Summary of well water sampling in California to detect pesticide residues resulting from nonpoint-source applications

This report summarizes well sampling protocols, data collection procedures, and analytical results for the presence of pesticides in ground water deve loped by the California Department of Pesticide Regulation (DPR). Specific well sampling protocols were developed to meet regulatory mandates of the P esticide Contamination Prevention Ad (PCPA) of 1986 and to provide further understanding of the agronomic, chemical, and geographic factors that contr ibute to movement of residues to ground water. The well sampling data have formed the basis for the DPR's regulatory decisions, For example, a samplin g protocol, the Four-Section Survey, was developed to determine if reported detections were caused by nonpoint-source agricultural applications, a det ermination that can initiate formal review and subsequent regulation of a p esticide. Selection of sampling sites, which are primarily rural domestic w ells, was initially based on pesticide use and cropping patterns. Recently, soil and depth-to-ground water data have been added to identify areas wher e a higher frequency of detection is expected, In accordance with the PCPA, the DPR maintains a database for all pesticide well sampling in California with submission required by all state agencies and with invitations for su bmission extended to all local and federal agencies or other entities. To d ate, residues for 16 active ingredients and breakdown products have been de tected in California ground water as a result of legal agricultural use. Re gulations have been adopted for all detected parent active ingredients, and they have been developed regardless of the level of detection.