IN-STREAM SEDIMENT AND NUTRIENT RESPONSE TO STORM EVENTS IN AN URBAN STREAM

The Wissahickon Creek is a 3rd order urban stream located north of Philadelphia and is classified as impaired by the US EPA due to turbidity and nutrients. The Stormwater Management Model (SWMM) is being developed for the Wissahickon Creek, and data showing nutrient and sediment dynamics during storms is critical to this calibration. In an effort to quantify the impacts of wastewater treatment plant effluent, two locations upstream and downstream of a wastewater treatment plant were monitored for turbidity, water level, nitrate, and phosphate using high frequency loggers. The upstream site is narrower and the streambed is more embedded with a lower percentage of fine sediment than the downstream site. Despite the differences in streambed sediment, the two sites had similar timing in turbidity response for both large and small storms, which suggests upstream sediment may have a greater control on turbidity than local sediment. Typically, turbidity tracked the water level rise, but fell more rapidly than water level decline. However, the turbidity levels were consistently higher at the upstream site, which was unexpected due to the higher embeddedness and grain size. Dilution from the wastewater treatment plant may explain lower turbidity downstream. Despite the presence of effluent at the downstream site, nitrate and phosphate had a similar response to most storms at the two sites, typically exhibiting a short peak on the rising limb then falling across peak discharge due to dilution. The initial peak suggests a source of nutrients entering the stream through runoff, most likely overland flow from the surrounding urban land use. At both sites the turbidity and nutrients followed a different pattern than the water level, which suggests that when calibrating the SWMM model turbidity, nitrate, and phosphate need different input functions. The temporal data collected during this research will help determine the controls on sediment and nutrient concentrations and dynamics in the Wissahickon Creek in order to better model the impact of storm events in the watershed.