Comparisons of E. coli Standards


The current  E. coli water quality standard for swimming (class B) waters in Vermont is 77 organisms/100 ml--a single sample maximum.  There are no criteria for number of samples nor sampling conditions.  Although standards approved in June of 1999, effective July of 2000, maintain the same standard, discussion regarding modification of the recreational water quality standard for E. coli continues.  In order to illustrate the potential impact of changing the current water quality standard, three scenarios are compared using the current Vermont standard (77 organisms/100 ml), an alternative standard (geometric mean of 77 organisms/100 ml with a single sample maximum of 143 organisms/100 ml), and the EPA recommended standard (geometric mean of 126 organisms/100 ml with a single sample maximum of 235 organisms/100 ml).  Data used for these comparisons were collected weekly at 15 sites in tributaries of the Mad River from May 2-November 3, 1998.  Total number of violations and number of sites in violation of geometric mean and single sample standards are summarized below.  Violations of the single sample maximum for each of three standards is compared by site and for all sites.  The effects of streamflow on the violation rate of three single sample maxima and two geometric mean standards are also analyzed.  Finally, the number of violations are calculated for a geometric mean based on three time periods (30, 60, and 90 days). 



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created by Deb Sargent under the direction of Dr. Leslie Morrissey
School of Natural Resources, University of Vermont
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