MODELING NITRATE TRANSPORT IN SPANISH SPRINGS VALLEY, WASHOE COUNTY, NEVADA

Rapid urbanization in the Spanish Springs Valley, north of Sparks, Nevada has lead to nitrate contamination of municipal wells in the area. As the population of Reno-Sparks grows, urbanization has extended into the valleys north of the metropolitan area causing a marked increasing in the population of Spanish Springs Valley over the last 30 years. Monitoring of Washoe County Department of Water Resources (WCDWR) municipal wells beginning in the late 1980s indicates rising nitrate levels (WCDWR 2001). This contamination has been linked to septic tank effluent (Seiler 1999). In 2000, the Nevada Department of Environmental Protection (NDEP) directed WCDWR to address the groundwater degradation (WCDWR 2001).

A groundwater flow and transport model was developed as part of the response to the NDEP directive. MODFLOW – 2000 and MT3D were used to construct the model. The model simulates a 36 year time period from 1979 to 2015. Nitrate accumulation in the valley has been a gradual process, with the rate of accumulation increasing as development increased. By starting the simulated time period close to the beginning of urbanization, the model better represents both the nitrate loading over time, and the spatial distribution of the nitrate contamination in the valley. Mass loading can be verified via comparison with observed nitrate values.

TPROGS (Carle 1999) was implemented to generate a spatial distribution of subsurface lithology. The generalized likelihood uncertainty estimator (GLUE) methodology was used to calibrate the model and characterize uncertainty (Beven and Binley 1992). This process creates a better representation of heterogeneity in the aquifer, which influences the transport of nitrate contamination.

This model will be used to implement several scenarios modifying sewer installations. The response of the nitrate plume to various scenarios will be used as a management tool by WCDWR.