Beaches including Dockweiler State Beach and El Segundo Beach were closed as officials tested the water for elevated bacteria

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Hyperion Water Reclamation Plant plant
Hyperion Water Reclamation Plant
| Credit: DAVID MCNEW/EPA-EFE/Shutterstock

A cluster of California beaches was closed on Monday after nearly 20 million gallons of unfiltered sewage from a local plant spilled into the ocean.

A "mechanical failure" at Hyperion Water Reclamation Plant led to the "massive discharge" of 17 million gallons of untreated sewage on Sunday through pipes extending one mile and five miles offshore, according to Los Angeles County Public Supervisor Janice Hahn and the Los Angeles County Department of Public Health.

"I understand that the plant was able to prevent an even larger spill, but we are going to need answers about how and why this happened," Hahn wrote on Twitter.

As a precautionary measure, four nearby beaches were closed to the public: Dockweiler State Beach at Water Way Extension and at Hyperion Plant, El Segundo Beach and Grand Ave. Storm Drain, the Los Angeles County Department of Public Health said in a press release.

The release said that residents were advised to avoid contact with ocean water, and that the beaches would remain closed until water samples were confirmed negative for elevated bacteria, a process that should take about 24 hours.

Dockweiler State Beach
Dockweiler State Beach
| Credit: Mel Melcon / Los Angeles Times via Getty Images

Hyperion Executive Plant Manager Timeyin Dafeta said in a statement that the plant "became inundated with overwhelming quantities of debris, causing backup of the headworks facilities," according to KCBS.

Dafeta's statement said that the plant's relief system was subsequently triggered and "sewage flows were controlled through use of the plant's one-mile outfall and discharge of untreated sewage into Santa Monica Bay."

He said that the 17 million gallons that spilled were just 6 percent of the plant's daily load, and that its discharge was an emergency measure that helped prevent even more spillage.

"Thanks to these efforts, and significant equipment improvements that have been made in recent years in partnership with several environmental groups, the amount of flow into the ocean was mitigated and the plant remained online," Dafeta's statement said.

He reportedly told the Los Angeles Times that as of Monday, the mechanical issue had been resolved.

Heal the Bay, a local environmental group, told the outlet in a statement that it seems as though debris such as tampons and plastic trash was successfully filtered out of the spill before it made it to the Bay.

The plant has been in operation since 1894, and the new Hyperion Reclamation Plant opened in 1950, according to the city's sanitation department. It discharges "digested sludge" into Santa Monica Bay through a seven-mile ocean outfall.

On a dry weather day, the plant receives an average of 275 million gallons of wastewater — a number that can double on rainy days.