Pesticide and Nitrate Trends in Domestic Wells where Pesticide Use Is Regulated in Fresno and Tulare Counties, California.

The California Department of Pesticide Regulation initiated regulations on pesticide use in 1989 to mitigate groundwater contamination by atrazine [6-chloro--ethyl-'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and subsequently for simazine (6-chloro-,'-diethyl-1,3,5-triazine-2,4-diamine), diuron ['-(3,4-dichlorophenyl)-,-dimethylurea], bromacil [5-bromo-6-methyl-3-(1-methylpropyl)-2,4(1,3)-pyrimidinedione], and norflurazon [4-chloro-5-(methylamino)-2-[3-(trifluoromethyl)phenyl]-3(2)-pyridazinone]. Annual water samples from 2000 to 2012 were obtained from domestic wells in Fresno and Tulare counties in regulated areas designated either as leaching groundwater protection areas (GWPAs), where residues move downward in percolating water, or runoff GWPAs, where residues move offsite in rain or irrigation runoff water to sensitive sites such drainage wells. Concentrations decreased below the reporting limit, so maximum likelihood estimation methodology for left-censored data was used. Decreasing trends in concentration were measured in both GWPA designations for simazine, its breakdown products desisopropyl atrazine (ACET, 2-amino-4-chloro-6-ethylamino--triazine) and diamino chlorotriazine (DACT, 2,4-diamino-6-chloro--triazine), and diuron. Bromacil crop use was predominant in runoff GWPAs, where decreases over time were also measured. In contrast, increased trends were observed for norflurazon and its breakdown product desmethyl norflurazon [DMN, 4-chloro-5(amino)-2-(α,α,α trifluorometa-tolyl] in runoff GWPAs. Use of simazine, diuron, and bromacil was regulated before norflurazon, so patterns of detection represent a shift to use of unregulated products. For NO, 22 of 67 wells indicated linear decreases in concentration coinciding with decreases in pesticide residues in those wells. Concentration of ACET, DACT, diuron, and NO in well water was two to five times greater when located in runoff GWPAs. Greater amounts of herbicide were applied to crops grown in runoff GWPAs, but high concentrations in runoff water entering ponds or drainage wells could also be a factor for increased well water concentration. Initial regulatory measures appear to have been effective in reducing groundwater concentrations, but continued monitoring is needed to evaluate changes made to the regulatory approach in 2004.