E. coli standards comparisons:  30 DAy, 60 day, and 90 day
Geometric Mean E. coli

WATER QUALITY VIOLATIONS OF MONTHLY GEOMETRIC MEANS

Water quality violations for monthly geometric means were compared for all 15 Mad River sites.  May and June had no violations of the geometric mean while August had the most sites in violation of the geometric means.  Violations of the alternative geometric mean standard were more than twice those of the EPA geometric mean standard.  There were 11 violations of the monthly geometric mean for the alternative standard and 5 for the EPA standard.

COMPARISON OF VIOLATIONS OF 30 DAY, 60 DAY, AND 90 DAY GEOMETRIC MEANS

The violation rate of the geometric mean standards differs when the geometric mean is calculated for different time periods.  In general, the longer the time period and greater sample size, the fewer the violations.  The geometric mean was calculated for a 30 day period in August, a 60 day period in mid-July to mid-September, and a 90 day period in mid-June to mid-September.  For the alternative geometric mean standard, there were 2 violations of the 60 day and 90 day geometric means, but 7 violations of the monthly geometric mean.  For the EPA geometric mean standard, there was one violation of the 60 day and 90 day geometric means, but 3 violations of the monthly geometric mean.  As a rule, the  geometric mean is insensitive to extreme values, or in this case, high E. coli measurements acquired during peak streamflow.  As a result, a geometric mean calculated from E. coli measurements over a 60 day and 90 day period give less weight to high E. coli counts acquired during peak flows and more weight to the more numerous and lower E. coli counts.

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Standards Comparisons