Pesticides in Eastern North Carolina Rural Supply Wells: Land Use Factors and Persistence

Water samples were collected from 171 rural domestic well supplies in eastern North Carolina and analyzed for eight pesticides. Information on borehole depth, well-easing depth, distance to nearest pesticide mixing area, types of pesticides used, and distance to nearest field application was obtained for each site. Four herbicides [alachlor, 2-chloro-2′-6′diethyl-N-(methoxymethyl)-acetanilide; atrazine, 2-chloro-4-ethylamino-6-isopropylamino-s-triazine; metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide; trifluralin, α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine] were detected in the samples, with detection frequencies of 8.8, 8.2, 3.6, and 1.8%, respectively. About 15% of the samples contained at least one of these herbicides, with resampling indicating persistence throughout the year. Only alachlor concentrations were in excess of maximum contaminant levels (MCLs; 2.0 μg L⁻¹) or Health Advisory Levels (HALs; 0.4 μg L⁻¹) established by the U.S. Environmental Protection Agency (USEPA). Neither atrazine nor alachlor detection exhibited statistical correlation with well depth, although both were rarely detected in wells >100 feet deep. Atrazine concentrations and detection frequencies did not correlate with distance to nearest application site, while alachlor had a significantly greater detection frequency for wells further from the nearest application site. For nearly one-half of the wells with detectable atrazine and alachlor, there was no reported usage of either herbicide on the same farm during the previous three years, possibly indicating herbicide transport in groundwater or long times before degrading. No statistically significant relationships were observed between the presence of alachior or atrazine, and distance from the well to the nearest pesticide handling and storage area. Although inconclusive by itself, this indicates that at least some contamination originated from other than point-source spills. Nitrate-N concentrations in well water were poor predictors for atrazine and alachlor presence in this study.