About lake bacteria
Some bacteria, especially intestinal bacteria discharged in feces, can pose a health risk
to bathers and swimmers in public waterways such as lakes and rivers. Fecal coliform bacteria are found in the
feces of most mammals and birds, whereas Escherichia coli (abbreviated E. coli) are found almost exclusively in
humans. These bacteria enter lakes and streams from leaking sewer lines, discharges from sewer overflow systems
during periods of heavy rainfall, effluent from improperly functioning septic systems, feces-contaminated runoff
in streams, and discharges from "heads" on boats, especially in marinas. The significant role of pets
in contaminating surface waters is now being recognized. In a 1993 study of the ribonucleic acid in bacteria in
Piper's Creek, Seattle, the main source of bacteria was discovered to be domestic cats.
King County's Division of Water and Land Resources collected data on bacterial counts and
water temperature at a large number of public beaches around Lake Washington and Lake Sammamish, east of downtown
Seattle. Juanita Beach on the northeast side of Lake Washington was sampled approximately every week from May through
September, 2000, at a depth of 1 meter. Bacterial concentrations are given in colony forming units (CFUs) per 100
milliliters (0.1 liters) of water. Typical bacterial counts in the middle of Lake Washington away from public beaches
average around 20 CFU/100 ml. King County uses the "10 state standard" for water quality for swimmers;
the mean of all fecal coliform samples cannot exceed 200 CFU/100 ml and no single sample can exceed 1000 CFU/100
ml. As can be seen from the data, this water quality standard was exceeded during the summer of 2000 at Juanita
Beach.
The graph shows fecal coliform abundance plotted against E. coli abundance, to see how these
two bacterial types are correlated. There is a strong linear relationship between the two, suggesting a similar
source for these bacteria, or the same environmental control on the abundance of these two bacterial types. A 1998
study of the genetic makeup of fecal coliform bacteria at Juanita Beach showed that ducks and geese were the primary
source of contamination, with seagulls and dogs as secondary sources. Note that there is a spike in bacterial counts
in early July, following a warm water period at the end of June. It would be interesting to look at rainfall and
runoff from this period. The 12 July data have been left off the graph for ease of viewing the rest of the data;
is this a true outlier?
Reference: King County (Washington) Division of Water and Land Resources
http://dnr.metrokc.gov/wlr/waterres/lakes/
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