## Facts

- EPA listed the Matthiessen & Hegeler Zinc Company hazardous-waste site (the M&H Site) on the National Priorities List (NPL) under CERCLA using the Hazard Ranking System (HRS).
- Carus Chemical Company owned part of the M&H Site and operated a plant on land formerly within the larger site near La Salle, Illinois.
- The site contained two large slag piles adjacent to and partly within the Little Vermilion River; contaminants included cadmium and other metals.
- In scoring the site, EPA used cadmium’s highest available toxicity value (inhalation), even though surface-water migration was asserted to be the more likely exposure route but carried a lower toxicity value.
- Carus submitted more recent sampling data (generated by M&H) and argued it conflicted with earlier Illinois EPA sampling data used by EPA.
- Carus petitioned for review, alleging the NPL listing was arbitrary and capricious under the Administrative Procedure Act.

## Issues

1. Whether EPA acted arbitrarily or capriciously by interpreting the HRS to require use of a contaminant’s highest toxicity value regardless of the most likely exposure pathway.
2. Whether EPA acted arbitrarily or capriciously by relying on older Illinois EPA sampling data rather than newer data submitted by Carus/M&H.

## Decision

- The D.C. Circuit denied the petition for review and upheld the NPL listing.
- The court held EPA’s interpretation and application of the disputed HRS regulation were reasonable.
- The court held EPA considered the newer data submitted by Carus and reasonably concluded it did not undermine the listing decision.

## Legal Principles

- Courts give substantial deference to an agency’s interpretation of its own technical regulations, particularly in scientifically technical programs such as HRS scoring.
- HRS scoring for NPL listing functions as a screening and prioritization tool rather than a site-specific risk assessment; conservative assumptions are permissible if consistent with the regulation.
- Under arbitrary-and-capricious review, an agency may choose among competing technical data sources if it reasonably explains its evaluation and conclusion.
- NPL listing triggers further study and prioritization and does not itself constitute an order to perform a remedial action, supporting a restrained scope of judicial review of technical judgments at the listing stage.

## Conclusion

The court upheld EPA’s decision to place the M&H Site on the NPL, concluding that EPA reasonably interpreted the HRS to use cadmium’s highest toxicity value and reasonably determined that newer sampling submissions did not call the listing score into question under the APA’s deferential standard.


Carus Chem. Co. v. EPA, 395 F.3d 434 (D.C. Cir. 2005)