Identifying the sources of fecal contaminants in surface water bodies such as rivers, lakes and beaches is of importance for environmental safety, public health safety, food safety and regulatory purposes. Nutrient utilization patterns (NUPs) were used as a bacterial source tracking technique to identify the possible sources of fecal coliform bacteria, Escherichia coliin Silver Lake, Delaware County, Iowa.  A total of three hundred (300) E. coli isolates collected from different sources (water, birds, geese, cattle, hogs and soil contaminated by feces) were analyzed.  A database was built from these isolates by using discriminant analysis to identify the nutrient utilization patterns that best classify all 300 isolates by source. The average rate of correct classification by source was 89.5% when applying the nutrient utilization patterns database. After this verification, the NUP for E. coli isolates from Silver Lake water were measured.  Based on the NUPs of the Silver Lake isolates, 73.1% were found to originate from cattle and hogs. Smaller percentages were predicted to be coming from birds and geese.  None of the isolates were predicted to be originating from the human source. The results indicate that livestock are the primary contributors to fecal pollution in this hypereutrophic Iowa lake.

Key words: Nutrient utilization patterns, Escherichia coli, bacterial source tracking, Silver Lake, Delaware, Iowa.