Integrating Watershed-Scale Hydrologic and Water Quality Evaluations with the Contaminant Source Inventory for Philadelphia's Source Water Protection Program

The Philadelphia Water Department's Office of Watersheds Source Water Protection Program embarked upon a comprehensive Source Water Assesssment (SWA) project to identify challenges to, and priorities for, the protection of raw drinking water supplies in the region. The project is intended to identify and prioritize potential sources of contamination across the watersheds and to assess the vulnerability and susceptibility of the water supplies to these sources. An extensive watershed-wide inventory of potential contaminant sources, supported by significant database and GIS efforts, was central to the development of the SWA process. The need to provide estimates of potential nonpoint source contaminant loads from subbasins over the two watersheds presented one of the more difficult challenges in the execution of the SWA project. To meet this challenge, comprehensive, watershed-wide, long-term continuous simulation models of watershed hydrology and water quality loading were applied. The runoff loading models combined with GIS, displays spatially, the areas with high pollutant loads and of high protection priority. The use of a complex deterministic model that attempts to represent rainfall-runoff, groundwater, and water quality contaminant contributions realistically at this scale is unique among the SWAs being performed across the United States today.