## Facts

- Union Oil operated facilities subject to a Clean Water Act NPDES permit and submitted certified discharge monitoring reports (DMRs) under the self-monitoring system.
- Sierra Club filed a Clean Water Act citizen suit alleging 76 NPDES permit exceedances between April 1979 and December 1983, relying largely on Union Oil’s own DMRs.
- Union Oil argued many reported exceedances were excused by extraordinary rainfall (asserting an “upset” defense) or were not real exceedances due to sampling error.
- After a bench trial, the district court found no violations, accepting upset, sampling-error explanations, and a judicial “de minimis” concept for small exceedances.
- The district court also denied Sierra Club leave to amend its complaint to add additional alleged permit violations.
- Sierra Club appealed the no-liability judgment and the denial of leave to amend.

## Issues

1. Whether a permittee may invoke an “upset” defense to excuse NPDES limit exceedances when the permit lacks an upset clause and the permittee did not pursue administrative challenges or modification.
2. Whether a permittee may defeat liability by recharacterizing DMR-reported exceedances as sampling error.
3. Whether courts may treat minor exceedances as non-violations under a de minimis exception to NPDES permit limits.
4. Whether denying leave to amend to add additional alleged permit violations was an abuse of discretion in a Clean Water Act citizen suit.

## Decision

- The Ninth Circuit reversed the district court’s no-liability determination for 74 of the 76 alleged exceedances and held Union Oil liable for those violations.
- The court rejected the district court’s use of an implied upset defense to excuse exceedances where the permit did not contain such a provision and the permittee had not pursued the administrative process to obtain it.
- The court rejected broad acceptance of sampling-error claims to negate violations reflected in the permittee’s certified DMRs.
- The court rejected a de minimis exception to Clean Water Act permit limitations.
- The court held the district court abused its discretion by denying leave to amend, except that the district court could deny amendment as to alleged violations Sierra Club knew or should have known about when it filed the original complaint.
- The case was remanded for penalties on the 74 violations, further findings on two remaining alleged violations, and reconsideration of amendment consistent with the appellate ruling.

## Legal Principles

- NPDES permits impose binding effluent limitations; exceedances shown on a permittee’s certified DMRs ordinarily establish violations for enforcement purposes.
- A permittee generally cannot use an enforcement action to retroactively alter permit obligations or add defenses that should have been raised through permit issuance, challenge, or modification procedures; failure to pursue administrative remedies bars such attempts.
- Post hoc assertions of sampling error cannot be used broadly to negate DMR-reported exceedances, because the Clean Water Act’s enforcement structure depends on reliable self-monitoring and reporting.
- The Clean Water Act does not authorize a judicially created de minimis exception to NPDES permit limits; any exceedance is a cognizable violation.
- Leave to amend in Clean Water Act citizen suits should be granted absent adequate reason to deny it; denial may be appropriate for allegations the plaintiff knew or should have known at the time of the original filing.

## Conclusion

The Ninth Circuit treated DMR-reported NPDES exceedances as enforceable Clean Water Act violations, rejected implied upset and de minimis excuses and expansive sampling-error defenses, and required a more permissive approach to amendments adding related violations, remanding for penalties and further proceedings.


Sierra Club v. Union Oil Co. of Cal., 813 F.2d 1480 (9th Cir. 1987)