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Historical trends and concentrations of fecal coliform bacteria in the Brandywine Creek basin, Chester County, Pennsylvania

September 1, 2001

The Brandywine Creek in Chester County is used for recreation and as an important source of drinking water. For this study, 40 sites were established for collection of water samples for analysis of fecal coliform and Escherichia coli bacteria in 1998-99. Samples were collected during base-flow conditions and during five storms in which rainfall exceeded 0.5 inch. During base- flow conditions, the median concentrations of fecal coliform bacteria exceeded 200 col/100 mL at 26 of the 40 sites (65 percent). During stormflow conditions, the median concentration of fecal coliform bacteria exceeded the Pennsylvania Department of Environmental Protection (PaDEP) criterion of 200 col/100 mL at 30 of 33 sites sampled (91 percent). Trends in fecal coliform bacteria concentrations were downward for the period 1973-99 at three long-term water-quality monitor stations, the result of upgrades in wastewater treatment plants, decreases in point-source discharges, and a decrease in agricultural land. A positive relation exists between streamflow and concentrations of fecal coliform bacteria at two of the long-term stations, but concentrations are elevated in base flow and stormflow at all three stations.

Factors affecting bacteria concentrations in the Brandywine Creek Basin include nonpoint-source contaminants, reservoirs, seasonality, and stormflow. Nonpoint sources of bacterial contamination in the basin include, but are not limited to, land-surface runoff, urbanization, agricultural processes, groundwater contamination, and wildlife. Bacteria concentrations in streams that flow directly from the reservoirs are much lower than the concentrations in the streams flowing into the reservoirs. During March, April, May, October, and November, the Brandywine Creek tends to have lower water temperatures and bacteria concentrations than during June, July, August, and September. The 10-year median concentrations of bacteria at West Branch Brandywine Creek at Modena and East Branch Brandywine Creek below Downingtown exceed the criterion of 200 col/100 mL established by the PaDEP during the swimming season. The 10-year median concentrations of bacteria at Brandywine Creek at Chadds Ford exceed the criterion of 200 col/100 mL only during June. None of the stations exceed the criterion of 2,000 col/100 mL as established by the PaDEP for the remainder of the year.