Ocean Standards - CWA §316(b) Regulation

Cooling Water Intake Structures
Once-Through Cooling

On May 4, 2010, the State Water Resources Control Board (State Water Board) adopted a Policy on the Use of Coastal and Estuarine Waters for Power Plant Cooling (Once-Through Cooling or OTC Policy). The OTC Policy became effective on October 1, 2010.

The OTC Policy establishes technology-based standards to implement federal Clean Water Act section 316(b) and reduce the harmful effects associated with cooling water intake structures for power generating facilities on marine and estuarine life. The OTC Policy applied to 19 existing power plants (including two nuclear plants) with the combined ability to withdraw almost 16 billion gallons of water per day from the state’s coastal and estuarine waters using a single-pass system, also known as once-through cooling (OTC). Closed-cycle wet cooling has been selected as Best Technology Available (BTA). Permittees must either reduce intake flow and velocity (Track 1) or reduce impacts to aquatic life comparably by other means (Track 2).

Ten plants have ceased their once-through cooling operations. Eight of the remaining plants have informed the State Water Board that they are planning to comply by retiring their existing OTC plants’ equipment. In some cases, plants will repower their locations with modern non-water-cooled systems. Dynegy’s Moss Landing Power Plant is the only OTC plant that is complying with the OTC Policy through Track 2.

Once-Through Cooling – Current Version – Effective November 30, 2020

 » Go to the Official Policy Documentation


The OTC Policy amendment to revise the compliance schedules for Alamitos, Huntington Beach, Ormond Beach, and Redondo Beach Generating Stations and Diablo Canyon Nuclear Power Plant was adopted by the State Water Board on September 1, 2020, as Resolution No. 2020-0029.  With the approval of the Office of Administrative Law on November 30, 2020, these amendments are now effective.  The approval letter from the Office of Administrative Law, Form 400, and the concise regulatory summary are available on the Official Policy Documentation page.  The newly effective 2020 OTC Policy is available through the link above titled “Once-Through Cooling Policy – CURRENT VERSION” and on the Official Policy Documentation page.

Public Meetings

None at this time.

Other Relevant Documents


The two committees listed below, were convened by the Executive Director of the State Water Board, as required by the OTC Policy. The committees are subject to the Bagley-Keene Open Meeting Act.

Statewide Advisory Committee on Cooling Water Intake Structures (SACCWIS)
SACCWIS was established to advise the State Water Board on the implementation of the OTC Policy to ensure that implementation plans and schedules established by the OTC Policy are realistic and will not cause disruption to the State’s electrical power supply. SACCWIS includes representatives from the California Energy Commission, California Public Utilities Commission, California Coastal Commission, California State Lands Commission, California Air Resources Board, California Independent System Operator, and the State Water Board.

Review Committee for Nuclear Fueled Power Plants
This Review Committee was established to oversee special studies, which investigated ability, alternatives, and cost, for the two nuclear-fueled power plants to meet OTC Policy requirements. The special studies were conducted by an independent third party with engineering experience with nuclear power plants, selected by the Executive Director of the State Water Board, and paid for by the two nuclear-fueled power plants subject to the OTC Policy.  The Review Committee included representatives from the SACCWIS agencies, the nuclear power companies, the environmental community, and staffs of the State Water Board and appropriate Regional Water Boards.

Permitting of Affected Power Plants

The OTC Policy is implemented through National Pollutant Discharge Elimination System (NPDES) permits. The applicable Regional Water Boards are responsible for implementing the OTC Policy requirements in NPDES permits for existing coastal and estuarine power plants. The Water Boards determine requirements to implement best technology available for cooling water intake structures to be included in the permits. State Water Board staff provides technical support to the Regional Water Boards to ensure a high level of statewide consistency in implementing the OTC Policy.

»»   For more information on each power plant required to comply with the OTC Policy, visit the Power Plants That Are Affected page.

Questions or Comments

  Subscribe online to receive email updates on Ocean Issues – Once-Through Cooling (see the Water Quality topics section)