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Fecal coliform are bacteria that live in the digestive tract of warm-blooded animals including humans, horses, cows, chickens, cats, dogs, and waterfowl. The bacteria are excreted in the solid waste of humans and other animals and enter waterways via failing septic systems, sewer overflows, inadequate treatment of municipal waste, runoff from animal pastures, or inadequate human waste disposal associated with camping or other outdoor activities.

Unless human sewage is being discharged into the lake from failing septic systems or sanitary sewer overflows, most bacteria present are typically assumed to be of non-human origin. For most lakes, geese, gulls, and ducks are speculated to be a major source of bacteria, especially where large resident bird populations have become established.

Looking for contamination

Fecal coliform bacteria is a good indicator of sewage pollution of water and is routinely monitored as an indicator of the possible health risk to people who may be swimming in or drinking contaminated water.

When sewage is present in the waters, elevated counts of fecal coliform bacteria occur. However, the source of the high bacteria counts may not originate with human sewage. Many other mammals as well as birds can also contribute this type of bacteria to the water. To identify if the bacteria are from human sewage, tests for more specific bacteria types or analysis of genetic material must be completed. Through additional testing, the species of animal that added the bacteria to the water can sometimes be determined.

Monitoring for bacteria in King County lakes

Beginning in 1996 swimming beaches on Lake Sammamish, Lake Washington, and Green Lake were surveyed to determine levels of bacterial pollution and relative human health risks. Prior to this survey little local data on bacterial levels at these swimming beaches existed. Based on this monitoring, periodic closure of area beaches have occurred.

During 1998 and 1999, one-day bacteria surveys were conducted through the Lake Stewardship Program at area lakes. Samples were collected from 27 King County small lakes with public access to evaluate levels of bacterial pollution. Most lakes surveyed had low levels of bacteria. Table 1 summarizes the minimum, maximum, and geometric mean values among the 27 lakes surveyed.

Funding remains available to monitor large lake public beaches. No funding is available to regularly monitor bacteria levels in the small lakes. Bacteria monitoring is periodically conducted at the County’s three small lake swimming beaches (Fivemile, Pine, and Wilderness).

State standards

According to the Washington Water Quality Standards WAC 173-201A-030, fecal coliform levels in lakes shall not exceed a geometric mean value of 50 colonies/100 mL and not have more than 10 percent of all samples exceeding 100 colonies/100 mL. Based on limited data, these bacteria concentrations were not exceeded at most of the small lakes surveyed in King County during 1998 and 1999.

Current beach closure protocol

The Seattle-King County Public Health Department is responsible for determining the public health implications of bacteria monitoring results, conveying this information to elected officials and the public. Under current regulations, local health officers determine whether a waterbody is polluted or not.

Because no exact standards have been adopted by the state Department of Health to determine when a beach should be closed, the local health department must make beach closure decision on a case-by-case basis. Generally, the health department will use one of two standards in making their determination:

  1. the fecal coliform standard developed by the federal Water Pollution Control Federation, which is a geometric mean of 200 fecal coliform/100 mL, with not more than 10 percent of the samples exceeding 400.
  2. the 10 state standard which is a geometric mean of 200 fecal coliform/100 mL, with no single sample exceeding 1000.

Reducing bacterial contamination

Lake residents can do their part to ensure that they do not contribute to bacterial contamination of the lake. Properly maintaining septic systems is a good start. Cleaning up after pets and properly managing waste from farm animals will also help. Finally, discouraging waterfowl use of adjacent lawns and docks by planting vegetation along the shoreline will minimize bacteria from birds. (See below.)

Don't get goosed! What you should know

Geese and ducks love to feed on the succulent grasses of well kept lawns. With few natural enemies to keep their numbers in balance, waterfowl have become a nuisance for many lakeside residents and a major source of bacteria entering the water.

Waterfowl prefer to rest and feed on open, grassy areas next to water, and they generally walk onto land using routes that allow them easy access. One way to discourage these web-footed creatures is to create a buffer of native plants between the water’s edge and the lawn. Fearing hidden predators, waterfowl are reluctant to step through the vegetation to dine on the grass. Large boulders placed in front of the buffer may enhance the effectiveness.

Back to fact sheets

For questions about lakes in King County, please contact or call the Water and Land Resources Division front desk at 206-477-4800.