WATERSHED MODELING OF ONSITE WASTEWATER SYSTEMS IN SUMMIT COUNTY, COLORADO

WARMF, a GIS-based watershed model, was applied to the 840 km2 (325 mile2) Dillon Reservoir Watershed in Colorado where approximately 1500 onsite wastewater systems (OWS) can be found along the Blue River. WARMF was set up with digital elevation model (DEM), meteorology, point source, and land use data. Additional data collected in the watershed for this project included surface water quality, soil properties, well data, and the spatial distribution of OWS in the watershed. A biozone module was developed to simulate the treatment processes taking place in the biologically active soil layer that develops in a soil receiving septic tank effluent. The build-up of bacteria increases field capacity, and decreases porosity and the infiltration rate over time leading to potential hydraulic failure of the OWS. The biozone module was tested with laboratory column data collected under accelerated dosing conditions. The module was also tested under field conditions to produce a frequency distribution of OWS time to failure under various dosing rates and septic tank effluent concentrations. The tested module was then incorporated into WARMF to process the septic effluent before releasing it to a soil layer. Hydrology and water quality simulations were run and the model was calibrated to available observed data. After establishing a base case, various management scenarios related to OWS were tested. These scenarios included the conversion of existing OWS to centralized sewers. The scenario runs provide information to evaluate the trade-offs between OWS and centralized sewer systems as well as the general impact of OWS on surface water quality.