Thornton State Beach

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San Mateo County conducts weekly sampling at over 43 ocean and fresh water sites throughout the year. Samples are collected on Mondays and analyzed by the San Mateo County Public Health Laboratory for the levels of three indicator bacteria. Results are posted to their website within 48 hours. Swim Guide checks for the latest results daily. The standards for indicator bacteria are as follows: Single sample maximum Total Coliform > 10,000 MPN (Most Probable Number) / 100 ml of water Enterococcus > 104 MPN / 100 ml E.coli > 400 MPN / 100 ml Geometric Mean (5 samples collected over a 30 day period) Total Coliform > 1,000 MPN / 100 ml of water Enterococcus > 35 MPN / 100 ml E.coli > 200 MPN / 100 ml A beach is marked Green when bacteria levels for a single day and geometric mean are below the standards mentioned. A beach is marked Red when bacteria levels for a single day or the geometric mean are above the standards mentioned. A beach is marked Grey when there is no current or reliable monitoring information available. Please contact Greg Smith of County Environmental Health by email GJSmith@smcgov.org or phone (650) 372-6200 if you are interested in becoming a volunteer to help collect water samples at your favorite beach. If you think you’ve contracted an water related illness please send us a pollution report, see your doctor and contact County Environmental Health Services at (650) 372-6200 to make a report. They will need your name, the dates and areas you were swimming, symptoms and contact information for your or your health care provider.