Styrofoam cups and other trash washes up on Santa Monica's beaches during rain storms, creating a health hazard for simmers and surfers. (photo by Heal The Bay)

SM BEACH — On the heels of Los Angeles County’s first significant rainfall since March, Santa Monica-based Heal the Bay is urging people to not go into the water at local beaches for at least 72 hours.

The environmental watchdog issued a warning Wednesday as coastal areas and valleys were expected to be soaked by anywhere from a half-inch to an inch of rainfall, according to the National Weather Service.

The main source of pollution to Santa Monica and San Pedro bays is urban runoff carried through the 5,000 mile-long storm drain system. Unlike sewage, this runoff typically receives no treatment and flows freely onto shorelines and the sea through the network of open channels, catch basins and drain pipes and streams, officials with Heal the Bay said.

After the “first flush,” debris and toxins that have been accumulating for months on sidewalks, roadways and riverbeds are now being washed into the storm drains. More than 70 major outfalls funnel manmade debris, animal waste, toxic chemicals, automotive fluids and aerial outfall into the marine ecosystem. It is estimated that more than 80 percent of marine debris comes from inland sources, scientists with the environmental watchdog said.

“This pollution poses human health risks, harms marine life and dampens the tourist economy by fouling shorelines,” Heal the Bay said in a press release.

The National Weather Service warned drivers to be alert during the storm as roads can become slippery from accumulated oil on the asphalt.

Exposure to runoff can cause a variety of illness, most frequently respiratory infection and stomach flu. Human pathogens of unknown origins are also frequently carried in storm-drain gutters.

Experts agree after a major rainfall that local beachgoers should stay out of the water entirely for at least 72 hours. During dry months, Heal the Bay and county health officials urge swimmers to stay 100 yards from flowing storm drains, which have been shown to have elevated levels of pathogens and other pollutants.

Swimmers are also advised to avoid contact with storm water that pools or streams along the beach. Near flowing storm drain outlets, bacteria indicator counts are approximately 10 times higher at ankle depth — where small children typically play — than at chest depth.

“Our region’s water bodies are likely to see significantly higher pollution levels after this rain event,” said Kirsten James, Heal the Bay’s director of water quality. “With so-many year-round ocean users now, it’s critical that they stay out of the water for a while.”

Ocean users can get the latest water quality grades for more than 500 beaches statewide throughout the year by visiting

During the rainy season, Heal the Bay reminds residents that they can take steps in their own home to take pressure off an already taxed storm drain system. Among them: keep trash out of gutters and storm drains, dispose of animal waste and automotive fluids properly, and avoid hosing off driveways and sidewalks.

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