General Information on Bacteria Monitoring for NPDES Permits
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.
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.