The most-polluted beaches have higher levels of bacteria in the water, according to the group. The Marina Lagoon in Lakeshore Park in San Mateo County was the state's second most-polluted beach because of poor water circulation and high bacteria counts. Source
Half Moon Bay's Surfer's Beach also with high bacteria when it rains. "Rain flushes contaminant and bacteria sources from our streets including fertilizer, pet waste and automotive fluids into our waterways," Source
After three years of claiming the number one spot, Cowell Beach dropped to number 3 in 2017. The reason: poor circulation and runoff from polluted rivers or storm drains. Heal the Bay says getting in the water at a polluted beach can lead to a respiratory or gastrointestinal illness. Source
Alameda beach has suffered from a lot of polution from the result for a lot of visitors leaving lots of trash. Having many reports of trash from on-goers.