WATER-QUALITY EFFECTS OF STORMWATER MANAGEMENT ON A GROWING COLLEGE CAMPUS

Recent construction of a large athletic arena at the downstream terminus of the University of Virginia campus in the watershed of Meadow Creek, an impaired stream that drains ultimately to the Chesapeake Bay, motivated an assessment of upstream stormwater management. Past expansion with new residence halls, classroom and research buildings, and parking lots resulted in a greater volume of stormwater runoff downstream and a lesser volume of stormwater able to infiltrate into the ground. In 2004, an innovative design was adopted that daylighted a 1100-foot section of culverted stream, inserted a 0.64-acre wet pond that served as an aesthetic feature, and built a sinuous channel leading to the pond planted with floodplain vegetation. Post-construction performance of the stormwater-control measure (SCM) was accomplished at the stream just prior to entry into the pond and at the outflow of the pond over the course of 11 storm events spanning a 5-month period. Combining chemical and hydrological measurements, event mean concentration (EMC) of nitrate, phosphate, and total suspended solids (TSS) was calculated for each site. The percentage of contaminant removed over the duration of each storm event was expressed as removal efficiency for the system. Removal efficiencies ranged from 44% - 96% for TSS, 56% - 100% for phosphate, and 23% - 78% for nitrate, suggesting great variability in SCM performance. No strong statistical relationships for system performance as a function of environmental factors such as number of antecedent dry days and rainfall amount, intensity, and duration were quantified, although duration of rainfall was most influential on TSS and phosphate removal. The lower values of removal efficiencies tended to be associated with rain events of longer duration. The design criterion for pollutant removal based on a 10-year storm event was 50%, so the SCM assessment typically showed even greater-than-planned improvement. Concerns about overall pollutant removal remain, however, because all the monitored events were smaller than that for a 2-year return interval. Even though the outlet of the pond is water of good quality, further inputs of stormwater at downstream locations resulted in degraded water quality leaving university property.