Groundwater Vulnerability Assessments for the San Joaquin Valley
Precious groundwater resources across the U.S. have been contaminated due to decade-long nonpoint source applications of agricultural chemicals. Assessing the impact of past, ongoing, and future chemical applications for large-scale agriculture operations is indeed timely for designing best management practices to reduce subsurface pollution. Presented here are results from a series of regional-scale groundwater vulnerability assessments for the San Joaquin Valley. Two relatively simple indices, the retardation and attenuation factors, are used to estimate groundwater vulnerabilities based upon the chemical properties of 32 pesticides (e.g., EDB, DBCP, Atrazine, Diuron) and the variability of both soil characteristics (e.g., water content, fraction of organic carbon) and recharge rates across the valley. The uncertainties inherit to these vulnerability assessments, derived from the uncertainties within the chemical, soil, and, recharge data, was estimated using first-order analyses. The groundwater vulnerability maps generated for the entire San Joaquin Valley, based upon deterministically-crisp applications of retardation and attenuation indices, show considerable variations in leaching potential for the 32 pesticides. In some cases the uncertainty within the vulnerability assessments is significant, thereby identifying serious limitations within the data. Assuming that these data shortfalls can be satisfied, revised estimates of groundwater vulnerability, in the spirit of those shown here, will be useful to those in the decision-management arena charged with regulating the future use of agriculture chemicals in the San Joaquin Valley. Furthermore, the areas/pesticides shown to have the greatest leaching potential, based upon a simple screen/rank approach, could be targeted for future measure and model investigation, driven by risk-adverse motives, with detailed field observations and comprehensive physics-based simulation (forensic or predictive) focused on individual events.