County of Maui v. Hawaii Wildlife Fund, 590 U.S. 165

County of Maui operates a wastewater reclamation facility that pumps treated effluent into four deep injection wells; the effluent travels through groundwater and reaches the Pacific Ocean.
Respondent environmental groups brought a citizen suit under the Clean Water Act, alleging the County discharged pollutants to navigable waters without an NPDES permit.
The district court found the groundwater pathway to the ocean was "functionally one into navigable water" and granted summary judgment for respondents.
The Ninth Circuit affirmed using a "fairly traceable" / functional-equivalent framing; courts of appeals had adopted conflicting standards (e.g., "direct hydrological connection," exclusion of groundwater).
The Supreme Court held that the CWA requires a permit for direct discharges and for discharges that are the "functional equivalent" of a direct discharge through groundwater, vacated the Ninth Circuit, and remanded for further proceedings.

Issue	Plaintiff's Argument	Defendant's Argument	Held
Whether the CWA requires an NPDES permit when pollutants originate at a point source but reach navigable waters via groundwater	Respondents: permit required if pollutants are "fairly traceable" to a point source (proximate cause standard)	Maui: no permit if any nonpoint source (e.g., groundwater) intervenes; point source must be the means of delivery (bright-line)	A permit is required for direct discharges and when the discharge through groundwater is the "functional equivalent" of a direct discharge (middle-ground multi-factor test)
Whether EPA's Interpretive Statement excluding all groundwater releases controls and merits deference	Respondents: EPA's blanket exclusion is unreasonable / not controlling here	Solicitor General / EPA: releases to groundwater are excluded from NPDES, so no permit even if pollutants later reach surface waters	Court declines Chevron deference here and rejects EPA's all-groundwater exclusion as unpersuasive and inconsistent with statutory text and purpose
Which standard courts should apply to middle cases and how to assess functional equivalence	Respondents: prefer standards that capture long-chain traceability; seek workable tests	Maui/EPA/dissent: favor bright-line tests for administrability and predictability	Court provides non-exhaustive multi-factor approach (time, distance most important; also medium, dilution, identity, amount, manner of entry) and remands for factual application

EPA v. California ex rel. State Water Resources Control Bd., 426 U.S. 200 (describing federal permitting focus under the Clean Water Act)
Milwaukee v. Illinois, 451 U.S. 304 (establishing the broad permitting scheme and that point-source discharges require permits)
Rapanos v. United States, 547 U.S. 715 (plurality opinion discussing indirect discharges and relevance of conveyances)
South Fla. Water Management Dist. v. Miccosukee Tribe, 541 U.S. 95 (clarifying that a point source need only convey pollutants to navigable waters)
Chevron U.S.A. Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837 (framework for agency deference; Court here declines to apply it)
United States v. Mead Corp., 533 U.S. 218 (agency interpretations considered under Skidmore; Court discusses but finds EPA's position unpersuasive)
Landgraf v. USI Film Products, 511 U.S. 244 (clear-statement principle referenced regarding statutory messages)
CSX Transp., Inc. v. McBride, 564 U.S. 685 (discussion of proximate cause and limits of importing tort concepts)
Utility Air Regulatory Group v. EPA, 573 U.S. 302 (clear-statement and limits on expansive agency interpretations)