Effects of combined-sewer overflows and urban runoff on the water quality of Fall Creek, Indianapolis, Indiana

In 1986, the U.S. Geological Survey and the Indianapolis Department of Public Works began a study to evaluate the effects of combined-sewer overflows and urban runoff discharging to Fall Geek on the White River. This report describes the effects of combined-sewer overflows and urban runoff on the water quality of Fall Creek during summer 1987 by comparing the water quality during base flow with that during storm runoff and by comparing water quality in the urbanized area with that in the less urbanized area upstream from the combined-sewer overflows. Data were collected at three streamflow-gaging stations located upstream from, downstream from, and in the middle of 27 combined-sewer overflows on Fall Creek. The most downstream station also was immediately downstream from the discharge of filter backwash from a water-treatment plant for public supply.

Specific conductance and concentrations of major ions and dissolved solids in base flow increased downstream in response to surface-water withdrawn for public supply, ground-water inflow, and the discharge of filter backwash. Concentrations of dissolved oxygen were least in the reach of Fall Creek in the middle of the combined- sewer overflows where black sludge deposits covered the stream bottom. Concentrations of nitrate plus nitrite and ammonia steadily increased downstream, whereas concentrations of organic nitrogen, phosphorus, and orthophosphate only increased at the most downstream station. Nearly all concentrations of chromium, copper, lead, nickel, and zinc at the upstream and middle stations were less than the detection limit of 10 micrograms per liter. Detectable concentrations of these metals and high concentrations of suspended solids in base-flow samples at the most downstream station were caused by the discharges from the water-treatment plant.

Concentrations of dissolved oxygen measured at the station in the middle of the combined-sewer overflows were less than the Indiana minimum ambient water-quality standard of 4.0 milligrams per liter during all storms. Concentrations of ammonia, oxygen demand, copper, lead, zinc, and fecal coliform bacteria at the stations downstream from the combined-sewer overflows were much higher in storm runoff than in base flow. Increased concentrations of oxygen demand in runoff probably were caused by combined-sewer overflows, urban runoff, and the resuspension of organic material deposited on the streambed. Some of the increased concentrations of lead, zinc, and probably copper can be attributed to the discharge and resuspension of filter backwash