Study 240: Monitoring Ground Water in Sections with Reported Detections Outside Existing Ground Water Protection Areas
Abstract
From January 2007 to June 2008, Environmental Monitoring Branch (EM) staff sampled 176 wells in 11 counties to: (1) determine if previously detected pesticide residues were due to legal agricultural use (LAU), (2) obtain well sampling data on the presence and distribution of pesticide residues in areas outside existing GWPA sections, and (3) identify current CALVUL model factors that may need to be modified or incorporated into the model to encompass all areas of the State that might be vulnerable to pesticide contamination of ground water. This study used a transect sampling design to determine whether previously detected pesticide residues are the result of LAU and provides additional data about what soil types, depths to ground water, and pesticide use patterns allow known pesticide contaminants to reach ground water in areas that are not captured by the current model. Sixty-eight of the 176 sampled wells had at least one of 11 different pesticide residues above its detection limit. All of these pesticide or degradate residues were of pesticides regulated by Title 3 California Code of Regulations (3 CCR) section 6800(a) except hexazinone, which was found in five wells.
The hexazinone detections were investigated further in 2008. A four-section field study (Z573) was conducted around two detections in a single section in Fresno County and an additional well with hexazinone residues was found (Nordmark, 2008). An LAU use determination was conducted and the residues were found not to pollute at the levels detected (Reardon, 2011). DPR has sampled wells near each of the other three wells in this study with hexazinone detections but none of these additional wells had hexazinone residues present.
All pesticides residues detected were at concentrations below known health levels. The maximum concentration for any pesticide residue was 2.14 ppb for DACT in a Fresno County well.
This report presents the results for monitoring ground water in sections with reported detections outside existing ground water protection areas (Study 240). These results include limited analysis. The complete analysis and modeling of the data will be included in a subsequent report when the revised statewide California Vulnerability (CALVUL) analysis and modeling is complete. EM staff are incorporating the results from this study into the CALVUL modeling program. Sampling results from this study will be used to complete 38 four-section surveys (Z-Studies) that DPR would have typically conducted, thus saving DPR resources and making results available more quickly. DPR also plans to use the data to make recommendations for the creation of 70 Ground Water Protection Areas (GWPA) based on verified detections.