About 5,000 gallons of tritium-laced wastewater flowed into a storm drain pathway toward the Pacific Ocean at the shuttered San Onofre Nuclear Generating Station on Sept. 23, 2026, after a routine wastewater transfer ran well past its planned stop time. Southern California Edison detected tritium in the water at roughly 3,000 picocuries per liter, and the Nuclear Regulatory Commission logged the release the next day as a non-emergency event.

San Onofre stopped generating electricity more than a decade ago. What happened this week is a decommissioning-site accident, not a reactor incident, and it went to federal regulators through the same event-reporting system that tracks routine problems at retired plants across the country. Tritium, a radioactive form of hydrogen, is a routine byproduct of nuclear plant operations and shows up in low concentrations in groundwater and cooling-system water long after a reactor stops running.

A wastewater transfer that ran past its stop time

Southern California Edison told the NRC that a planned transfer of mostly groundwater and concrete-cutting water continued between roughly 8 p.m. and 10 p.m. Pacific time on Sept. 23 after it should have stopped, sending the excess into the plant’s Unit 2 storm drain and intake conduit rather than completing the discharge path the company had planned to use. The utility notified the NRC’s event notification system the following afternoon, citing an earlier notice it had already made to the San Diego Regional Water Quality Control Board.

That sequencing matters under federal rules: NRC licensees report certain events to the agency once a separate regulator has already been told, rather than as the first call. Southern California Edison’s report reached the NRC at 2:57 p.m. Pacific time on Sept. 24, roughly a day after the discharge itself, and identified the pathway as leading from the Unit 2 conduit toward the ocean near San Onofre State Beach, on the San Diego County line in San Clemente.

Tritium at 3,000 picocuries, and a pH that broke the site’s own limit

The 3,000 picocuries-per-liter tritium reading is far below the EPA’s 20,000 picocurie-per-liter limit for drinking water, but the discharge broke a separate threshold: its pH measured 10.6, above the permit limit of 9 set for water leaving the site. Total suspended solids came in at 16 milligrams per liter, and results for oil and grease were still pending when the report was filed.

Wastewater leaving a nuclear site into public waters is governed by discharge permits the San Diego Regional Water Quality Control Board issues under the same National Pollutant Discharge Elimination System framework that covers ordinary industrial dischargers, not by NRC radiological limits alone. A pH violation on its own is a permit matter; combined with an off-path release of tritiated water, it drew reports to both agencies at once.

Thirteen years into a decommissioning that won’t finish until 2051

San Onofre’s two working reactors, Units 2 and 3, went online in the 1980s and supplied power alongside an older Unit 1 that dates the site’s operating history back to 1968. Southern California Edison permanently retired the plant on June 7, 2013, after replacement steam generators installed a year earlier developed unusual wear that the utility ultimately judged too costly to fix.

The site has been in an active dismantlement phase, known as DECON, ever since, according to the NRC’s decommissioning tracker for the site. Spent fuel finished moving into an underground dry-storage system on Aug. 6, 2020, and the NRC’s timeline calls for the site to reach unrestricted use by 2028, with full decommissioning — the point at which SCE’s oversight ends entirely — projected for 2051.

Southern California Edison says the release posed no threat

“Based on these preliminary analytical results and currently available information, Southern California Edison does not believe the release poses a threat to ocean water quality,” the utility told the NRC in its report on the Sept. 23 discharge. The agency’s own classification — non-emergency — reflects the same read: no evacuation, no public health order, no operating reactor involved.

It is not the only time SCE has had to explain an unplanned wastewater release at San Onofre. A 2020 discharge from the site’s sewage treatment plant, which sent partially treated wastewater more than a mile offshore after dilution problems, drew a sharper company response at the time: SCE called that earlier incident “unacceptable” and said it was reviewing whether its own procedures had failed to prevent it. Six years later, with the plant a third of the way through a 38-year decommissioning that has already outlasted its final decade of power generation, wastewater controls at the site remain the recurring weak point regulators keep coming back to.

Neither agency has said whether the Sept. 23 release will trigger a fine or a corrective-action order. The NRC’s report leaves that determination for later review, and Southern California Edison’s decommissioning timeline continues on the same schedule regardless — spent fuel stays in dry storage on site for decades longer than the reactors themselves ever ran.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.