The frequency of occurrence of alachlor and several other agrichemicals in water from private, rural domestic wells was determined for all counties in the United States where alachlor is sold. The three-stage, stratified, unequal probability selection procedure yielded water samples from 1430 wells in 89 counties. Of the estimated 6 million existing private domestic wells in the target area, less than 1% have detectable levels of alachlor. Similar occurrence frequencies were found for metolachlor and simazine. Atrazine was the most commonly detected pesticide in the alachlor use area with an occurrence frequency near 12%. Concentrations of all detected pesticides in rural well water are very low and are rarely expected to exceed any health-based standard. The occurrence of nitrate/nitrite, however, is more common. Over 50% of the wells in the alachlor use area have detectable levels of nitrate/nitrite. Nearly 5% exceed the 10 mg/L maximum contaminant level for nitrate/nitrite (expressed as total nitrogen). The occurrence of all chemicals is correlated with various measures of chemical use near the well and with measures of groundwater vulnerability. The likelihood of finding other agrichemicals in a well is significantly increased when nitrate and atrazine are also present.
