Study 300: Groundwater Monitoring for DCPA and Its Degradation Products MTP and TPA
Abstract
Chlorthal-dimethyl (DCPA) is the active ingredient of a pre-emergent herbicide sold in California under the tradename Dacthal®. It is primarily used for control of annual grasses and certain broadleaved weeds in various fruit and vegetable crops and in ornamental turf. DCPA has two major degradation products: monomethyl tetrachloroterephthalate (MTP) and 2,3,5,6tetrachloroterephthalic acid (TPA). This report summarizes the findings of Groundwater Study 300 (Study 300): groundwater sampling from water wells in California for DCPA, MTP, and TPA. The motivation and preliminary field sampling plan for Study 300 were documented in the protocol: “Study #300: Protocol for Groundwater Monitoring of DCPA and its Degradates MTP and TPA” (Ruud, 2017). For this study, 45 unique water wells located in six different groundwater basins/sub-basins spanning five counties were sampled from January 2017 through April 2018: 13 wells in the Salinas Valley Groundwater Basin in Monterey County; 18 wells in the Santa Maria River Valley Groundwater Basin in San Luis Obispo and Santa Barbara counties; five wells in the Santa Ynez River Valley Groundwater Basin in Santa Barbara County; one well in the Oxnard Aquifer Sub-basin in Ventura County; one well in the Pleasant Valley Groundwater Basin in Ventura County; and seven wells in the Coachella Valley Groundwater Basin in Riverside County. Forty-three wells were sampled once and two were sampled twice for a total of 47 groundwater samples from the 45 wells. All groundwater samples were analyzed by the California Department of Food and Agriculture’s Center for Analytical Chemistry. None of the sampled wells contained detections of DCPA above the reporting limit of 0.05 parts per billion (ppb). Five wells contained detections of MTP with concentrations ranging from 0.056 to 0.13 ppb. Nineteen wells contained detections of TPA with concentrations ranging from 0.121 to 159 ppb. In particular, one well near the city of Greenfield in the Salinas Valley contained a TPA detection of 101 ppb and three wells around the city of Guadalupe in the Santa Maria River Valley Groundwater Basin contained TPA detections of 66.5, 133, and 159 ppb. All detections were well below the health-protective drinking water level of 2,500 ppb set for TPA by the Office of Environmental Health Hazard Assessment (OEHHA, 2018).