Source Information
Every week from mid-May through mid-September, we test the water at many popular public swimming beaches on Lake Washington, Lake Sammamish, Green Lake, and other lakes. We test for bacteria, which tells us whether there is poop in the water from people, pets, or wildlife. And at many beaches, we also test for toxic algae. Poop or toxic algae can make people sick from swimming or playing in the water.
King County measures bacteria concentrations at the swimming beaches each week. We collect three water samples from different parts of the beach. Bacteria are measured as colony forming units (CFU), which is a count of the number of bacteria, per 100 mL of water (about a half-cup of water).
If the bacteria concentrations are high, Public Health – Seattle & King County will review the monitoring data and other information about the beach, and may recommend that beach managers close the beach to public use.
If 2 or 3 samples today have bacteria concentrations above 320 bacteria/100 mL, Public Health will usually recommend closing the beach (marked as Red).
If only 1 sample today is above 320 bacteria/100 mL, Public Health will also evaluate all bacteria results from the last 30 days: If there is at least 1 other sample above 320 bacteria/100 mL in the last 30 days, Public Health will usually recommend closing the beach (marked as Red). If the average of all samples (geometric mean) in the past 30 days is above 100 bacteria/100 mL, Public Health will usually recommend closing the beach (marked as Red).
Otherwise, the beach will usually remain open.
If no samples today are above 320 bacteria/100 mL, Public Health will usually recommend that the beach remains open, or that the beach may reopen if it is currently closed (marked as Green).
A beach may also be marked Red if there is a Harmful Algal Bloom (HAB) reported at the beach. A beach is marked Grey when current water quality data is not available.
For more information go to King County Swimming Beach Bacteria Monitoring website: kingcounty.gov/swimbeach.
