The Clean Water Act is the principal federal law aimed at controlling pollution of the nation’s water resources, yet it does not provide comprehensive oversight of pollutants entering groundwater, the subsurface water that often feeds into rivers, lakes, and oceans. This Note examines a recent Supreme Court decision, County of Maui v. Hawaii Wildlife Fund, which appeared to endorse a theory of federal regulation of groundwater discharges under the Clean Water Act. County of Maui established a “functional equivalent” standard, under which a discharge through groundwater is subject to the Clean Water Act’s permitting requirements if it is the functional equivalent of a direct discharge into jurisdictional surface waters. While the Court outlined several factors for courts to consider in making a functional equivalent determination, the decision offers limited guidance for lower courts applying the test. Moreover, it leaves an important regulatory question unanswered. This Note aims to address some of the persisting uncertainties by proposing that Justice Kennedy’s “significant nexus” standard from his Rapanos v. United States concurrence can be illuminating. This Note argues that overlaying the significant nexus standard on the functional equivalent test offers a practical strategy for lower courts applying the test in difficult cases.

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