the available . . . shall initial imple
best practicable include consideration of mentation
control technology the total cost of phase.
currently available.” application of technology
33 U. S. C. § 1311(b) in relation to the effluent
(1)(A) (emphasis reduction benefits to be
added). achieved.” 33 U. S. C.
§ 1314(b)(1)(B).
BCT:
“[E]ffluent limitations
. . . which . . . shall
require application of
the best conventional
pollutant control
“Factors relating to the
assessment of best
conventional pollutant
control technology . . .
shall include
consideration of the
Existing point
sources that
discharge
“conventional
pollutants” as
defined by the
ity available to new facilities for selecting the location of their intakes and
installing technologies at lower costs relative to the costs associated with
retrofitting existing facilities,” and because “economically impracticable
impacts on energy prices, production costs, and energy production . . .
could occur if large numbers of Phase II existing facilities incurred costs
that were more than ‘significantly greater’ than but not ‘wholly out of
proportion’ to the costs in the EPA’s record.” 68 Fed. Reg. 13541 (2003).

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Appendix to opinion of the Court
Statutory Standard Statutorily Mandated
Factors
Entities
Subject to
Regulation
BCT (continued): reasonableness of the EPA under
technology.” relationship between the 33 U. S. C.
33 U. S. C. costs of attaining a § 1314(a)(4).
§ 1311(b)(2)(E) reduction in effluents
(emphasis added). and the effluent
reduction benefits
derived.” 33 U. S. C.
§ 1314(b)(4)(B).
BATEA:
“[E]ffluent
limitations . . .
which . . . shall
require application of
the best available
technology
economically
achievable . . . which
will result in
reasonable further
progress toward the
national goal of
eliminating the
discharge of all
pollutants.” 33
U. S. C.
§ 1311(b)(2)(A)
(emphasis added).
“Factors relating to the
assessment of best
available technology
shall take into
account . . . the cost of
achieving such effluent
reduction.” 33 U. S. C.
§ 1314(b)(2)(B).
Existing point
sources that
discharge toxic
pollutants and
non-conven
tional
pollutants.
BADT:
“[A] standard for the
control of the
discharge of
“[T]he Administrator
shall take into
consideration the cost of
achieving such effluent
New point
sources within
the categories
of sources

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Appendix to opinion of the Court
Statutory Standard Statutorily Mandated
Factors
Entities
Subject to
Regulation
BADT (continued):
pollutants which
reflects the greatest
degree of effluent
reduction which the
Administrator
determines to be
achievable through
application of the best
available
demonstrated control
technology.” 33
U. S. C. § 1316(a)(1)
(emphasis added).
reduction, and any
non-water quality,
environmental impact
and energy
requirements.” 33
U. S. C. § 1316(b)(1)(B).
identified by
the EPA under
33 U. S. C.
§ 1316(b)(1)(A).
BTA:
“Any standard . . .
applicable to a point
source shall require
that the location,
design, construction,
and capacity of
cooling water intake
structures reflect the
best technology
available for
minimizing adverse
environmental
impact.” 33 U. S. C.
§ 1326(b).
N/A Point sources
that operate
cooling water
intake
structures.

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230 ENTERGY CORP. v. RIVERKEEPER, INC.
Opinion of Breyer, J.
Justice Breyer, concurring in part and dissenting in
part.
I agree with the Court that the relevant statutory lan
guage authorizes the Environmental Protection Agency
(EPA or Agency) to compare costs and benefits. Ante, at
217–223. Nonetheless the drafting history and legislative
history of related provisions, Pub. L. 92–500, §§ 301, 304, 86
Stat. 844, 850, as amended, 33 U. S. C. §§ 1311, 1314, make
clear that those who sponsored the legislation intended the
law’s text to be read as restricting, though not forbidding,
the use of cost-benefit comparisons. And I would apply that
text accordingly.
I
Section 301 provides that, not later than 1977, effluent lim
itations for point sources shall require the application of
“best practicable control technology,” § 301(b)(1)(A), 86 Stat.
845 (emphasis added); and that, not later than 1983 (later
extended to 1989), effluent limitations for categories and
classes of point sources shall require application of the
“best available technology economically achievable,” § 301(b)
(2)(A), ibid. (emphasis added). Section 304(b), in turn, iden
tifies the factors that the Agency shall take into account in
determining (1) “best practicable control technology” and
(2) “best available technology.” 86 Stat. 851 (emphasis
added).
With respect to the first, the statute provides that the
factors taken into account by the Agency “shall include
consideration of the total cost of application of technology in
relation to the effluent reduction benefits to be achieved from
such application . . . and such other factors as the Administra
tor deems appropriate.” § 304(b)(1)(B), ibid. With respect
to the second, the statute says that the Agency “shall take
into account . . . the cost of achieving such effluent reduction”
and “such other factors as the Administrator deems appro
priate.” § 304(b)(2)(B), ibid.

