Two of the dirtiest beaches in California are located in Los Angeles County—Santa Monica Pier and Mother’s Beach at Marina del Rey, according to environmental advocacy group Heal the Bay.

It released its annual California Beach Report Card today, and a full half of the “Beach Bummers”—Heal the Bay’s list of the 10 most bacteria-laden beaches—are located right here in Southern California. The two LA beaches came in at No. 6 and No. 9 on the list, respectively.

That’s an improvement over last year, when three LA County beaches made the list. Redondo Pier dropped off the list this year, and, in a stunning turnaround, received all A and B grades in water quality.

Also in the good news category: 93 percent of beaches in LA County received A grades during the hottest months of the year, between April and October. Meanwhile, 95 percent of beaches in Orange County and all of Ventura County’s beaches received A grades during the same period.

The grades are based on the presence of fecal bacteria in the water that can cause a host of gastrointestinal illnesses. Higher grades mean that beachgoers have a lower chance of getting sick after taking a swim, says Heal the Bay.

That means that it’s generally safe to swim at most Southern California beaches during the months when people are most likely to do so. However, during the other half of the year, things change dramatically.

During the abnormally wet winter, Heal the Bay gave almost half of LA County’s beaches F grades, as stormwater runoff drained into the ocean and dirtied up the coastal waters.

To address the runoff problem, Heal the Bay’s policy staff is advocating for better infrastructure to capture and reuse stormwater, rather than allowing it to “flow uselessly to the sea.”

In Santa Monica, construction is set to begin this year on an underground storage tank that will hold up to 1.6 million gallons of captured stormwater, which will then be transferred to a water recycling facility. Heal the Bay expects this will improve water quality at the pier, which is also affected by the presence of birds and by the underside of the pier itself, which offers a friendly climate for bacteria.

Four beaches in LA County made this year’s honor roll list of beaches with A-plus cleanliness ratings: Malibu Point, El Matador State Beach, Bluff Cove, and Portuguese Bend Cove. All 32 beaches that made the honor roll are located in Southern California.