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Beginning in 1996, swimming beaches on Lake Sammamish, Lake Washington and Green Lake have been monitored during the summer months 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. Substantial amounts of bacterial data are collected in lakes Sammamish and Washington from the King County Lake Assessment long term monitoring program, but these data are collected to monitor the sewage collection and transfer system and are not collected within designated swimming beaches or boat anchorages. A portion of the 1996 bacteria study was conducted to determine the impact of overnight recreational boat anchoring on bacterial concentrations in Andrews Bay, located on the southwest shore of Lake Washington.
Fecal coliform bacteria is routinely sampled as an indicator of sewage pollution in water, and as an indicator of the human health risk from swimming in contaminated waters. Elevated counts of fecal coliform bacteria 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. To identify if the bacteria are from human sewage, tests for the presence of specific bacteria such as Escherichia coli or Pseudomonas aeruginosa have been used. Another technique to identify the source of the fecal bacteria is to use the genetic material RNA (ribonucleic acid) from the bacteria. This provides information on the species of animal which added the bacteria to the water. A recent study using this RNA method conducted in Piper's Creek in Seattle's Carkeek Park (1993) identified domestic cats as the major source of bacteria.
According to the Washington Water Quality Standards WAC 173-201A-030, freshwaters rated AA (extraordinary) shall not have fecal coliform levels exceeding 50 colonies/100 ml and not have more than 10% of all samples exceeding 100 colonies/100 ml (for Class A excellent - 100 colonies/100 ml and less than 10% of all samples exceeding 200 colonies/100 ml). These concentrations are often exceeded in urban streams in King County.
The Seattle - King County Public Health Department determines the public health implications of the bacteria data collected from this program and conveys this information to elected officials and the public.
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