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Title: Data related uncertainty in near-surface vulnerability assessments for agrochemicals in the San Joaquin Valley

Author
item LOAGUE, KEITH - Stanford University
item BLANKE, JAMES - Rmc Water And Environment, Inc
item MILLS, MELISSA - Stanford University School Of Medicine
item DIAZ-DIAZ, RICARDO - University Of Las Palmas De Gran Canaria
item Corwin, Dennis

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/2011
Publication Date: 12/1/2012
Citation: Loague, K., Blanke, J.S., Mills, M.B., Diaz-Diaz, R., Corwin, D.L. 2012. Data related uncertainty in near-surface vulnerability assessments for agrochemicals in the San Joaquin Valley. Journal of Environmental Quality. DOI:10.2134/jeq2011.0443.

Interpretive Summary: Many highly productive agricultural areas use a wide variety of agricultural chemicals or agrochemicals (e.g., fertilizers, pesticides, amendments, etc.) that are to a great extent responsible for the high level of crop productivity. However, when used year-after-year these agrochemicals can pose a threat to groundwater due to leaching. A GIS-based regional-scale model was developed to assess groundwater vulnerability for a major agricultural region of California, the San Joaquin Valley (SJV). The assessment of groundwater vulnerability for the SJV to 32 pesticides showed high vulnerability to 1,2-Dichloropropane and Tebuthiuron, and moderate vulnerability to Prometon, DBCP, and EDB. Maps of the entire SJV were created that show areas of greatest potential groundwater vulnerability. The reliability of these maps is shown in associated maps of uncertainty, which indicate to what level of confidence the vulnerable areas can be trusted. The GIS-based model can be applied at any location where groundwater is suspected to be threatened by the use of agrochemicals. This information is an extremely valuable tool for policymakers, producers, and ag consultants for the regulation and responsible use of these chemicals to minimize their detrimental environmental impact on groundwater resources in the SJV. California Water Resources Control Board, California Department of Food and Agriculture, extension specialists, and Natural Resource Conservation Service field staff are the beneficiaries of these groundwater vulnerability maps for the SJV.

Technical Abstract: Precious groundwater resources across the USA have been contaminated due to decades-long nonpoint-source applications of agricultural chemicals. Assessing the impact of past, ongoing, and future chemical applications for large-scale agriculture operations is timely for designing best-management practices to prevent subsurface pollution. Presented here are the results from a series of regional-scale vulnerability assessments for the San Joaquin Valley (SJV). Two relatively simple indices, the retardation and attenuation factors, are used to estimate subsurface vulnerabilities based upon the chemical properties of 32 pesticides and the variability of both soil characteristics and recharge rates across the SJV. The uncertainties inherit to these assessments, derived from the uncertainties within the chemical and soil data bases, are estimated using first-order analyses. The vulnerability maps generated for the SJV show considerable variations in mobility / leaching potential. The data related uncertainties for some vulnerability assessments are significant, identifying limitations within the underlying data.