California just proved it’s the golden state for a reason — it swept all ten spots on the list of America’s cleanest beaches.

San Gregorio State Beach in San Mateo County came out on top, boasting near-pristine water quality that tested so pure, it couldn’t even be measured.

The new analysis, from tide-tracking platform Tideschart, reviewed 15 months of Environmental Protection Agency (EPA) data measuring bacterial levels in coastal waters.

Using the EPA’s “Most Probable Number” (MPN) system — which estimates how many organisms exist per 100 milliliters of water — the study found every single beach in the nation’s top ten sits right here in California.

Orange County and San Diego County dominated the list, claiming nine of the top ten cleanest beaches.

Dana Point and Aliso County Beach tied for second place with readings of just 1.00 MPN per 100mL — far below the EPA’s safety threshold of 35. South Laguna Beach followed close behind at 1.75 MPN, proving even smaller coves can maintain spotless standards.

In San Diego County, Marine Street Beach, Tourmaline Surfing Park, and Pacific Beach all tied for fourth at 2.00 MPN. Despite being some of San Diego’s most popular spots, they still turned in impressively low bacteria counts. La Jolla Shores and Torrey Pines also made the list, joined by South Casa Beach, Mission Beach, and Sunset Cliffs Park — along with Monterey Beach Hotel on the central coast.

Altogether, it’s a full California sweep — a win for the state’s coastal beauty, local tourism, and environmental care.

America’s 10 cleanest beaches

1. San Gregorio State Beach (San Mateo County) – -1.04 MPN
   =2. Dana Point (Orange County) – 1.00 MPN
   =2. Aliso County Beach (Orange County) – 1.00 MPN
2. South Laguna Beach (Orange County) – 1.75 MPN
   =4. Marine Street Beach (San Diego County) – 2.00 MPN
   =4. Tourmaline Surfing Park (San Diego County) – 2.00 MPN
   =4. Pacific Beach (San Diego County) – 2.00 MPN
3. La Jolla Shores Beach (San Diego County) – 2.25 MPN
4. Torrey Pines State Beach (San Diego County) – 3.00 MPN
5. South Casa Beach (San Diego County) – 5.00 MPN
6. Mission Beach (San Diego County) – 6.50 MPN
7. Monterey Beach Hotel (Monterey County) – 6.70 MPN
8. Sunset Cliffs Park (San Diego County) – 7.75 MPN

But the study also revealed the other side of the story — California is home to both the cleanest and dirtiest beaches in America.

The Tijuana Slough National Wildlife Refuge near the U.S.-Mexico border ranked as the most polluted, testing at 5,174 MPN per 100mL — nearly 50 times the EPA’s safety limit. Border Field State Park wasn’t far behind, while Islais Creek Channel and Mission Bay Channel also made the top five most contaminated spots.

At those levels, experts warn swimmers face potential exposure to bacteria that can cause everything from stomach bugs to skin infections.

America’s 5 dirtiest beaches

1. Tijuana Slough National Wildlife Refuge (San Diego County) – 5,174 MPN
2. Border Field State Park (San Diego County) – 3,195 MPN
3. Islais Creek Channel (San Francisco County) – 2,973 MPN
4. Imperial Beach (San Diego County) – 2,665 MPN
5. Mission Bay Channel (San Francisco County) – 2,219 MPN

Ryan Blundell, founder of Tideschart, said the results highlight just how unpredictable coastal conditions can be:

“The difference between the cleanest and dirtiest beaches is massive. San Gregorio’s water is so pure it doesn’t even register, while others in the same state are way over the safety limit. A heavy rainstorm can flush bacteria straight into the ocean — it can take days for levels to settle. People forget how much local weather affects the water they’re swimming in.”

Despite the contrast, Blundell added that the results are also encouraging: “Busy spots like Pacific Beach and Mission Beach show that when beaches are managed well, even high-traffic areas can stay remarkably clean.”

Bottom line: California’s coastlines are delivering the best of both worlds — breathtaking views and the cleanest swimming waters in the U.S. Just remember to check local advisories before diving in.