SCE Submits Wastewater Release Response to Water Quality Control Board

Southern California Edison has been posting updates to this page following the March 25 wastewater release from the San Onofre nuclear plant’s sewage treatment plant (STP). Another update is available here.

What happened?

On March 25, up to 7,000 gallons of partially treated sewage was diluted and inadvertently released to the ocean a little more than a mile offshore. The minimum dilution factor was greater than 224-to-1, which is important for minimizing any impacts of the release to the local environment. The release was further diluted once it reached the ocean. This was a non-radiological release.

Latest update

The San Diego Regional Water Quality Control Board issued a notice of violation in April for the release and asked us to respond by June 15. Today we submitted that report to the board for review and potential follow-up questions, which we will answer in a timely manner.

Key findings

After a thorough review by SCE and third-party experts, we determined the most probable source of the influx was the site’s potable (suitable for drinking) water system. A specific cause for the large influx of water into the STP that initially triggered the event has not been conclusively identified.

Because of the dilution levels, the San Diego County Department of Environmental Health believed at the time there would be little environmental impact from this release, and indeed there were no beach closures required at the time. The report submitted to the board today confirms that ecological impacts were low, there was no evidence of human health impacts, and no evidence of physical impacts from this release.

Our response

SCE has taken multiple actions since the release to restore the STP to compliance and prevent reoccurrence.

Following discovery of the release, we immediately took the STP pumps out of service, which prevented further release to the outfall; closed the site restrooms and staged portable toilets and sinks for employee use. We also turned off the transfer pumps at the area known as the Mesa, located across Interstate 5 from the plant, as a precaution to limit inflows, and made the proper notifications to state and federal agencies.

Before bringing the sewage treatment plant back online we:

  • Improved the timing of alarm notifications by revising procedures
  • Modified the contract with the state-certified sewage treatment plant operator to require emergency response within a two-hour window
  • Installed a submersible pump with auto start capability (float switch) to automatically divert excess influx to the STP to reserve storage capacity of approximately 40,000 gallons
  • Prior to putting the plant back in service, treated plant wastewater, or effluent, was sampled for two consecutive days while the plant was in a recirculation mode to ensure the effluent would meet our National Pollution Discharge Elimination System (NPDES) water quality permit. As soon as the plant was put in service, additional samples verified the plant was running in 100% compliance.

Going forward, we have outlined in our response to the board additional actions we will take to improve the operation of the STP and prevent reoccurrence, including instrumentation to automate the communication of STP alarm conditions to the state-certified STP operator.

SCE values being a good steward of the environment and we will continue our work to minimize impacts as we move through the decommissioning of San Onofre.

The full response to the board is available here.

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