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Bacteria Sampling

General Requirements for Bacteria Sampling:

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:

  • Samples must be spaced over one calendar month.
  • No more than one sample can be collected on any one day.
  • There must be a minimum of two days between each sample.
  • No more than two samples may be collected in a period of seven consecutive days.

Guidance for Laboratory Analysis of Bacteria Samples:

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.

Calculation of the Geometric Mean:

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.


Enter the five samples here:
Press the calculate button:   or  

Here is the geometric mean to be reported on the MOR:

Note: For another calculation, hit "Start Over" and enter 5 new samples. The geometric mean will then update using the "Calculate" button.

If you are having problems with the calculator, please contact the webmaster at: