## Facts

- Dioxin is a potentially carcinogenic compound that bioaccumulates in fish tissue, raising human-health and ecological concerns.
- Under the Clean Water Act (CWA), states adopt water-quality standards to protect designated uses; EPA reviews and approves or disapproves those standards.
- Maryland and Virginia revised their dioxin criteria to an ambient concentration of 1.2 parts per quadrillion (ppq), substantially less stringent than EPA’s national guidance value of 0.0013 ppq.
- The states supported 1.2 ppq by relying on a less conservative cancer-potency factor derived from FDA materials and other scientific evidence disputing EPA guidance assumptions.
- EPA approved the state criteria as scientifically defensible, protective of human health, and compliant with the CWA.
- Environmental groups and individuals challenged the approval, alleging that the criteria (and EPA’s approval) underestimated exposure and risk by:
  - Using insufficient fish-consumption rates for heavy fish-consuming subpopulations,
  - Using an outdated bioconcentration factor (BCF), and
  - Failing to protect aquatic life and wildlife adequately.
- The district court upheld EPA’s approval as not arbitrary or capricious, and the challengers appealed.

## Issues

1. Whether EPA may approve state water-quality criteria that materially differ from EPA’s national guidance when EPA explains why the criteria remain protective under the CWA.
2. Whether EPA’s approval was arbitrary or capricious due to alleged underestimation of fish-consumption rates and use of an outdated BCF in human-health risk assessment.
3. Whether EPA acted arbitrarily or capriciously by approving criteria alleged to be insufficient to protect aquatic life and wildlife.

## Decision

- The Fourth Circuit affirmed the district court and upheld EPA’s approval of Maryland and Virginia’s 1.2 ppq dioxin criteria.
- The court held that EPA’s national criteria guidance is advisory and does not bind states or EPA to a fixed numeric outcome.
- The court found EPA reasonably evaluated competing scientific evidence (including the FDA-derived cancer-potency factor) and explained its conclusions.
- The court held EPA reasonably addressed uncertainties and methodological choices regarding fish consumption and BCF and permissibly treated them as matters of technical judgment within a reasonable range.
- The court held EPA reasonably concluded the criteria protected designated aquatic uses, and the record did not show EPA ignored relevant ecological evidence.

## Legal Principles

- Under APA arbitrary-and-capricious review, courts do not replace an agency’s technical judgment; they assess whether the agency considered relevant factors and provided a reasoned explanation connecting the record to its decision.
- The CWA’s cooperative federalism framework allows states to propose water-quality standards; EPA may approve standards that differ from EPA guidance if EPA reasonably finds them protective of designated uses and consistent with statutory requirements.
- EPA’s national water-quality criteria guidance does not establish mandatory floors or ceilings; it informs, but does not dictate, EPA’s approval decisions.
- When scientific evidence conflicts, an agency may choose among reasonable interpretations if it explains its choice and the choice is supported by the administrative record.

## Conclusion

The Fourth Circuit sustained EPA’s approval of Maryland and Virginia’s dioxin criteria, holding that EPA permissibly exercised technical discretion under the CWA and adequately explained its scientific judgments so that the approval was not arbitrary or capricious under the APA.


Nat. Res. Def. Council, Inc. v. U.S. Env't Prot. Agency, 16 F.3d 1395 (4th Cir. 1993)