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 andylirel results far the presence of peslicides in ground water developed by the California Department of Pesticide Regulation (DPR). SpeciRr well sampling protocols were developed to meet regulatory mandates of the Pesticide Canlaminatian Prevention Act (PCPA) of 1986 snd to provide further understanding of the agronomic, chemical, and geographic faelors that contribute to movement of residues lo ground water. The well sampling data have formed the basis far the DPR's regulatory decisions. For example, a sampling protocol, the Four-Seclion Survey, was developed to determine if reported detertions were caused by nonpoint.saurce agricultural applications, a determinalion that can iniliate formal review and subsequent regulation of a pesticide. Selection of sampling sites, which are primarily rural domestic wells, was initially bssed on pesticide use and cropping patterns. Recenlly, sail and depth-to-ground water data have been added to idenlify areas where a higher frequency of detection is expeeled. In sccordsnce with Ihe PCPA, the DPRm maintains a database far all pesticide well sampling in California with submission required by all state agencies and with invitations for submission extended to all local and federal agencies or other entities. To date, residues for 16 active ingredients and breakdown products have been deteeted in California ground water as a result of legal agricultural use. Regulations have been adapled for all deterted parent active ingredients, and they have been developed regardless of the level of detection.