Study 318: Cooperative Groundwater Monitoring in Region 3 with the Central Coast Regional Water Quality Control Board
Abstract
This report summarizes the findings of Study 318: groundwater sampling from water wells located in areas within the jurisdiction (Region 3) of the Central Coast Regional Water Quality Control Board. For this study, 39 unique water wells located in three different groundwater basins spanning three counties in Region 3 were sampled over two sampling events in 2019: 17 wells in the Salinas Valley Groundwater Basin in Monterey County; 19 wells in the Gilroy-Hollister Valley Groundwater Basin in San Benito and Santa Clara counties; and three wells in the Pajaro Valley Groundwater Basin in northern Monterey County. All groundwater samples were analyzed by the California Department of Food and Agriculture’s Center for Analytical Chemistry for 69 pesticide active ingredients (AIs) and seven degradation products. Overall, only three pesticide AIs and one degradation product were detected in the analyzed groundwater samples. No pesticide AIs or degradation products were detected in 19 of the 39 sampled wells. In particular, the chlorthal-dimethyl (DCPA) degradation product 2,3,5,6tetrachloroterephthalic acid (TPA) was detected in 20 wells with concentrations ranging from 0.072 to 27.6 parts per billion (ppb). All measured concentrations of TPA were below its health-protective drinking water level of 2,500 ppb. The other DCPA degradation product monomethyl tetrachloroterephthalate (MTP) was not detected in any well. Bromacil was detected in one well in the Salinas Valley Groundwater Basin at a concentration of 0.054 ppb. The bromacil detection was measured in a well located in a Ground Water Protection Area (GWPA) where bromacil is regulated as a restricted-use pesticide for agricultural, outdoor industrial, and outdoor institutional uses to protect groundwater. The bromacil detection was also significantly below its human health reference level of 197 ppb. Since the detections of TPA and bromacil were less than their respective health reference levels, no follow-up investigation of these wells by the California Department of Pesticide Regulation (DPR) is necessary at this time. As required by Food and Agricultural Code (FAC) § 13152(a)(1), DPR will continue to conduct future groundwater monitoring of DCPA, MTP, and TPA in areas of Region 3 where DCPA is used.
Additionally, a trace concentration of mefenoxam/metalaxyl was detected in one well in the Pajaro Groundwater Basin and a trace concentration of tebuthiuron was detected in one well in the Hollister-Gilroy Valley Groundwater Basin. A ‘trace’ detection is defined as a measured concentration between a chemical’s respective method detection limit and DPR’s reporting limit of 0.05 ppb. Since the trace concentrations of mefenoxam/metalaxyl and tebuthiuron are both below DPR’s reporting limit, no follow-up investigation of these wells by DPR is also necessary at this time.