Coeur Alaska sought to reopen the Kensington gold mine near Juneau, Alaska, using a froth-flotation mill that would generate a slurry of tailings (a mixture of crushed rock and water) as process wastewater. The company proposed to discharge the tailings via pipeline into Lower Slate Lake, a 23-acre navigable water, where the tailings would settle and raise the bottom elevation by roughly 50 feet, effectively burying the existing lakebed. Under the Clean Water Act (CWA), discharges of pollutants generally require a permit. The Act creates two principal permitting pathways: (1) §402's National Pollutant Discharge Elimination System (NPDES), administered by EPA, covering most pollutant discharges; and (2) §404, administered by the Army Corps of Engineers, covering "dredged or fill material." In 2002, EPA and the Corps jointly defined "fill material" to include materials (such as mine tailings) that have the effect of replacing water with dry land or changing the bottom elevation of a waterbody. Coeur Alaska obtained a §404 permit from the Corps to deposit tailings in the lake, and EPA separately issued an NPDES permit for treated water leaving the lake's outlet after operations. Environmental groups sued, arguing the discharge was process wastewater from a new source subject to EPA's NSPS for froth-flotation gold mining (40 C.F.R. pt. 440, subpt. J), which prohibit any discharge of process wastewater to navigable waters from new sources. The district court upheld the permits, but the Ninth Circuit reversed, holding that the NSPS applied and barred the discharge notwithstanding the §404 permit. The Supreme Court granted certiorari.
When a mining operation discharges tailings slurry that will raise the bottom elevation of a navigable lake, is the discharge governed by the Clean Water Act's §404 permitting regime for "fill material" (administered by the Army Corps of Engineers), or by EPA's §402 NPDES program and its New Source Performance Standards for froth-flotation mills, which would prohibit the discharge?
Under the Clean Water Act, the Corps has authority under §404 to issue permits for the discharge of "dredged or fill material" into navigable waters. EPA administers §402's NPDES program for other pollutant discharges and promulgates technology-based effluent limitations, including §306 New Source Performance Standards. Pursuant to a 2002 joint EPA–Corps regulation, "fill material" includes materials that change the bottom elevation of a waterbody, such as mine tailings. EPA's regulations also provide that discharges of dredged or fill material regulated under §404 are not subject to NPDES permitting. Where a discharge qualifies as fill material, permitting authority lies with the Corps under §404, and EPA's §402-based effluent limitations and NSPS do not apply to that discharge, absent contrary direction consistent with the statute and regulations. Courts defer to an agency's reasonable interpretation of the CWA and its own regulations (Chevron/Auer deference).
The tailings slurry to be discharged into Lower Slate Lake is "fill material" under the joint 2002 definition; therefore, the discharge is governed by §404, and the Army Corps of Engineers had authority to issue the permit. EPA's §306 New Source Performance Standards for froth-flotation mills do not apply to a §404 discharge of fill material. The Ninth Circuit's judgment was reversed.
1) Classification as fill material: The 2002 joint EPA–Corps rule defines "fill material" based on effect: material that has the effect of replacing water with dry land or changing the bottom elevation of a waterbody. The proposed slurry would raise Lower Slate Lake's bottom approximately 50 feet by depositing tailings on the lakebed. The rule's illustrative list includes mine tailings and slurries. Thus, the discharge is fill material. 2) Allocation of permitting authority: The CWA establishes a division of labor—§404 covers dredged or fill material (Corps), while §402 covers other discharges (EPA). EPA's NPDES regulation expressly excludes from §402 discharges of dredged or fill material regulated under §404. Reading the statute and regulations together, the Court concluded that the regime channels fill discharges to §404 and out of §402. 3) Applicability of NSPS: Environmental groups argued that even if the discharge qualifies as fill, EPA's NSPS for new froth-flotation mills independently bar any process wastewater discharge. The Court disagreed. EPA and the Corps had determined in the permitting process that NSPS do not apply to §404 discharges, and EPA's own regulations direct that NPDES-based effluent limits do not govern where §404 applies. The Court found at least ambiguity in how §306 standards interact with §404 permits and deferred to EPA's reasonable interpretation that NSPS do not apply to discharges classified and permitted as fill under §404. EPA maintains oversight of §404 through the §404(b)(1) Guidelines and its §404(c) veto authority, and, here, separately regulated post-lake-outlet water under §402. This structure preserves environmental protections without collapsing the statutory division of programs. 4) Rejection of contrary arguments: The Ninth Circuit's approach would have allowed §306 to override §404 whenever a discharge could be characterized as process wastewater, undermining the 2002 fill rule and the agencies' deliberate allocation of authority. The Court also rejected arguments that §404(b)(1) Guidelines or other provisions implicitly import §306 NSPS into §404 permitting; the applicable guidelines do not incorporate NSPS for fill discharges, and EPA did not exercise its veto authority. 5) Deference and administrative coherence: Given the interlocking statutory provisions and joint regulations, the Court deferred to EPA's interpretation of the CWA and its own rules allocating program responsibility. The agencies' reading was long-standing, coherent, and reasonably reconciled the statutory scheme.
The decision clarifies the boundary between the CWA's §404 and §402 programs and confirms that when a discharge qualifies as fill material, it is exclusively governed by §404 and the Corps' permitting process, not EPA's §402 NPDES regime or NSPS. For practitioners and students, the case is a prime example of Chevron/Auer deference, interagency coordination, and the regulatory consequences of definitional rules. It also highlights a policy tension: classifying certain mine waste as "fill" can avoid stringent technology-based limits, spurring debate about potential regulatory "loopholes" and prompting attention to EPA's oversight tools (e.g., §404(b)(1) Guidelines and §404(c) veto) and future rulemaking.
Coeur Alaska underscores the practical and doctrinal significance of statutory and regulatory definitions in environmental law. By endorsing the agencies' allocation of Clean Water Act authority, the Court confirmed that classification as "fill material" is determinative of both the permitting pathway and the substantive standards that apply.