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Bacteria must be monitored at quarterly intervals. During each three month period (quarter), a total of five samples must be taken. The following requirements apply to the individual samples collected in one calendar month:
For E. coli samples, if the effluent has been disinfected using chlorine, ultraviolet light (UV), or any other process intended to disrupt the biological integrity of the E. coli, the samples shall be analyzed using the Most Probable Number method found in Standard Method 9223B (Colilert® or Colilert-18® made by IDEXX Laboratories, Inc). If the effluent has not been disinfected, the samples may be analyzed using either the MPN method above or EPA Method 1603: Escherichia coli (E. coli) in water by membrane filtration using modified membrane- thermotolerant E.coli agar (modified mTEC) or mColiBlue-24® made by the Hach Company.
For additional guidance on the holding time for bacteria samples, see the DNR Guidelines for Bacteria Analysis
The geometric mean must be calculated using all valid sample results collected during a month. The geometric mean formula is as follows:
Geometric Mean = (Sample one * Sample two * Sample three * Sample four * Sample five…Sample N)^(1/N), which is the Nth root of the result of the multiplication of all of the sample results where N = the number of samples.
If a sample result is a less than value, the value reported by the lab without the less than sign should be used in the geometric mean calculation.
The following calculator can be used to calculate the geometric mean from five samples. Please enter the five samples in the spaces below and press the calculate button. The geometric mean that should be reported on the Monthly Operating Report will then be calculated for you.