As Mother Nature continues to turn up the heat this summer, those looking to cool off along the California coast might want to reconsider where they're swimming.

Ten California and Baja California beaches, including around the Santa Monica Pier and a stretch of Mother's Beach in Marina del Rey, received the poorest water quality grades based on measurements taken last year during dry periods between April and October. Four locations in San Diego County, two in Baja and two in San Mateo County also made the list, which was part of Heal the Bay's 34th annual report card of the state's beaches released Wednesday.

Of California's more than 500 beaches tested last year, 89% received top marks for water quality for the warmer seasons between April and October, when people tend to spend the most time in the ocean. However, water sampling conducted during dry periods from November 2023 through March 2024 produced results well below average, with only 66% of beaches receiving A and B grades, according to the report.

The poor grades during dry winter periods are the result of heavy bouts of rain flushing pollutants through storm drains, which can trigger sewage spills. The report notes that such spills resulted in about 38 million gallons of sewage flowing into the ocean and coastal waterways.

"One of the things that contributes to the poor water quality that we see in a lot of Southern California is how urbanized our communities are," said Heal the Bay Chief Executive and President Tracy Quinn.

"Nearly every square inch is covered with buildings or concrete or asphalt, and that means we have more stormwater collecting more pollution and heading for coastal waterways."

Heal the Bay's report included a beach bummer list of the 10 beaches that consistently receive poor marks throughout the year.

Here are the 10 dirtiest beaches

1. Tijuana River Mouth (San Diego County)
2. Playa Blanca (Baja California, Mexico)
3. Santa Monica Pier (Los Angeles County)
4. Tijuana Slough (San Diego County)
5. Linda Mar Beach (San Mateo County)
6. Lakeshore Park (San Mateo County)
7. Imperial Beach at Seacoast Drive (San Diego County)
8. Border Field State Park (San Diego County)
9. Marina del Rey Mother's Beach at Lifeguard Tower (Los Angeles County)
10. El Faro (Baja California, Mexico)

Despite the heavy rains last winter, 12 beaches— nine in Orange County, two in San Diego County and one in Ventura County—met the highest clean-water standards for the entire year. That is an improvement from last year's report, when only two beaches made the organization's "honor roll." However, it's still far below the usual 30 to 50 beaches typically included on the list.

"While we had some really big storms this year it wasn't quite the same as the record-breaking year for rain that we saw last year, which I think definitely contributed to the very low level of only two honorable beaches," said Annelisa Moe, associate director of science and policy at Heal the Bay.

Some areas in California, including parts of San Diego County, shifted to a new water sampling protocol that doesn't align with how Heal the Bay calculates water grades, which is why some beaches may not have made this year's honor roll. Other California beaches had poor grades during dry periods in the winter, a lingering effect of strong winter storms.

Some Northern California beaches aren't tested during winter months when people are less likely to be re-creating in the ocean because of the weather. Beaches need to be tested weekly, year-round to qualify for the honor roll, Moe said.

Here are California's 'honor roll' beaches

1. Dana Point Harbor Fuel Dock (Orange County)
2. Huntington Harbor, Seagate Lagoon (Orange County)
3. Huntington Harbor, Trinidad Lane Beach (Orange County)
4. Newport Bay, Promontory Point (Orange County)
5. Dana Point, South Capistrano Bay Community Beach (Orange County)
6. Riviera Beach (Orange County)
7. Emerald Bay Beach (Orange County)
8. Marine Science Institute Beach (Orange County)
9. Salt Creek Beach (Orange County)
10. Point Loma Lighthouse (San Diego County)
11. Encinitas, Moonlight Beach, Cottonwood Creek (San Diego County)
12. County Line Beach (Ventura County)

The beach rankings are based on routine water-quality testing conducted by local officials and then analyzed for concentrations of fecal indicator bacteria that signals pollution from numerous sources including human and animal waste.

Officials recommend staying at least 100 yards away from storm drains and river outlets when entering the water and to always wait at least 72 hours after it rains before going into the ocean.

The report noted that water quality near the Santa Monica Pier continues to vex officials despite efforts like installing a stormwater capture system in 2018 and adding bird netting beneath the pier to manage pollution from bird feces.

"Unfortunately, those efforts aren't working particularly well," Moe said, adding that the organization is discussing assembling a task force with the city to make recommendations for the area.

"Just upon visual inspection we can see the bird netting is in disrepair, so there are certain things that can still be done to kind of try to bring down that particular source of pollution," she said.

Beaches with poor water circulation also continue to be an issue. Mother's Beach in Marina del Rey and Lakeshore Park in San Mateo are both enclosed by urbanized channels and harbors, which reduces water flow and allows bacteria to linger.

In other areas, sewage and runoff following heavy rainstorms is primarily to blame for the poor water quality.

Imperial Beach in San Diego County has faced water quality issues for years because of its proximity to the mouth of the Tijuana River where raw sewage flows into the ocean daily. This is the first time the location has made it onto the beach bummer list, a sign that scientists say shows the sewage pollution was significant enough this year to overwhelm natural dispersal mechanisms that typically keep open ocean beaches off the list.

Construction began this year on a new wastewater treatment plant in Mexico that officials say will dramatically reduce the discharge of sewage.

2024 Los Angeles Times. Distributed by Tribune Content Agency, LLC.