Pollutant Source Assessment and Model Development for Wissahickon Creek – An Urban TMDL Case Study

Wissahickon Creek drains an urbanized watershed in southeast Montgomery County and northeast Philadelphia, Pennsylvania, and is currently included (with tributaries) on the State's 303(d) list of impaired waters due to elevated levels of nutrients, organic enrichment, low dissolved oxygen levels, habitat modification, extreme flow variability, and noticeable problems resulting from siltation. The Wissahickon is designated for trout stocking, and therefore is subject to seasonal dissolved oxygen standards as set by the Pennsylvania Department of Environmental Protection (PA DEP). From analysis of continuous sensor data and diurnal data collected in 1998, high nutrient levels in the creek were linked (due to eutrophication and excess periphyton growth) to relatively large diurnal fluctuations in oxygen concentration causing violations of standards. In addition, due to rapid development associated with urbanization, high storm flows have resulted in much streambank erosion and siltation of the streambed. This form of habitat modification compounds the impact on aquatic life resulting from nutrient loading.Increased development of the Wissahickon watershed has resulted in an increase in point source discharge, and a rise in the magnitude of storm flows resulting from runoff from impervious areas. From an analysis of streamflows and water quality data, higher nutrient levels show a positive correlation to low flows, suggesting levels are dominated by point source contributions. To address impacts of urbanization, modeling analysis focused on both dry and wet conditions to analyze impacts of pollutants associated with each respective period. For dry weather, point sources were the dominant contributor to streamflow, resulting in reduced assimilative capacity of the stream, higher nutrient levels, and greater impacts on aquatic life. A modified, low-flow, steady-state WASP model was used to determine the nutrient TMDL and develop wasteload allocations to point sources on the Wissahickon and tributaries. This modified version of WASP included an added sub-routine that simulated processes associated with attached algae. For wet weather, a modified version of the GWLF watershed model was used for determination of the siltation TMDL and wasteload allocations. This modified version included a streambank erosion routine to estimate the siltation loads resulting from higher peak flows associated with increased urbanization.For development of nutrient and siltation wasteload allocations, considerations were made regarding capabilities of stakeholders to reduce loads to meet TMDLs, regulatory measures through discharge permitting (including stormwater MS4 permits), and watershed management options through best management practices. To ensure that all such considerations were complete, stakeholder involvement was a key component in the success of the TMDL and provided assurance that resulting wasteload allocations were reasonable and attainable through proper implementation and watershed management. Public meetings with the United States Environmental Protection Agency, PA DEP, dischargers, environmental groups, municipalities, and concerned citizens were critical to ensure that such considerations were included throughout the process of TMDL development.