Turbidity and Nutrient Response to Storm Events in the Wissahickon Creek, Suburban Philadelphia, PA

Abstract

The Wissahickon Creek is an urban stream that runs through Montgomery and Philadelphia Counties and discharges to the Schuylkill River in Philadelphia. A majority of stream segments in the Wissahickon watershed are considered impaired by the USEPA due to sediment and nutrients. Total Maximum Daily Loads (TMDLs) were implemented in 2003 for nutrients (NO3-, PO43-, NO2-, and CBOD5) and siltation. A new TMDL for total phosphorus (TP) was proposed in 2015, despite minimal data on the effectiveness of the 2003 TMDLs. This new proposal was met with concern, suggesting more data must be collected to better understand impairment in the Wissahickon Creek. The purpose of this research was to study turbidity and nutrient responses to storm events, as storm events are known to contribute significant loads of both sediment and nutrients. Twelve sites were chosen for high frequency turbidity and water level monitoring along the Wissahickon Creek and one of its main tributaries, Sandy Run. These sites were selected around three of the major wastewater treatment plants (WWTPs) to determine the relative roles of WWTPs and overland flow as sources of turbidity and nutrients during storm events. The upstream site and first downstream site at each WWTP were monitored for nutrients during storms using high frequency loggers and ISCO automatic samplers. Stream assessments were done at each site to characterize in-stream physical parameters, bank vegetation, and algae cover. High frequency turbidity data suggests that the turbidity is locally sourced, as turbidity peaks at the same time as water level, or within an hour or two, at all sites regardless of storm size. Comparisons of the turbidity response with in-stream parameters and land cover helped determine that the main factor driving the turbidity response is discharge, although bank topping and impervious cover, particularly roads, may increase turbidity responses at some sites. Similarities in nutrient, turbidity, and conductivity responses upstream and downstream of the WWTPs strongly suggest that overland flow, not WWTP effluent, is the major source of nutrients and sediment during storm events. Finally, a strong relationship between total phosphorus and high turbidity suggests that only during high discharge events is there a significant increase in TP in the Wissahickon Creek. Results from this research identify the source of turbidity and nutrients to the Wissahickon Creek during storms as primarily coming from overland flow, that the primary factor controlling the turbidity response is discharge, with some secondary influence from over-banking and the contribution of roads to land use, and a close link between TP concentrations and sediment during storms in the stream.