Expert Support for Development of NPDES Permit for Diablo Canyon Power Plant

The Pacific Gas and Electric Company (PG&E) owns and operates the Diablo Canyon Nuclear Power Plant, which produces over 8 percent of California’s electrical supply. The facility discharges up to 2,760 million gallons per day of once-through cooling water and other commingled waste streams to the Pacific Ocean. The Central Coast Regional Water Quality Control Board (Water Board) last issued a National Pollutant Discharge Elimination System (NPDES) permit for the facility in 1990 and administratively extended it in 1995. In the years since, the Water Board delayed permit renewal for several reasons, including PG&E’s plans to decommission the facility. However, in 2022, California’s legislature passed a bill extending facility operation until 2030, necessitating renewal of the NPDES permit. The Water Board needed expert assistance to develop the permit in a timely manner due to the complex technical work required to evaluate the effects of the cooling water intake structure and thermal discharges on the ocean environment.

ERG’s NPDES permitting experts supported the Water Board with development of the NPDES permit through a unique “third party” contract with the California Marine Sanctuary Foundation. With ERG’s support, the Water Board was able to issue the permit in February 2026, approximately 10 months following submission of the permit application. ERG’s support included:

• Evaluating compliance with effluent limitations and monitoring requirements from 1990 to 2024.
• Determining consistency with the state’s Once-Through Cooling Policy and Clean Water Act Section 316(b) requirements for cooling water intake structures.
• Reviewing PG&E’s mixing zone study to determine the appropriate dilution.
• Developing the permit and fact sheet.
• Developing responses to public comments on the draft permit.
• Coding permit requirements into EPA’s Integrated Compliance Information System (ICIS) database.