Modeling the impact of subsurface nutrient flux on water quality in the Lower Truckee River, Nevada

The impacts of regional groundwater quality and local agricultural activiti es on in-stream water quality in the Lower Truckee River, Nevada, were asse ssed through a detailed program of monitoring and computer simulation. An a gricultural diversion and return-flow were monitored in great detail to det ermine mass loading rates of nutrients from agriculture in the area. Once c haracterized, the cumulative impacts of agricultural diversions and return- flows were evaluated using the Water Quality Assessment Program (WASP) to m odel nitrogen, phosphorus, periphyton, and dissolved oxygen. Monitoring sho wed that a significant proportion of the water diverted for agricultural pu rposes returned to the river as surface point return-flow (estimated at 13. 9 percent +/- 0.1 percent), and as groundwater diffuse return-flow (estimat ed at 27 percent +/- 6 percent). Modeling efforts demonstrated the signific ant effect of assumed regional groundwater quality (nitrate) upon predicted periphyton growth and associated diel fluctuations of dissolved oxygen.