Trash deposited on the beach at the Pico-Kenter storm drain after a rainstorm in 2007. Note the small bits and pieces of styrofoam which are often mistaken as food by birds. Ingested plastic can block a bird's intestines leading to death.

Avoid Santa Monica, Los Angeles Beaches For Next 72 Hours: Heal The Bay:

With the first significant rainfall on Wednesday since March, Heal the Bay is urging residents and tourists to avoid Santa Monica and Los Angeles beaches until the weekend.

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.

“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.”

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.

After the so-called 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 man-made 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.

This pollution poses human health risks, harms marine life and dampens the tourist economy by fouling shorelines.

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.