Site Links:
Bacteria and Temperature Data
What Do These Numbers Mean?
Beach Closures
FAQs
Related Links
About the Swimming Beach Program
Green Algae Bloom
|
What do these numbers mean?
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.
Fecal coliforms always occur at very high counts (several thousand to several million CFU's –colony forming units) when sewage is present in the water; an example is when a sewer line leaks or breaks. However, high bacteria counts in water do not necessarily indicate human sewage pollution because many other mammals and birds also contribute to this type of bacteria. 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 >200 cfu/100 ml or higher under normal flow conditions, and during storm events these counts can exceed 1000 cfu/100 ml.
The numbers we use to evaluate the swimming beaches were adopted by the EPA in 1976; a geometric mean of <200 cfu/100 ml, 90% of samples <400 cfu/100 ml. This program also uses the criteria of all samples <1000 cfu/100 ml.
What is a CFU/100mL?
CFU’s are a unit of measure and stand for Colony Forming Units. After incubation, bluish colonies of bacteria should appear on the sample dish. It is then necessary to count all of the colonies that are in each dish. A calculation must be performed to determine the number of colony forming units per 100 milliliters of sample.
|
