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 activities on in-stream water quality in the Lower Truckee River, Nevada, were assessed through a detailed program of monitoring and computer simulation. An agricultural diversion and return-flow were monitored in great detail to determine mass loading rates of nutrients from agriculture in the area. Once characterized, the cumulative impacts of agricultural diversions and return-flows were evaluated using the Water Quality Assessment Program (WASP) to model nitrogen, phosphorus, periphyton, and dissolved oxygen. Monitoring showed that a significant proportion of the water diverted for agricultural purposes returned to the river as surface point return-flow (estimated at 13.9 percent +/- 0.1 percent), and as groundwater diffuse return-flow (estimated at 27 percent +/- 6 percent). Modeling efforts demonstrated the significant effect of assumed regional groundwater quality (nitrate) upon predicted periphyton growth and associated diel fluctuations of dissolved oxygen.