QUANTITATIVE ASSESSMENT OF POLLUTANT SOURCE AND TRANSPORT IN THE BLUE RIVER WATERSHED COLORADO

The Blue River supplies Dillon Reservoir, a primary water supply for Denver. Eutrophication of Lake Dillon is a concern, primarily due to phosphorus (P). There is little agriculture in the watershed. Thus, many local officials attribute the P loading to onsite wastewater systems (OWS). Data from a field study and a watershed model are used to assess the potential influence of nonpoint OWS sources to the total P loading in the Blue River watershed (the most developed of three watersheds that supply Lake Dillon). A variety of hydrologic and chemical-transport measurements and estimations were integrated using various methods to construct the mathematical model: domestic well-log analyses, monitoring-well information, aquifer slug tests, precipitation measurements, evapotranspiration estimates, streamflow measurements, stream-concentration measurements, land-use, soil-chemical analyses, septic-tank effluent chemical analyses, P sorption isotherms, statistical evaluation of literature values for pollutant transport parameters, GIS and others. Graduate and undergraduate students collected the data. The physical hydrologic model results are most sensitive to the snowmelt and the orographic effects on precipitation and evapotranspiration. The chemical-hydrology model is very sensitive to uncertainties in P sorption coefficient, P availability index, and P enrichment ratio (a measure of P in runoff sediments compared to immobile sediments). The watershed model was calibrated to measured streamflow rates and stream P concentrations. An excellent hydrologic calibration was achieved. The pollutant transport calibration was problematic. Nonetheless, results are appropriate for understanding the contribution of OWS to the watershed pollutant load. Modeling results indicate that OWS are not a primary source of P to Lake Dillon via the Blue River. This work has resulted in a better understanding of P chemical parameters required to simulate watershed-scale transport.