Legal Agricultural Use Determination for DCPA Degradate Detections in California
Abstract
Chlorthal-dimethyl (DCPA) is the active ingredient of a pre-emergent herbicide commonly known by the trade name Dacthal®. It is primarily used for control of annual grasses and certain broadleaved weeds in various fruit and vegetable crops and ornamental turf. DCPA was originally registered with the United States Environmental Protection Agency (US EPA) in 1958 and has been used in California since the late 1960s. DCPA has two major degradation products: monomethyl tetrachloroterephthalate (MTP) and 2,3,5,6-tetrachloroterephthalic acid (TPA). The California Department of Pesticide Regulation (DPR) conducted groundwater monitoring between 1990 and 1997 in different areas of the state where DCPA had been used, looking specifically for DCPA, MTP, and TPA. None of the laboratory analyses of the collected samples yielded a confirmed detection of DCPA or MTP. However, the analyses did confirm detections of TPA in samples collected in several counties, including five detections in the Salinas Valley of Monterey County and six detections in a region that spans Arroyo Grande in San Luis Obispo County to Lompoc in Santa Barbara County. To assess current levels of these chemicals, sampling was conducted during 2017 in the same general areas of Monterey, San Luis Obispo, and Santa Barbara counties where the previous TPA detections were found. During the winter and spring of 2017, thirteen unique wells were sampled in the Salinas Valley for DCPA, MTP, and TPA. No detections of DCPA were found in any of the wells above DPR’s reporting level of 0.05 parts per billion (ppb). However, two wells contained MTP concentrations of 0.056 and 0.13 ppb and five wells contained TPA concentrations in the range of 0.916 to 101 ppb. The 101 ppb concentration exceeded the lifetime Health Advisory Level (HAL) of 70 ppb set for DCPA by the US EPA and adopted by DPR as a Reference Health Level for TPA. During the fall of 2017, twenty-three wells were sampled in areas located in southern San Luis Obispo County and Santa Barbara County. Again, no detections of DCPA were found. However, three wells contained MTP concentrations in the range of 0.063 to 0.101 ppb and thirteen wells contained TPA concentrations in the range of 0.121 to 159 ppb. Two wells, in particular, contained TPA concentrations (133 and 159 ppb) that exceeded the lifetime HAL of 70 ppb. The wells with the three highest detections (66.5, 133, and 159 ppb) were located individually in three consecutively adjacent sections in the east to west direction.
From review of data describing the environmental fate properties of DCPA and its degradation products, DCPA was found to be neither mobile nor persistent in soil. MTP is considered highly mobile in soil but not very persistent. However, TPA was determined to be both mobile and persistent in soil and considered to have a high potential to contaminate groundwater. As such, TPA has been detected in groundwater in eight counties in California and in groundwater in several other states, including Washington, Oregon, Idaho, New York, and Michigan. Given the historical use of DCPA in California and the locations of TPA detections in groundwater relative to those use patterns, it is the conclusion of this report that these data satisfy the criteria to make a Legal Agricultural Use determination that the detections of TPA found in wells between 1991 and 2017 are due to legal agricultural use of the parent active ingredient DCPA in those regions.
DISCLAMIER: The mention of commercial products, their source, or use in connection with material reported herein is not to be constructed as an actual or implied endorsement of such product.