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Opinion of Breyer, J.
The drafting history makes clear that the statute reflects
a compromise. In the House version of the legislation, the
Agency was to consider “the cost and the economic, social,
and environmental impact of achieving such effluent reduc
tion” when determining both “best practicable” and “best
available” technology. H. R. 11896, 92d Cong., 2d Sess.,
§§ 304(b)(1)(B), (b)(2)(B) (1972) (as reported from Commit
tee). The House Report explained that the “best available
technology” standard was needed—as opposed to mandating
the elimination of discharge of pollutants—because “the dif
ference in the cost of 100 percent elimination of pollutants
as compared to the cost of removal of 97–99 percent of the
pollutants in an effluent can far exceed any reasonable bene
fit to be achieved. In most cases, the cost of removal of the
last few percentage points increases expo[n]entially.” H. R.
Rep. No. 92–911, p. 103 (1972).
In the Senate version, the Agency was to consider “the
cost of achieving such effluent reduction” when determining
both “best practicable” and “best available” technology.
S. 2770, 92d Cong., 1st Sess., §§ 304(b)(1)(B), (b)(2)(B) (1971)
(as reported from Committee). The Senate Report explains
that “the technology must be available at a cost . . . which
the Administrator determines to be reasonable.” S. Rep.
No. 92–414, p. 52 (1971) (hereinafter S. Rep.). But it said
nothing about comparing costs and benefits.
The final statute reflects a modification of the House’s lan
guage with respect to “best practicable,” and an adoption
of the Senate’s language with respect to “best available.”
S. Conf. Rep. No. 92–1236, pp. 124–125 (1972). The final
statute does not require the Agency to compare costs to
benefits when determining “best available technology,” but
neither does it expressly forbid such a comparison.
The strongest evidence in the legislative history support
ing the respondents’ position—namely, that Congress in
tended to forbid comparisons of costs and benefits when de
termining the “best available technology”—can be found in

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a written discussion of the Act’s provisions distributed to
the Senate by Senator Edmund Muskie, the Act’s principal
sponsor, when he submitted the Conference Report for the
Senate’s consideration. 118 Cong. Rec. 33693 (1972). The
relevant part of that discussion points out that, as to “best
practicable technology,” the statute requires application of
a “balancing test between total cost and effluent reduction
benefits.” Id., at 33696; see § 304(b)(1)(B). But as to “best
available technology,” it states: “While cost should be a fac
tor in the Administrator’s judgment, no balancing test will
be required.” Ibid.; see § 304(b)(2)(B). And Senator Musk
ie’s discussion later speaks of the Agency “evaluat[ing] . . .
what needs to be done” to eliminate pollutant discharge and
“what is achievable,” both “without regard to cost.” Ibid.
As this language suggests, the Act’s sponsors had reasons
for minimizing the EPA’s investigation of, and reliance upon,
cost-benefit comparisons. The preparation of formal cost
benefit analyses can take too much time, thereby delaying
regulation. And the sponsors feared that such analyses
would emphasize easily quantifiable factors over more quali
tative factors (particularly environmental factors, for exam
ple, the value of preserving nonmarketable species of fish).
See S. Rep., at 47. Above all, they hoped that minimizing
the use of cost-benefit comparisons would force the develop
ment of cheaper control technologies; and doing so, whatever
the initial inefficiencies, would eventually mean cheaper,
more effective cleanup. See id., at 50–51.
Nonetheless, neither the sponsors’ language nor the under
lying rationale requires the Act to be read in a way that
would forbid cost-benefit comparisons. Any such total pro
hibition would be difficult to enforce, for every real choice
requires a decisionmaker to weigh advantages against disad
vantages, and disadvantages can be seen in terms of (often
quantifiable) costs. Moreover, an absolute prohibition
would bring about irrational results. As the respondents
themselves say, it would make no sense to require plants to

