Detection of Atrazine, Simazine, and Their Breakdown Products in Public Water Supply Wells

Atrazine and simazine are pre-emergence herbicides that are known to contaminate ground water from normal agricultural use. According to a recent U.S. Environmental Protection Agency (EPA) re-registration eligibility decision for atrazine, the breakdown products of atrazine and simazine were determined to be as toxic as the parent chemical. Consequently, EPA concluded that the concentrations of parent and breakdown triazine residues in a well sample should be summed and that value compared to established health standards. Atrazine, simazine, and their breakdown products have been frequently detected in wells sampled by the Environmental Monitoring Branch, Department of Pesticide Regulation (DPR), California EPA. Most wells sampled by DPR staff were domestic, single family wells but some small public water supply (PWS) wells have also been sampled and found to contain triazine residues. The California Department of Public Health (CDPH) requires public water agencies to include atrazine and simazine in their sampling schedule because they are state and federally regulated potential contaminants. Public water agencies are not required to sample for the triazine breakdown products because they have not yet been included in established health standards. DPR conducted this study to compare the presence of parent atrazine and simazine residues to breakdown product residues in PWS wells. DPR sampled PWS wells that had previous detections of dibromochloropropane (DBCP) in areas of high simazine use. DBCP is a banned agricultural soil fumigant that was widely detected in drinking water wells beginning in 1977. Residues of atrazine, simazine, or their breakdown products deethyl-atrazine (DEA), deethyl­ simazine (ACET), or diamino chlorotriazine (DACT) were detected in 15 of 49 PWS wells sampled in eastern Fresno and Tulare Counties. Parent residues were present in 3 of the 15 wells whereas the breakdown products were detected in all of the 15 wells. When compared to previous results from domestic wells, the breakdown products occurred more frequently in the absence of parent chemical in the PWS wells. The higher detection frequency is likely due to the deeper depth of PWS wells in comparison to domestic wells. The greater travel time for residues to reach these water supply wells facilitates conversion of the parent to breakdown products. With respect to established health standards for triazine herbicides, the total residue detected in each well was below California’s current atrazine maximum contaminant level (MCL) of one microgram per liter (μg/L). Although this study surveyed only a small sample of the PWS wells in the Fresno and Tulare county area, factors that contributed to a greater chance of detecting residues in a well were location in areas with greater reported use of simazine, previous detection of DBCP at relatively high concentrations, and sampling of PWS wells that were shallower in total borehole depth. Sampling over time will be required to establish potential temporal trends in concentrations, especially with respect to the concentration of the breakdown products.