E. coli, Sampling Procedures and Total Body Contact Standards
E. Coli Bacteria
E. coli is a bacteria naturally found in the intestinal tracts of humans and other animals. The presence of e. coli in the water indicates possible contamination with human or animal waste.
Sampling and Testing Procedures
Samples are collected twice weekly and, in accordance with established procedures, are collected one foot below the surface in water that is three to six feet deep. A minimum of three samples are collected at a beach during each visit.
Collected samples are brought to a laboratory that has been certified by the Michigan Department of Environmental Quality, Drinking Water and Radiological Protection Division for analysis. The Idexx Colilert System is used for sample analysis.
The Macomb County Health Department uses the Geometric Mean in estimating a 30-day average from individual samples taken during five or more sampling events. A Geometric Mean is another way to average a set of values. It is commonly used with parameters such as bacterial water assessment, which show a great deal of variability, in order to reduce the effect that occasional high or low values have on the average. Technically, the Geometric Mean is calculated by taking the nth root of the multiplication product of a string of e. coli data, where n equals the number of sample results.
Example: Given three sample results (colonies/100ml) of 20, 80, 40
- Calculation: 20 x 80 x 40 = 64,000
- Since there are three sample results, we take the 3rd root of 64,000 which is 40 colonies per 100 milliliters.
- If there were ten sample results, the geometric mean would be the 10th root of the product; twenty samples would be the 20th root, and so on.
Total Body Contact Standards
Sample results are compared to the Total Body Contact Standards contained in Part 125 of the Michigan Public Health Code which are as follows:
-300 e. coli colonies per 100 milliliters, as a geometric mean of all samples collected at a beach during one sample event;
-130 e. coli colonies per 100 milliliters, as a geometric mean of all samples collected over a 30-day period.
If either of the above standards are exceeded at a bathing beach, the owner/operator is contacted and instructed to close the beach.
If you have any questions, call us at 586-469-5236, Monday-Friday, 8:30 a.m. - 5:00 p.m. or E-mail us now.
Center for Disease Control - Healthy Swimming information
Great Lakes Information Network - Regional watershed information
Environmental Protection Agency - Beach standards and national monitoring