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Opinion of Breyer, J.
“spend billions to save one more fish or plankton.” Brief for
Respondent Riverkeeper, Inc., et al. 29. That is so even if
the industry might somehow afford those billions. And it is
particularly so in an age of limited resources available to
deal with grave environmental problems, where too much
wasteful expenditure devoted to one problem may well mean
considerably fewer resources available to deal effectively
with other (perhaps more serious) problems.
Thus Senator Muskie used nuanced language, which one
can read as leaving to the Agency a degree of authority to
make cost-benefit comparisons in a manner that is sensitive
both to the need for such comparisons and to the concerns
that the law’s sponsors expressed. The relevant statement
begins by listing various factors that the statute requires
the Administrator to take into account when applying the
phrase “practicable” to “classes and categories.” 118 Cong.
Rec. 33696. It states that, when doing so, the Administrator
must apply (as the statute specifies) a “balancing test be
tween total cost and effluent reduction benefits.” Ibid. At
the same time, it seeks to reduce the likelihood that the Ad
ministrator will place too much weight upon high costs by
adding that the balancing test “is intended to limit the appli
cation of technology only where the additional degree of ef
fluent reduction is wholly out of proportion to the costs of
achieving” a “marginal level of reduction.” Ibid.
Senator Muskie’s statement then considers the “different
test” that the statute requires the Administrator to apply
when determining the “ ‘best available’ ” technology. Ibid.
(emphasis added). Under that test, the Administrator “may
consider a broader range of technological alternatives.”
Ibid. And in determining what is “ ‘best available’ for a
category or class, the Administrator is expected to apply
the same principles involved in making the determination of
‘best practicable’ . . . except as to cost-benefit analysis.”
Ibid. (emphasis added). That is, “[w]hile cost should be a
factor . . . no balancing test will be required.” Ibid. (empha

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Opinion of Breyer, J.
sis added). Rather, “[t]he Administrator will be bound by
a test of reasonableness.” Ibid. (emphasis added). The
statement adds that the “ ‘best available’ ” standard “is in
tended to reflect the need to press toward increasingly
higher levels of control.” Ibid. (emphasis added). And
“the reasonableness of what is ‘economically achievable’
should reflect an evaluation of what needs to be done to move
toward the elimination of the discharge of pollutants and
what is achievable through the application of available tech
nology—without regard to cost.” Ibid. (emphasis added).
I believe, as I said, that this language is deliberately nu
anced. The statement says that where the statute uses the
term “best practicable,” the statute requires comparisons of
costs and benefits; but where the statute uses the term “best
available,” such comparisons are not “required.” Ibid. (em
phasis added). Senator Muskie does not say that all efforts
to compare costs and benefits are forbidden.
Moreover, the statement points out that where the statute
uses the term “best available,” the Administrator “will be
bound by a test of reasonableness.” Ibid. (emphasis added).
It adds that the Administrator should apply this test in a
way that reflects its ideal objective, moving as closely as is
technologically possible to the elimination of pollution. It
thereby says the Administrator should consider, i. e., take
into account, how much pollution would still remain if the
best available technology were to be applied everywhere—
“without regard to cost.” Ibid. It does not say that the
Administrator must set the standard based solely on the re
sult of that determination. (It would be difficult to reconcile
the alternative, more absolute reading of this language with
the Senator’s earlier “test of reasonableness.”)
I say that one may, not that one must, read Senator Musk
ie’s statement this way. But to read it differently would put
the Agency in conflict with the test of reasonableness by
threatening to impose massive costs far in excess of any ben
efit. For 30 years the EPA has read the statute and its his

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Opinion of Breyer, J.
tory in this way. The EPA has thought that it would not be
“reasonable to interpret Section 316(b) as requiring use of
technology whose cost is wholly disproportionate to the en
vironmental benefit to be gained.” In re Pub. Serv. Co. of
N. H. (Seabrook Station, Units 1 and 2), 1 E. A. D. 332, 340
(1977), remanded on other grounds, Seacoast Anti-Pollution
League v. Costle, 572 F. 2d 872 (CA1 1978) (emphasis added);
see also In re Central Hudson Gas & Elec. Corp., EPA Gen
eral Counsel Opinions, NPDES Permits, No. 63, p. 371 (July
29, 1977) (also applying a “wholly disproportionate” test); In
re Pub. Serv. Co. of N. H., 1 E. A. D. 455 (1978) (same).
“[T]his Court will normally accord particular deference to an
agency interpretation of ‘longstanding’ duration.” Barn
hart v. Walton, 535 U. S. 212, 220 (2002). And for the last
30 years, the EPA has given the statute a permissive reading
without suggesting that in doing so it was ignoring or
thwarting the intent of the Congress that wrote the statute.
The EPA’s reading of the statute would seem to permit it
to describe environmental benefits in non-monetized terms
and to evaluate both costs and benefits in accordance with
its expert judgment and scientific knowledge. The Agency
can thereby avoid lengthy formal cost-benefit proceedings
and futile attempts at comprehensive monetization, see 69
Fed. Reg. 41661–41662 (2004); take account of Congress’
technology-forcing objectives; and still prevent results that
are absurd or unreasonable in light of extreme disparities
between costs and benefits. This approach, in my view,
rests upon a “reasonable interpretation” of the statute—
legislative history included. Hence it is lawful. Chevron
U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467
U. S. 837, 844 (1984). Most of what the majority says is con
sistent with this view, and to that extent I agree with its
opinion.
II
The cases before us, however, present an additional prob
lem. We here consider a rule that permits variances from

