Survival of Escherichia Coli and Enterococcus in Stream Water.

E. coli and Enterococcus indicate fecal contamination and are used for monitoring of lakes, streams, and rivers.  Transport of bacteria from manured or pastured lands can result in large bacterial loads from both small and large runoff events and the persistence of bacteria following these loadings is important to modeling transport at the watershed scale.  Our objective was to examine the survival of E. coli and Enterococcus in stream water and to determine if seasonal trends could be detected. Samples were obtained from 6 sites within the South Fork of the Iowa River in June, July, August, October, and November of 2007.  The watershed is largely agricultural, primarily cropped to corn and soybeans, with swine and cattle present.  Initial population measurements were made on the day of sample collection and samples were then incubated at the same temperatures as the stream.  First-order decay equations described survival of both bacteria.  Averaged over all sampling times and locations, the mean half life of E coli was 18.4 ± 5 hr which was significantly different (p< 0.05) from the 23.9 ± 8.9 hr half-life for Enterococcus.  Few differences were observed due to sampling location, but differences were found in survival over time for both E. coli and Enterococcus.  Survival was shorter in summer months, which correspond to the periods when greatest stream water populations are found, but with relatively few runoff events.  Increased survival results in greater transport at any given stream flow.