skip to main content

King County Swimming Beach Monitoring


If you have question on this site, please contact Debra Bouchard (206) 477-4650.

About the Swimming Beach Monitoring Program

Image of kids playing at Newport Beach Photo courtesy of Jonathan Frodge

Beginning in 1996, selected swimming beaches on Lake Sammamish, Lake Washington and Green Lake have been monitored weekly from mid-May through Labor Day weekend to determine levels of bacterial pollution. In 2005 algal toxin monitoring was added to this program. Bacteria and algal toxin values are compared to standards to determine if there are health risks to swimmers. If an unacceptable risk is present, Seattle/King County Public Health department will advise swimming beach managers, usually city parks departments, to post signs on a beach warning of the risk to swimmers. These signs will advise people not to swim at the affected beaches. See Beach Closure Protocols for more information about what protocols are used for bacteria and algal toxins.

Bacteria standard:
The bacteria water quality standard used for the King County Swimming Beaches Program is called the Ten State Standard for fecal coliform bacteria. The standard is: a running geometric mean of the five most recent samples of 200 CFU/100ml (colony forming units per 100 mililiter) fecal coliform with no single sample exceeding 1000 CFU/100ml.

Algal Toxin Guidance Values:
Algal toxin results are compared to the Washington State Department of Health recommended recreational guidance values. The value for the liver toxin microcystin is 6 ug/L. The interim guidance value for the nerve toxin, anatoxin-a, is 1.0 ug/L.

What are fecal coliform bacteria?

Image of a fecal coliform

Fecal coliforms are a group of intestinal bacteria that are routinely used as an indicator of sewage pollution in water, and as an indicator of the human health risk. Epidemiological studies involving recreational swimming waters have shown predictive associations between several swimming-associated health effects and various microbial indicators or pathogens. Fecal coliform bacteria are used to detect the possible presence of microbial contamination of water by human waste.

Elevated counts of fecal coliform bacteria (several thousand to several million cfu/100 ml) always occur when sewage is present in the waters. However, high bacteria counts do not necessarily indicate human sewage pollution because many other mammals and birds can also contribute this type of bacteria to the water. Risks of getting sick are much higher if a swimmer is exposed to human source bacteria, but it’s much safer to avoid all high concentrations of bacteria. To identify if the bacteria are from human sewage, several tests have been developed over the years. Often these tests are expensive and inconclusive. However, King County has been monitoring test developments and we are currently testing a few different methods, which we hope will identify bacteria sources so that we can better understand if there are risks to swimmers getting sick.

Low counts of fecal coliform bacteria, less than around 50 - 100 CFU’s, are routinely found in high quality water. Typical fecal coliform bacteria counts from the middle of lakes Washington and Sammamish during the summer are less than 20 CFU's. Unfortunately, many of our urban streams in King County typically have counts of greater than 200 cfu/100 ml or higher under normal flow conditions, and during storm events these counts can exceed 1000 cfu/100 ml.

What are algal toxins?

Image of a shadow lake
Photo courtesy of Beth Cullen

In the fall of 1997, a toxic bloom of the cyanobacteria, Microcystis aeruginosa in Lake Sammamish led to park advisories at Lake Sammamish State Park, Idylwood Park, and Marymoor Park. In 1999, Green Lake was closed due to a toxic algae bloom. These events prompted the King County Science and Technical Support Section to include cyanotoxin monitoring as part of its Swimming Beach Monitoring Program beginning in 2005.

Cyanobacteria, formerly called "blue-green algae", are simple life forms closely related to bacteria. Although they are similar to algae, they are not true algae. When conditions are right, including sunlight and plenty of nutrients, a "clear" body of water can become very turbid with a green, blue-green or reddish-brown growth within just a few days as shown in this picture of Shadow Lake. Cyanobacteria can produce toxins that are potentially lethal to people and animals. Some cyanobacteria produce liver toxins and others produce nerve toxins (neurotoxins), which can manifest from minutes to days after exposure.

Poisoning from cyanobacteria nerve toxins can appear within 15-20 minutes after ingestion. In people, symptoms may include numbness of the lips, tingling in fingers and toes and dizziness. In animals symptoms from neurotoxin exposure include weakness, staggering, difficulty in breathing, and convulsions. Liver toxin poisoning may take hours or days for symptoms to appear. Symptoms of liver toxin exposure include pain, diarrhea and vomiting in humans and potentially death in animals.

King County’s Cyanotoxin Monitoring Program takes a proactive approach by testing for toxins both routinely and when cyanobacteria blooms or scums are observed at public beaches. In addition, the King County Science Section is playing a major role in a regional wide monitoring of toxic algae. Beginning in 2009, King County, in conjunction with the State Department of Ecology, State Department of Health, Snohomish and Pierce counties, is participating in a tri-county project to understand and quantify the extent of toxic cyanobacteria blooms in our lakes. This project is funded through a grant from the Center for Disease Control (CDC).

For more information about toxic cyanobacteria, visit King County Lakes toxic algae page.

Reference for the "Ten State Standard":

Recommended Standards for Bathing Beaches. Policies for the Review and Approval of Plans and Specifications for Public Bathing Beaches. 1990 Edition. A Report of the Committee of the Great Lakes – Upper Mississippi River, Board of State Public Health and Environmental Managers. Member States and Province: Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, New York, Ohio, Ontario, Pennsylvania, Wisconsin. Published by: Health Education Service, PO Box 7126, Albany, NY 12224. Phone 518-438-7286

Reference for the Washington State algal toxin guidelines:

Washington State Recreational Guidance for Microcystins (Provisional) and Anatoxin-a (Interim/Provisional). July 2008. Prepared by: Joan Hardy, Ph.D., Division of Environmental Health Office of Environmental Health Assessments.