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236 ENTERGY CORP. v. RIVERKEEPER, INC.
Stevens, J., dissenting
national standards if a facility demonstrates that its costs
would be “significantly greater than the benefits of comply
ing.” 40 CFR § 125.94(a)(5)(ii) (2008). The words “signifi
cantly greater” differ from the words the EPA has tradi
tionally used to describe its standard, namely, “wholly
disproportionate.” Perhaps the EPA does not mean to
make much of that difference. But if it means the new
words to set forth a new and different test, the EPA must
adequately explain why it has changed its standard. Motor
Vehicle Mfrs. Assn. of United States, Inc. v. State Farm Mut.
Automobile Ins. Co., 463 U. S. 29, 42–43 (1983); National
Cable & Telecommunications Assn. v. Brand X Internet
Services, 545 U. S. 967, 981 (2005); Thomas Jefferson Univ.
v. Shalala, 512 U. S. 504, 524, n. 3 (1994) (Thomas, J.,
dissenting).
I am not convinced the EPA has successfully explained the
basis for the change. It has referred to the fact that exist
ing facilities have less flexibility than new facilities with re
spect to installing new technologies, and it has pointed to
special, energy-related impacts of regulation. 68 Fed. Reg.
13541 (2003) (proposed rule). But it has not explained why
the traditional “wholly disproportionate” standard cannot do
the job now, when the EPA has used that standard (for exist
ing facilities and otherwise) with apparent success in the
past. See, e. g., Central Hudson, supra.
Consequently, like the majority, I would remand these
cases to the Court of Appeals. But unlike the majority I
would permit that court to remand the cases to the EPA so
that the EPA can either apply its traditional “wholly dispro
portionate” standard or provide an adequately reasoned ex
planation for the change.
Justice Stevens, with whom Justice Souter and Jus
tice Ginsburg join, dissenting.
Section 316(b) of the Clean Water Act (CWA), 33 U. S. C.
§ 1326(b), which governs industrial powerplant water intake

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237 Cite as: 556 U. S. 208 (2009)
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structures, provides that the Environmental Protection
Agency (EPA or Agency) “shall require” that such struc
tures “reflect the best technology available for minimizing
adverse environmental impact.” The EPA has interpreted
that mandate to authorize the use of cost-benefit analysis
in promulgating regulations under § 316(b). For instance,
under the Agency’s interpretation, technology that would
otherwise qualify as the best available need not be used if
its costs are “significantly greater than the benefits” of com
pliance. 40 CFR § 125.94(a)(5)(ii) (2008).
Like the Court of Appeals, I am convinced that the EPA
has misinterpreted the plain text of § 316(b). Unless costs
are so high that the best technology is not “available,” Con
gress has decided that they are outweighed by the bene
fits of minimizing adverse environmental impact. Section
316(b) neither expressly nor implicitly authorizes the EPA to
use cost-benefit analysis when setting regulatory standards;
fairly read, it prohibits such use.
I
As typically performed by the EPA, cost-benefit analysis
requires the Agency to first monetize the costs and benefits
of a regulation, balance the results, and then choose the reg
ulation with the greatest net benefits. The process is par
ticularly controversial in the environmental context in which
a regulation’s financial costs are often more obvious and eas
ier to quantify than its environmental benefits. And cost
benefit analysis often, if not always, yields a result that does
not maximize environmental protection.
For instance, although the EPA estimated that water in
take structures kill 3.4 billion fish and shellfish each year,1
1 To produce energy, industrial powerplants withdraw billions of gallons
of water daily from our Nation’s waterways. Thermoelectric powerplants
alone demand 39 percent of all freshwater withdrawn nationwide. See
Dept. of Energy, Addressing the Critical Link Between Fossil Energy and
Water 2 (Oct. 2005), http://www.netl.doe.gov/technologies/coalpower/ewr/

