The effect of storm runoff on the quality of water in the the Mill Creek drainage basin in west-central New Jersey is described in this report. The study area (23.7 square kilometers) consists of agricultural and undeveloped land and the residential community of Willingboro. From October 1975 to September 1976, stream discharge and 86 water-quality constituents were measured during five base flows and five storms at nine sites within the basin. Only 38 of the measured consitituents were detected in significant amounts.

Insignificant amounts of pesticides, except 2,4-D and silvex, and the metals cadmium, chromium, cobalt, copper, mercury, selenium, and carbonate were measured even during heavy runoff. All other constituents showed an increase in concentration with increasing streamflow and the corresponding loads usually correlated with streamflow.

Runoff from the nonresidential part of the study area in the upstream part of the drainage basin had a more significant impact on the stream quality than did the runoff from the residential area. The nonresidential area contributed more nutrients, common inorganics, sediment, and organic carbon than the residential area. The residential area contributed more calcium, nitrite, lead, iron, BOD, 2,4-D, and silvex.

With the exception of suspended iron, fecal coliform bacteria, suspended lead, and suspended phosphorus, all recorded concentrations under all streamflow conditions met the recommended limits set by the U.S. Environmental Protection Agency and the New Jersey Department of Environmental Protection for domestic water supplies or streams not discharging directly into a lake or reservoir. Very short-term exceptions were recorded during peak storm runoff.

Iron concentrations exceeded recommended limits of the U.S. Environmental Protection Agency. Occasional releases of suspended iron from the water-treatment plant in the lower part of the drainage basin contributed to this high concentration. However, Mill Creek and its tributaries will still have naturally occurring high suspended iron concentrations even after the water-treatment plant eliminates its sludge-holding pond.

Fecal coliform bacteria affected the quality of water in the Mill Creek basin more than any other constituent measured. It exceeded recommended limits by as much as two orders of magnitude and was found in equal amounts throughout the streams in the basin. The source of the bacteria could not be identified.

Runoff from the Willingboro residential area produced mean suspended lead concentrations up to three times greater than the 50 micrograms per liter criteria set for domestic water supplies. Mean concentrations of suspended phosphorus throughout the basin were 2.5 times greater than the recommended limit of 0.1 milligrams per liter. Sediment concentrations and loads increased with increasing streamflow. However, the amounts are not considered unusual for a developed area. The only sediment deposition occurred at the downstream end of the study area, which is slightly affected by tide. A significant part of the deposited material was composed of sludge material from the water-treatment plant in the lower end of the study area.