Assessment and Use of Indicator Bacteria to Determine Sources of Pollution to an Urban River

Elevated levels of indicator bacteria within a river system represent a significant impairment to surface water quality in many urban areas within the Great Lakes watershed. Outfalls from combined sewer systems are assumed to be the major source of bacteria to streams in many of these urban areas, including the Rouge River of southeastern Michigan. Current remediation strategies largely disregard other potential sources, including water entering the river from its headwaters region, rural runoff, and contaminated groundwater. These other potential sources of bacterial pollution to the Rouge River were assessed and compared with bacteria contributed from combined sewer outfalls (CSOs). Fecal coliform (FC) and fecal streptococci (FS) densities were determined at 28 locations in the Rouge River Watershed during the spring and summer of 1997, 1998, and 1999. The mean levels of both indicator groups vary considerably along each branch of the river and show no correlation to CSO locations. The magnitude of both FC and FS levels are, in fact, frequently greater at sites upstream of the part of the river influenced by discharges from CSOs. The highest FC levels coincided with rainfall events, but FC levels at various sites along each branch of the Rouge River violated acceptable water quality standards (200 colonies/100 mL) at nearly 50% of the sites even during dry weather. Total suspended solids were moderately correlated with FC (r = 0.79) throughout the watershed, suggesting that solids may play a role in transporting bacteria into the Rouge River. The data depict a strong influence of upstream water and rural runoff on the water quality of the Rouge River. FC to FS ratios (FC/FS) suggest the primary source of bacteria throughout the watershed is from domesticated animals and wildlife and not from sewage derived from CSOs.