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238 ENTERGY CORP. v. RIVERKEEPER, INC.
Stevens, J., dissenting
see 69 Fed. Reg. 41586 (2004), the Agency struggled to calcu
late the value of the aquatic life that would be protected
under its § 316(b) regulations, id., at 41661. To compensate,
the EPA took a shortcut: Instead of monetizing all aquatic
life, the Agency counted only those species that are commer
cially or recreationally harvested, a tiny slice (1.8 percent to
be precise) of all impacted fish and shellfish. This narrow
focus in turn skewed the Agency’s calculation of benefits.
When the EPA attempted to value all aquatic life, the bene
fits measured $735 million.2 But when the EPA decided to
give zero value to the 98.2 percent of fish not commercially
or recreationally harvested, the benefits calculation dropped
dramatically—to $83 million. Id., at 41666. The Agency
acknowledged that its failure to monetize the other 98.2 per
cent of affected species “ ‘could result in serious misallocation
of resources,’ ” id., at 41660, because its “comparison of com
plete costs and incomplete benefits does not provide an accu
rate picture of net benefits to society.” 3
Because benefits can be more accurately monetized in
some industries than in others, Congress typically decides
whether it is appropriate for an agency to use cost-benefit
analysis in crafting regulations. Indeed, this Court has rec
ognized that “[w]hen Congress has intended that an agency
engage in cost-benefit analysis, it has clearly indicated such
intent on the face of the statute.” American Textile Mfrs.
pubs/NETL_Water_Paper_Final_Oct.2005.pdf (all Internet materials as
visited Mar. 18, 2009, and available in Clerk of Court’s case file). The fish
and shellfish are killed by “impingement” or “entrainment.” Impinge
ment occurs when aquatic organisms are trapped against the screens and
grills of water intake structures. Entrainment occurs when these organ
isms are drawn into the intake structures. See Riverkeeper, Inc. v. EPA,
475 F. 3d 83, 89 (CA2 2007); 69 Fed. Reg. 41586 (2004).
2 EPA, Economic and Benefits Analysis for the Proposed Section 316(b)
Phase II Existing Facilities Rule, p. D1–4 (EPA–821–R–02–001, Feb.
2002), http://www.epa.gov/waterscience/316b/phase2/econbenefits.
3 EPA, Economic and Benefits Analysis for the Final Section 316(b)
Phase II Existing Facilities Rule, p. D1–5 (EPA–821–R–04–005, Feb. 2004),
http://www.epa.gov/waterscience/316b/phase2/econbenefits/final.htm.

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Stevens, J., dissenting
Institute, Inc. v. Donovan, 452 U. S. 490, 510 (1981). Ac
cordingly, we should not treat a provision’s silence as an im
plicit source of cost-benefit authority, particularly when such
authority is elsewhere expressly granted and it has the
potential to fundamentally alter an agency’s approach to
regulation. Congress, we have noted, “does not alter the
fundamental details of a regulatory scheme in vague terms
or ancillary provisions—it does not, one might say, hide ele
phants in mouseholes.” Whitman v. American Trucking
Assns., Inc., 531 U. S. 457, 468 (2001).
When interpreting statutory silence in the past, we have
sought guidance from a statute’s other provisions. Evidence
that Congress confronted an issue in some parts of a statute,
while leaving it unaddressed in others, can demonstrate that
Congress meant its silence to be decisive. We concluded as
much in American Trucking. In that case, the Court re
viewed a claim that § 109 of the Clean Air Act (CAA), 42
U. S. C. § 7409(a) (2000 ed.), authorized the EPA to consider
implementation costs in setting ambient air quality stand
ards. We read § 109, which was silent on the matter, to pro
hibit Agency reliance on cost considerations. After examin
ing other provisions in which Congress had given the
Agency authority to consider costs, the Court “refused to
find implicit in ambiguous sections of the CAA an author
ization to consider costs that has elsewhere, and so often,
been expressly granted.” 531 U. S., at 467. Studied si
lence, we thus concluded, can be as much a prohibition as an
explicit “no.”
Further motivating the Court in American Trucking was
the fact that incorporating implementation costs into the
Agency’s calculus risked countermanding Congress’ decision
to protect public health. The cost of implementation, we
said, “is both so indirectly related to public health and so full
of potential for canceling the conclusions drawn from direct
health effects that it would surely have been expressly men
tioned in [the text] had Congress meant it to be considered.”
Id., at 469.

