An integrated watershed modeling methodology for water quality restoration

The presence of fecal bacteria in a stream can pose serious risk to public health, especially for streams or lakes serving as recreation, agriculture or even water-supply sources. Sections of two creeks in the Piedmont region of North Carolina, the Main Creek and its major tributary, a Piedmont stream, have been found impaired with Fecal Coliform (FC) pollution and have been reported as 303(d) listed waters by NCDWQ since 2004. Programs have been developed to restore the polluted creeks to attain water quality standards. A TMDL program for the Main Creek watershed has been submitted by NC state agency, NCDWQ, to US EPA. Besides the TMDL analysis and plan, the program calls for further implementation activities, including pollution source assessment and modeling for reduction of FC loading, which invokes the dissertation research.;The research has conducted hydrology and water quality monitoring of the streams since 2005, and the data analysis have verified the frequent FC exceedance of water quality standards of the two streams, especially the tributary with urban contributing areas. The FC violations occur in both dry and wet weather events. Human-sources, related to SSOs or septic system failures, are the major FC pollution sources, especially in dry weather periods.;An integrated watershed and water quality stochastic model was developed to simulate the existing status of the FC pollutions of the streams and watersheds, which is composed of four interlinked models: watershed hydrology model, diffuse pollution model, septic tank system failures model, and SSOs model. The model was implemented in Monte Carlo Simulations (MCS) approach simulating the overall process of fate and transport of Fecal pollutant in consideration of varied environment factors, such as antecedent dry weather period, rain amount and intensity, land use, SSOs risks, septic tank failures risks, and one of main outputs of the model is the forecasted possibility of FC exceedance of the in-stream water quality, which links the characteristics of the FC pollution in the watersheds with the water quality standards.;The integrated watershed and water quality model was further used to discuss varied stream restoration and watershed management options, including reducing diffusing pollution sources in neighborhoods, reducing risk of septic tank systems failures, improving maintenance of sewer system and BMPs. Different restoration scenarios have been simulated, and the restoration simulation results suggest that Main Creek can be restored to fully satisfy the water quality standards if a well-planned stream restoration and watershed management program is fulfilled, which expects the mean efficiency of 90% FC removal by vegetation buffers, and the average reduction of 75% of the accumulated FC load between rain events, 75% of SSOs risks, and about 90% of risk of septic tanks systems failures of the existing status, respectively.;Key Words. Watershed modeling, water quality, fecal coliform, TMDL, Monte Carlo Simulations, GIS, BMPs, risk assessment.