Measured Event Water Quality Levels in Urban Karst Watersheds Compared to Standard Reference Values

Water quality samples were collected during 13 events from January 2009 to April 2011 at six locations in five urban karst watersheds located in the limestone geologic setting of the Nittany Valley in Centre County, Pennsylvania. The samples were analyzed for a variety of constituents including metals, nutrients, and sediment. For each event the event load and event mean concentration were calculated based on the time-based sampling. Amounts and concentrations of constituents were compared among study watersheds with different imperviousness and land uses. The study results were compared with other urban data including the National Urban Runoff Program (NURP) and a study combining NURP data with water quality data collected through National Pollutant Discharge Elimination System (NPDES) permits. Measured results were also compared with values obtained using the Simple Method to Calculate Urban Stormwater Loads. For total suspended solids (% difference: 45 - 353%), copper (% difference: 115 - 490%), lead (% difference: 1,720 - 4,330%), and zinc (% difference: 26 - 296%), the measured concentration data from this study were less than the data from the NURP studies. Annual nitrogen and phosphorus loads for these study watersheds as calculated by the Simple Method, using the measured concentration and runoff data, were much less than what would be expected for other non-karst watersheds with similar imperviousness. Based on watershed impervious area, the Simple Method overestimates the annual loads of phosphorus by 171% to 514% and of nitrogen by 831% to 3,157% for these karst watersheds. The event mean concentration data produced by this study can be used to more accurately predict water quality in karst watersheds than would be achieved using the NURP data, a standard reference for water quality modeling.