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240 ENTERGY CORP. v. RIVERKEEPER, INC.
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American Trucking’s approach should have guided the
Court’s reading of § 316(b). Nowhere in the text of § 316(b)
does Congress explicitly authorize the use of cost-benefit
analysis as it does elsewhere in the CWA. And the use of
cost-benefit analysis, like the consideration of implementation
costs in American Trucking, “pad[s]” § 316(b)’s environmen
tal mandate with tangential economic efficiency concerns.
Id., at 468. Yet the majority fails to follow American Truck
ing despite that case’s obvious relevance to our inquiry.
II
In 1972, Congress amended the CWA to strike a careful
balance between the country’s energy demands and its desire
to protect the environment. The Act required industry to
adopt increasingly advanced technology capable of mitigat
ing its detrimental environmental impact. Not all point
sources were subject to strict rules at once. Existing plants
were granted time to retrofit with the best technology while
new plants were required to incorporate such technology as
a matter of design. Although Congress realized that tech
nology standards would necessarily put some firms out of
business, see EPA v. National Crushed Stone Assn., 449
U. S. 64, 79 (1980), the statute’s steady march was toward
stricter rules and potentially higher costs.
Section 316(b) was an integral part of the statutory
scheme. The provision instructs that “[a]ny standard estab
lished pursuant to section 1311 of this title or section 1316 of
this title and applicable to a point source shall require that
the location, design, construction, and capacity of cooling
water intake structures reflect the best technology available
for minimizing adverse environmental impact.” 33 U. S. C.
§ 1326(b) (2006 ed.) (emphasis added).4 The “best technology
4 The two cross-referenced provisions, §§ 1311 and 1316, also establish
“best technology” standards, the first applicable to existing point sources
and the second to new facilities. The reference to these provisions in
§ 316(b) merely requires any rule promulgated under those provisions,
when applied to a point source with a water intake structure, to incorpo
rate § 316(b) standards.

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Stevens, J., dissenting
available,” or “BTA,” standard delivers a clear command: To
minimize the adverse environmental impact of water intake
structures, the EPA must require industry to adopt the best
technology available.
Based largely on the observation that § 316(b)’s text offers
little guidance and therefore delegates some amount of gap
filling authority to the EPA, the Court concludes that the
Agency has discretion to rely on cost-benefit analysis. See
ante, at 222–223. The Court assumes that, by not specifying
how the EPA is to determine BTA, Congress intended to
give considerable discretion to the EPA to decide how to
proceed. Silence, in the majority’s view, represents ambigu
ity and an invitation for the Agency to decide for itself which
factors should govern its regulatory approach.
The appropriate analysis requires full consideration of
the CWA’s structure and legislative history to determine
whether Congress contemplated cost-benefit analysis and, if
so, under what circumstances it directed the EPA to utilize
it. This approach reveals that Congress granted the EPA
authority to use cost-benefit analysis in some contexts but
not others, and that Congress intended to control, not dele
gate, when cost-benefit analysis should be used. See Chev
ron U. S. A. Inc. v. Natural Resources Defense Council, Inc.,
467 U. S. 837, 842–843 (1984).5
Powerful evidence of Congress’ decision not to authorize
cost-benefit analysis in the BTA standard lies in the series of
5 The majority announces at the outset that the EPA’s reading of the
BTA standard “governs if it is a reasonable interpretation of the statute—
not necessarily the only possible interpretation, nor even the interpreta
tion deemed most reasonable by the courts.” Ante, at 218. This obser
vation is puzzling in light of the commonly understood practice that, as a
first step, we ask “whether Congress has directly spoken to the precise
question at issue.” Chevron, 467 U. S., at 842. Only later, if Congress’
intent is not clear, do we consider the reasonableness of the agency’s ac
tion. Id., at 843. Assuming ambiguity and moving to the second step
reflects the Court’s reluctance to consider the possibility, which it later
laments is “more complex,” ante, at 220, that Congress’ silence may have
meant to foreclose cost-benefit analysis.

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standards adopted to regulate the outflow, or effluent, from
industrial powerplants. Passed at the same time as the
BTA standard at issue here, the effluent limitation standards
imposed increasingly strict technology requirements on in
dustry. In each effluent limitation provision, Congress dis
tinguished its willingness to allow the EPA to consider costs
from its willingness to allow the Agency to conduct a cost
benefit analysis. And to the extent Congress permitted
cost-benefit analysis, its use was intended to be temporary
and exceptional.
The first tier of technology standards applied to existing
plants—facilities for which retrofitting would be particularly
costly. Congress required these plants to adopt “effluent
limitations . . . which shall require the application of the best
practicable control technology currently available.” 33
U. S. C. § 1311(b)(1)(A). Because this “best practicable,” or
“BPT,” standard was meant to ease industry’s transition to
the new technology-based regime, Congress gave BPT two
unique features: First, it would be temporary, remaining in
effect only until July 1, 1983.6 Second, it specified that the
EPA was to conduct a cost-benefit analysis in setting BPT
requirements by considering “the total cost of application of
technology in relation to the effluent reduction benefits to be
achieved from such application.” 7 § 1314(b)(1)(B). Permit
ting cost-benefit analysis in BPT gave the EPA the ability
to cushion the new technology requirement. For a limited
6 Congress later extended the deadline to March 31, 1989.
7 Senator Muskie, the Senate sponsor of the legislation, described the
cost-benefit analysis permitted under BPT as decidedly narrow, asserting
that “[t]he balancing test between total cost and effluent reduction benefits
is intended to limit the application of technology only where the additional
degree of effluent reduction is wholly out of proportion to the costs of
achieving such marginal level of reduction for any class or category of
sources.” 1 Legislative History of the Water Pollution Control Act
Amendments of 1972 (Committee Print compiled for the Senate Commit
tee on Public Works by the Library of Congress), Ser. No. 93–1, p. 170
(1973) (hereinafter Leg. Hist.)

