Roughly two-thirds of the 70 million gallons of sewage spilled in Los Angeles County since 2007 ended up in a storm drain or a river connected to the Pacific Ocean, according to analysis of 15 years of data maintained by the county Public Health Department.

Of that total, the vast majority of the sewage that entered a waterway was inadvertently released in a single year: 2021.

A nearly catastrophic disaster at the Hyperion Water Reclamation Plant and the sudden collapse of a sewer system in Carson last year combined to make it the worst since the beginning of the data set in April 2007. The two spills, roughly six months and 15 miles apart, led to the total release of 25 million gallons of raw sewage either directly into the ocean or into waterways that empty into it.

In the 15-year period reviewed, there is no other year where more than 10 million gallons of untreated sewage made it to the sea, according to the data from the Public Health Department, which monitors ocean quality and tracks sewage spills reported by cities and other county agencies.

Spokespersons for L.A. city and county sanitation agencies — which combined own roughly half of the 17,000 miles of sewers in the county — say they are far below the state average for spills per 100 miles of sewers.

The largest of spills by volume have nearly all occurred in recent years and fewer of those spills have been prevented from reaching the ocean. Approximately 92 percent of the 42 million gallons of sewage that reached a waterway connected to the Pacific in the 15-year period were spilled after 2015, the data shows.

A comprehensive countywide assessment of Los Angeles County’s sewer infrastructure, approved by the Board of Supervisors in January, is underway in response to the Carson spill.

“The sewer pipe that collapsed in Carson and spilled 8 million gallons of sewage into our ocean in late December was nearly 60 years old,” Supervisor Janice Hahn said at the time. 

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