NUMERICAL MODELING TO CHARACTERIZE THE EFFECTS OF WASTEWATER POLLUTANTS ON WATER QUALITY IN MOUNTAINOUS WATERSHEDS

The mountainous regions of Colorado have experienced development and large population growth in the past thirty years. However, due to logistic and cost constraints, sewer systems have not been constructed in these mountainous areas. Consequently, all residents rely on soil-based wastewater treatment systems for disposal of their wastewater. This can pose problems both for the area residents and those down gradient of developed areas because of the potential for drinking-water supply contamination, exceeding regulated total maximum daily loads (TMDLs) in streams, and eutrophication of mountain lakes from wastewater pollutants such as nitrogen and phosphorus. It is clear that development of watershed-scale models is necessary for reliable assessment of the impacts of wastewater pollutants on water quality in the watershed. However, it is difficult to determine model input parameters, particularly for pollutant transport and transformation, and it is unclear if using literature values is sufficient. The primary objective of this study is to create and calibrate a numerical groundwater flow model for the Blue River watershed in Summit County, Colorado, and use the model to assess the influence of model-input-parameter variability on predicted water quality in the watershed. Specifically, the model is used to determine the sensitivities of predicted groundwater and surface-water pollution to typical variations in input wastewater concentrations and sorption coefficients. The appropriateness of using input values from previous research (i.e., from the literature) to characterize the pollutant transport in the complex watershed system is assessed.