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Stevens, J., dissenting
time, a technology with costs that exceeded its benefits
would not be considered “best.”
The second tier of technology standards required existing
powerplants to adopt the “best available technology econom
ically achievable” to advance “the national goal of eliminat
ing the discharge of all pollutants.” § 1311(b)(2)(A). In set
ting this “best available technology,” or “BAT,” 8 standard,
Congress gave the EPA a notably different command for de
ciding what technology would qualify as “best”: The EPA
was to consider, among other factors, “the cost of achieving
such effluent reduction,” but Congress did not grant it au
thority to balance costs with the benefits of stricter regula
tion. § 1314(b)(2)(B). Indeed, in Crushed Stone this Court
explained that the difference between BPT and BAT was the
existence of cost-benefit authority in the first and the ab
sence of that authority in the second. See 449 U. S., at 71
(“Similar directions are given the Administrator for deter
mining effluent reductions attainable from the BAT except
that in assessing BAT total cost is no longer to be considered
in comparison to effluent reduction benefits”).
The BAT standard’s legislative history strongly supports
the view that Congress purposefully withheld cost-benefit
authority for this tier of regulation. See ibid., n. 10. The
House of Representatives and the Senate split over the role
cost-benefit analysis would play in the BAT provision. The
House favored the tool, see H. R. Rep. No. 92–911, p. 107
(1972), 1 Leg. Hist. 794, while the Senate rejected it, see 2
id., at 1183; id., at 1132. The Senate view ultimately pre
vailed in the final legislation, resulting in a BAT standard
that was “not subject to any test of cost in relation to effluent
reduction benefits or any form of cost/ benefit analysis.” 3
Legislative History of the Clean Water Act of 1977: A Con
tinuation of the Legislative History of the Federal Water
Pollution Control Act (Committee Print compiled for the
8 Although the majority calls this “BATEA,” the parties refer to the
provision as “BAT,” and for simplicity, so will I.

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Senate Committee on Environment and Public Works by the
Library of Congress), Ser. No. 95–14, p. 427 (1978).
The third and strictest regulatory tier was reserved for
new point sources—facilities that could incorporate technol
ogy improvements into their initial design. These new facil
ities were required to adopt “the best available demon
strated control technology,” or “BADT,” which Congress
described as “a standard . . . which reflect[s] the greatest
degree of effluent reduction.” § 1316(a)(1). In administer
ing BADT, Congress directed the EPA to consider “the cost
of achieving such effluent reduction.” § 1316(b)(1)(B). But
because BADT was meant to be the most stringent standard
of all, Congress made no mention of cost-benefit analysis.
Again, the silence was intentional. The House’s version of
BADT originally contained an exemption for point sources
for which “the economic, social, and environmental costs bear
no reasonable relationship to the economic, social, and envi
ronmental benefit to be obtained.” 1 Leg. Hist. 798. That
this exemption did not appear in the final legislation demon
strates that Congress considered, and rejected, reliance on
cost-benefit analysis for BADT.
It is in this light that the BTA standard regulating water
intake structures must be viewed. The use of cost-benefit
analysis was a critical component of the CWA’s structure and
a key concern in the legislative process. We should there
fore conclude that Congress intended to forbid cost-benefit
analysis in one provision of the Act in which it was silent on
the matter when it expressly authorized its use in another.9
9 The Court argues that, if silence in § 316(b) signals the prohibition of
cost-benefit analysis, it must also foreclose the consideration of all other
potentially relevant discretionary factors in setting BTA standards.
Ante, at 222. This all-or-nothing reasoning rests on the deeply flawed
assumption that Congress treated cost-benefit analysis as just one among
many factors upon which the EPA could potentially rely to establish BTA.
Yet, as explained above, the structure and legislative history of the CWA
demonstrate that Congress viewed cost-benefit analysis with special skep
ticism and controlled its use accordingly. The Court’s assumption of

