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Environmental factors influencing the distribution of E. coli in water and sediments of a Lake Michigan swimming beach, Chicago, USA

RTS# MI-0109

By: R.L. Whitman1, M.L. Goodrich1, T. G. Horvath1, M.B. Nevers1, S.K.Haack2, M.J. Wolcott3, G.A. Olyphant4.


Swimming advisories due to excessive E. coli concentrations (>235 CFU/100 ml) are common at 63rd Street Beach, Chicago, IL. In order to characterize and understand the source and fate of bacteria we undertook an intensive study of the nearshore waters, submerged and foreshore sands, gull droppings and other possible sources of E. coli. In addition, we compared sand, water and gull E. coli and Salmonella enterica subsp. enterica ser. Typhimurium fingerprints and MARs. We took 3,900 E. coli samples from two media (sand and water), morning and afternoon from April through September, 2000. We also randomly took 10 days of hourly samples from 0700 to 1500 and another 10 days of intensive replicate samples. We found that morning sample bacteria numbers were significantly higher than afternoon (p<0.001) and that there was a general exponential decrease of E. coli in 8 of the 10 days sampled (p<0.001) strongly coinciding with insolation flux. In order of magnitude, foreshore sand, submerged sands, knee-deep water, waist-deep and offshore waters were all significantly different in mean E. coli concentration (p<0.001). Replicate samples showed that, on average, a relative error of 20% of the mean could be achieved with 80 samples taken in the morning or 30 samples in the afternoon (p=0.05 with outliers included). By modeling wind speed, rainfall, turbidity, air temperature, insolation and lake stage we were able to predict regulatory exceedences 79% of the time. There were correlations between knee-deep vs waist-deep or offshore water; morning and afternoon waters; foreshore sands vs submerged sands; submerged sands vs morning knee-deep or waist-deep water (p<0.001). E. coli numbers in foreshore sands were directly correlated with knee-deep and waist-deep water (R2 =0.43). While results of the E. coli and S. Typhimurium and MAR analysis remain mixed, there is no strong indication of humans as the dominant source of E. coli at this beach. These findings indicate that monitoring programs incorporating several environmental variables will be more effective than those simply documenting the numbers of indicator bacteria in water.

Whitman, R. L.1, M. L. Goodrich, 1, T. G. Horvath, 1, M. B. Nevers, 1, S. K. Haack, 2, M. J. Wolcott,3, G. A. Olyphant, 4, 2001. Environmental factors influencing the distribution of E. coli in water and sediments of a Lake Michigan swimming beach, Chicago, USA. Annual Meeting American Society for Microbiology, Orlando, FL, May 21-24, 2001. RTS# MI-0109

1U.S. Geological Survey, Porter, IN; 2U.S. Geological Survey, Lansing, MI; 3U.S. Geological Survey, Madison, WI; 4Indiana University, Bloomington, IN.

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