What is the proper test for determining whether the 'diligent prosecution bar' under 33 U.S.C. § 1319(g)(6)(A)(i) precludes citizen suits brought under 33 U.S.C. § 1365(a)?

QUESTION PRESENTED The Clean Water Act allows citizens to sue any person alleged to be in violation of an “effluent standard or limitation” under the Act. 33 U.S.C. § 1365(a). Because these “citizen suits” are meant to “supplement” not “supplant” the States’ primary role in regulating water quality, Gwaltney of Smithfield, Ltd. v. Chesapeake Bay Found., Inc., 484 U.S. 49, 60 (1987), Congress has barred them when a State has “commenced” and is “diligently prosecuting” an administrative penalty action “under a State law comparable to” the Clean Water Act’s administrative penalty provisions. 33 U.S.C. § 1819(g)(6)(A)(i) (‘diligent prosecution bar”). Over the last several decades, the Courts of Appeals have issued conflicting rules on how “comparable” a State’s law must be to trigger the bar. For example, the First and Eighth Circuits apply a deferential “overall comparability” test, while the Tenth and Eleventh Circuits employ a stricter “rough comparability” test, with still other Circuits applying variants of the two. Here, a divided panel of the Fourth Circuit adopted a third and even more demanding standard, according to which the diligent prosecution bar does not preclude citizen suits unless a State’s enforcement regime exactly follows the Clean Water Act’s administrative penalty provisions and implementing regulations. The question presented is: What is the proper test for determining whether the “diligent prosecution bar’ under 33 U.S.C. § 1319(g)(6)(A)Gi) precludes citizen suits brought under 33 U.S.C. § 1365(a)?