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Stevens, J., dissenting
See, e. g., Allison Engine Co. v. United States ex rel. Sand
ers, 553 U. S. 662, 671 (2008); Russello v. United States, 464
U. S. 16, 23 (1983) (“[W]here Congress includes particular
language in one section of a statute but omits it in another
. . . , it is generally presumed that Congress acts intention
ally and purposely in the disparate inclusion or exclusion”
(internal quotation marks omitted)). This is particularly
true given Congress’ decision that cost-benefit analysis
would play a temporary and exceptional role in the CWA to
help existing plants transition to the Act’s ambitious environ
mental standards.10 Allowing cost-benefit analysis in the
BTA standard, a permanent mandate applicable to all power
plants, serves no such purpose and instead fundamentally
weakens the provision’s mandate.11
Accordingly, I would hold that the EPA is without author
ity to perform cost-benefit analysis in setting BTA stand
equivalence is thus plainly incorrect. Properly read, Congress’ silence
in § 316(b) forbids reliance on the cost-benefit tool but does not foreclose
reliance on all other considerations, such as a determination whether a
technology is so costly that it is not “available” for industry to adopt.
10 In 1977, Congress established an additional technology-based stand
ard, commonly referred to as “best conventional pollutant control technol
ogy,” or “BCT,” to govern conventional pollutants previously covered by
the BAT standard. See 33 U. S. C. § 1311(b)(2)(E). The BCT standard
required the EPA to consider, among other factors, “the relationship be
tween the costs of attaining a reduction in effluents and the effluent reduc
tion benefits derived.” § 1314(b)(4)(B). That Congress expressly author
ized cost-benefit analysis in BCT further confirms that Congress treated
cost-benefit analysis as exceptional and reserved for itself the authority to
decide when it would be used in the Act.
11 The Court attempts to cabin its holding by suggesting that a “rigorous
form of cost-benefit analysis,” such as the form “prescribed under the stat
ute’s former BPT standard,” may not be permitted for setting BTA regu
lations. Ante, at 223. Thus the Court has effectively instructed the
Agency that it can perform a cost-benefit analysis so long as it does not
resemble the kind of cost-benefit analysis Congress elsewhere authorized
in the CWA. The majority’s suggested limit on the Agency’s discretion
can only be read as a concession that cost-benefit analysis, as typically
performed, may be inconsistent with the BTA mandate.

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ards. To the extent the EPA relied on cost-benefit analysis
in establishing its BTA regulations,12 that action was con
trary to law, for Congress directly foreclosed such reliance
in the statute itself.13 Chevron, 467 U. S., at 843. Because
we granted certiorari to decide only whether the EPA has
authority to conduct cost-benefit analysis, there is no need
to define the universe of considerations upon which the EPA
can properly rely in administering the BTA standard.
I would leave it to the Agency to decide how to proceed in
the first instance.
III
Because the Court unsettles the scheme Congress estab
lished, I respectfully dissent.
12 The “national performance standards” the EPA adopted were shaped
by economic efficiency concerns at the expense of finding the technology
that best minimizes adverse environmental impact. In its final rule
making, the Agency declined to require industrial plants to adopt closed
cycle cooling technology, which by recirculating cooling water requires less
water to be withdrawn and thus fewer aquatic organisms to be killed.
Riverkeeper, Inc. v. EPA, 358 F. 3d 174, 182, n. 5 (CA2 2004); 69 Fed. Reg.
41601, and n. 44. This the Agency decided despite its acknowledgment
that “closed-cycle, recirculating cooling systems . . . can reduce mortality
from impingement by up to 98 percent and entrainment by up to 98 per
cent.” Id., at 41601. The EPA instead permitted individual plants to
resort to a “suite” of options so long as the method used reduced impinge
ment and entrainment by the more modest amounts of 80 and 60 percent,
respectively. See 40 CFR § 125.94(b) (2008). The Agency also permitted
individual plants to obtain a site-specific variance from the national per
formance standards if they could prove (1) that compliance costs would be
“significantly greater than” those the Agency considered when establish
ing the standards, or (2) that compliance costs “would be significantly
greater than the benefits of complying with the applicable performance
standards,” § 125.94(a)(5).
13 Thus, the Agency’s past reliance on a “wholly disproportionate” stand
ard, a mild variant of cost-benefit analysis, is irrelevant. See ante, at
224–225 (majority opinion). Because “Congress has directly spoken to
the precise question at issue,” Chevron, 467 U. S., at 842, longstanding
yet impermissible agency practice cannot ripen into permissible agency
practice.