High-resolution simulation of basin-scale nitrate transport considering aquifer system heterogeneity

Nitrate contamination presents a growing threat to many groundwater basins relied upon for drinking water. This study combines geostatistical techniques, parallel computing of fl ow simulation, and particle tracking to develop realistic nitrate loading and transport scenarios. The simulation scenarios are patterned after the rapidly urbanizing Llagas groundwater subbasin in the south San Francisco Bay area of California. In the Llagas subbasin, groundwater is the sole municipal water supply. A key component of this study is the development of a highly resolved model of the heterogeneity in the aquifer system using a new geostatistical technique for simulating hydrofacies architecture that can incorporate uncertain or “soft” data, such as well driller logs. Numerical simulations of nitrate transport indicate the degree to which a heterogeneous conceptual model can account for dispersion relative to a conventional homogeneous model assuming typical dispersivity coeffi cients. The heterogeneous model transport results are found to be consistent with observed nitrate contamination patterns and depth distribution as well as groundwater-age trends with depth. The model provides a realistic test-bed for prediction of future nitrate concentrations, including the time frame for potential nitrate impacts to deep wells, given that geochemical data indicate that denitrifi cation is not likely to occur.