E. coli and enterococci levels in urban stormwater, river water and chlorinated treatment plant effluent

Abstract

Stormwater from the Ann Arbor, Michigan area affects the bacterial indicator organism quality of the Huron River. Investigations during the 1985 summer period involved sampling during dry and wet periods with parallel determination on each sample for fecal coliforms, fecal streptococci, E. coli and enterococci. Wet weather bacterial indicator densities were statistically significantly higher than dry weather levels, and downstream densities were statistically significantly higher than upstream densities. The FC/FS (fecal coliforms/fecal streptococci) ratios for the storm drains were low and suggestive of more animal than human sources. The geometric mean EC/FC (E. coli/fecal coliforms) ratios were in the range of 0.82-1.34, well above the ratio of 0.63 calculated using the U.S. EPA recommended level for E. coli of 126/100 ml to the presently accepted level for fecal coliforms of 200/100 ml. If the intent is to maintain the currently accepted illness rate, additional results from other areas are necessary to refine the E. coli and enterococci levels for water quality standard development purposes. In general, physical-chemical observations reflected the source of the sample.