## Facts

- The EPA issued Fayetteville, Arkansas an NPDES permit for a new sewage treatment plant authorizing discharge of treated effluent into a tributary that flows into the Illinois River and then into Oklahoma.
- The permitted discharge (up to about 6.1 million gallons per day) would enter the Illinois River roughly 22 miles upstream from the Arkansas–Oklahoma border.
- Oklahoma had adopted stringent “no degradation” water-quality standards for the upper Illinois River, including a segment designated as a scenic river near the state line.
- Oklahoma and other Oklahoma parties objected within EPA proceedings, contending the Arkansas discharge would violate Oklahoma’s downstream water-quality standards.
- EPA’s Chief Judicial Officer required permit limits sufficient to meet applicable state standards, but concluded Oklahoma’s standards would be violated only upon proof, by a preponderance of the evidence, of an “actual detectable” violation.
- After remand, an EPA administrative law judge made detailed factual findings and concluded the discharge would not cause an actual detectable violation; EPA upheld the permit.
- The Tenth Circuit reversed, holding the Clean Water Act barred issuance where the new discharge would “contribute” to conditions already violating standards, even if the incremental effect was not detectably measurable.
- The Supreme Court granted review and reversed the Tenth Circuit.

## Issues

1. Whether the Clean Water Act authorizes EPA to issue an NPDES permit for an upstream discharge that flows into a downstream state and to require compliance with the downstream state’s water-quality standards.
2. Whether the Clean Water Act categorically prohibits issuing a permit when the discharge would contribute to existing water-quality problems even if the contribution is not measurably detectable.
3. Whether the court of appeals improperly displaced EPA’s reasonable statutory and regulatory interpretations and set aside agency factual findings supported by substantial evidence.

## Decision

- The Supreme Court unanimously reversed the Tenth Circuit and upheld EPA’s issuance of the NPDES permit.
- The Court held EPA may include effluent limitations necessary to ensure compliance with applicable water-quality standards, including standards of a downstream state affected by the discharge.
- The Court accepted EPA’s interpretation that the relevant downstream standard is violated only upon an actual detectable or measurable violation, not by any non-detectable incremental contribution.
- The Court concluded the court of appeals failed to give proper deference to EPA’s reasonable interpretations and improperly substituted its own assessment for agency factfinding supported by substantial evidence.
- The case was remanded for further proceedings consistent with the Court’s decision.

## Legal Principles

- The Clean Water Act’s NPDES permitting framework allows EPA to impose effluent limitations necessary to comply with applicable state water-quality standards implicated by interstate waters.
- A downstream state cannot unilaterally block an upstream permit; objections are channeled through federal statutory procedures administered by EPA.
- EPA may reasonably interpret how a state’s anti-degradation or water-quality standards operate in the federal permitting context, including by requiring a showing of an actual detectable or measurable violation.
- The Clean Water Act does not impose a categorical ban on new discharges into already-impaired waters; permitting may proceed where EPA reasonably determines applicable standards will not be violated.
- Reviewing courts must defer to reasonable agency interpretations of the statute and must uphold agency factual findings supported by substantial evidence.

## Conclusion

The Court upheld EPA’s authority to issue an upstream NPDES permit affecting a downstream state and to implement downstream water-quality standards through permit limits, while rejecting a rule that would forbid any new discharge that merely contributes in an undetectable way to existing degradation.


Arkansas v. Oklahoma, 503 U.S. 91 (1992)