Tenth Circuit Weighs In on Maui’s “Functional Equivalent” Test
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Tenth Circuit Weighs In on Maui’s “Functional Equivalent” Test

Brownstein Client Alert, Jan. 24, 2023

The Tenth Circuit became the second federal appellate court—after the Ninth Circuit—to analyze how federal district courts should evaluate whether a discharge of pollutants to groundwater is the “functional equivalent” of a direct discharge and thus, subject to regulation and permitting under the Clean Water Act (CWA). In Stone v. High Mountain Mining Co., the Tenth Circuit reversed and remanded a district court decision for improperly applying the functional equivalence factors set forth in the U.S. Supreme Court’s 2020 decision in County of Maui v. Hawaii Wildlife Fund, 140 S. Ct. 1462, 1476 (2020). The Tenth Circuit’s decision comes on the heels of the comment deadline for the Environmental Protection Agency’s (EPA) draft Maui guidance on how to assess functional equivalence and considerations for determining whether a groundwater discharge requires a CWA discharge permit.

Tenth Circuit Decision

In Stone, citizen plaintiffs alleged that a Colorado mining operator required a CWA discharge permit on the basis that pollutants from the mining company’s settling ponds seeped through the bottom of the ponds, into the groundwater, and then to the Middle Fork of the South Platte River. After a four-day bench trial, the district court concluded that the mining company violated the CWA because seepage from the ponds to the Middle Fork was the functional equivalent of a discharge without a permit.

In its decision, the district court evaluated the seven non-exclusive Maui factors—(1) transit time, (2) distance traveled, (3) nature of the material through which the pollutant travels, (4) dilution or chemical changes, (5) relative amount of pollutant entering the water, (6) manner or area in which pollutant enters the water, and (7) the degree to which pollution maintains a specific identity—before finding that the first three factors supported the plaintiffs and there was limited or no evidence on the remaining four factors. 

The mining company appealed, arguing that the district court erred in applying the Maui legal framework.

The Tenth Circuit found that the district court erred in its legal analysis because it focused solely on three of the seven factors rather than considering all relevant geophysical factors. Although the Tenth Circuit largely agreed with the district court’s analysis of the transit time and distance traveled factors, it held that the district court should have made additional findings on the other Maui factors. It reasoned that although time and distance are, as the Supreme Court recognized, the most important factors, in closer cases such as the one presented, a court cannot rely solely on time and distance. Instead, the district court must consider all relevant geophysical factors. In particular, the Tenth Circuit noted that several factors, such as dilution and amount of discharge, might be important in this context. Moreover, by dismissing several factors as factually underdeveloped while still finding for the plaintiffs, the Tenth Circuit held that the district court impermissibly shifted the burden of proof away from the citizen plaintiffs to the mining company.

Colorado’s Complex Mining Regime

Underpinning the Tenth Circuit’s opinion appears to be a concern about interfering with Colorado’s regulatory regime. Colorado has a complex regulatory regime with multiple agencies responsible for different aspects of water quality regulation. The Colorado Mining Association argued in an amicus curiae brief that the district court’s decision would “create significant uncertainty as to the responsibilities of the agencies and the requirements to comply with the law.” In the concluding paragraphs of its decision, the Tenth Circuit recognized that the “complex topography of … our circuit writ law, and the complicated and overlapping regulatory regimes that surround the State of Colorado’s 10,380 active mines” required a “more comprehensive and rigorous application” of the Maui factors.

What Comes Next

The Tenth Circuit remanded the case to the district court, which now may reopen the evidentiary hearing or dismiss the plaintiffs’ claims or chart some other course. The parties and the district court must now grapple with the Maui factors once again.

Depending on the timing of the district court proceeding, inquiry into the Maui factors may be influenced by EPA’s draft guidance, which also concerns how to apply the Maui decision’s “functional equivalent” standard. Public comment ended on Dec. 27, 2023, and the ball is now in EPA’s court to evaluate not only the comments on the draft guidance but also the emerging appellate case law on the issue, including the Tenth Circuit’s decision in Stone and the Ninth Circuit’s decision in Cottonwood Env. Law Ctr. v. Edwards, 86 F.4th 1255 (9th Cir. 2023).

EPA’s draft guidance recognizes that the functional equivalence analysis is “highly dependent on site-specific features,” and that the list of factors is “illustrative,” not comprehensive. EPA contends that, in some cases, analysis may not require consideration of all the factors, particularly where time and distance are either very short or very long. But it does not define precisely when a comprehensive or more cursory analysis is appropriate.

Notably, EPA’s draft guidance also states that an operator’s intent or the existence of a state groundwater program are not relevant to the functional equivalence analysis, which arguably is at odds with both the Tenth Circuit opinion in Stone and the Supreme Court’s concern for decisions that “create serious risk[] … of undermining state regulation of groundwater” in Maui, see 140 S. Ct. at 1477.


This document is intended to provide you with general information regarding the application of County of Maui v. Hawaii Wildlife Fund. The contents of this document are not intended to provide specific legal advice. If you have any questions about the contents of this document or if you need legal advice as to an issue, please contact the attorneys listed or your regular Brownstein Hyatt Farber Schreck, LLP attorney. This communication may be considered advertising in some jurisdictions. The information in this article is accurate as of the publication date. Because the law in this area is changing rapidly, and insights are not automatically updated, continued accuracy cannot be guaranteed.

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