Simulating fecal coliform bacteria loading from an urbanizing watershed.

The fate and transport of fecal coliform bacteria in the urbanizing Polecat Creek watershed, located in Virginia, was simulated using the Hydrological Simulation Program-FORTRAN (HSPF). Both point and nonpoint sources of fecal coliform were included in the simulation. Hydrologic and water quality parameters of HSPF were calibrated and validated using observed data collected from October 1994 to June 2000 at three monitoring stations. The percent errors in total runoff volumes between observed and simulated values ranged from 0.4 to 4.2% for the calibration period, and 0.4 to 6.7% for the validation period. The geometric mean of simulated fecal coliform concentrations at the outlet of the watershed was 10% lower than that of observed values for the calibration period. HSPF moderately under-predicted the geometric mean concentration by 16.4% for one sub-watershed and slightly over-predicted by 7.3% for another. Observed fecal coliform concentrations were compared with the range defined by the minimum and maximum simulated concentrations occurring within a 3-day window centered on the day the water sample was collected. Over 42% of grab sample data collected at the three monitoring sites in the watershed fell within the max min range of simulated concentrations over the 3-days window for the calibration period. For all monitoring sites, 39.5% of the total samples taken during the validation period fell in the range of simulated concentrations over the 3-day window period. Results presented in this study demonstrate that HSPF reasonably represents the hydrology and water quality of an urbanizin watershed and that it could be utilized as a planning tool for future assessment of land use impacts on fecal coliform on in-stream concentrations.