An Assessment of Nitrate Occurrence and Transport in Washoe Valley, Nevada

Abstract

Using an unsewered subdivision in western Nevada as a field site, the occurrence and transport of nitrate from septic tanks was first studied using nitrogen and nitrate mass balance calculations, and was then simulated by using a solute transport model (MT3D) for two different input scenarios. The results from the transport simulations are compared with the results from the mass balance calculations, which predict that average nitrate concentrations are rarely above the federal drinking water standard of 10 mg/l NO 3 -N in the study area. The simulation using a uniform nitrogen-to-nitrate conversion ratio provides greater detail of nitrate behavior, but results are not compatible with observed values in portions of the study area. Hydrogeochemical data suggest that the study area is characterized by zones of contrasting redox conditions and associated contrasts in nitrogen speciation. These variable geochemical conditions provide the basis for regionalizing nitrogen→nitrate conversion ratios in the simulations. Resulting simulations are improved, and agree better with observed field data. We also address the capabilities and limitations of transport simulations. The limitations of simulations originated from the incomplete input information related to field conditions even though the adopted models (MODFLOW and MT3D) are robust themselves.