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The occurrence and distribution of selected pesticides and their metabolites were investigated through the collection of 837 water-quality samples from 303 wells across the Midwest. Results of this study showed that five of the six most frequently detected compounds were pesticide metabolites. Thus, it was common for a metabolite to be found more frequently in groundwater than its parent compound. The metabolite alachlor ethanesulfonic acid (alachlor- ESA; 2-[(2,6-diethylphenyl)(methoxymethyl)amino]-2- oxoethanesulfonic acid) was detected almost 10 times as frequently and at much higher concentrations than its parent compound alachlor (2-chloro-2',6'- diethyl-N-(methoxymethyl)acetamide). The median detectable atrazine (2-chloro-4-ethylamino-6- isopropylamino-s-triazine) concentration was almost half that of atrazine residue (atrazine plus the two atrazine metabolites analyzed). Cyanazine amide [2-chloro-4-(1-carbamoyl-1-methylethylamino)-6- ethylamino-s-triazine] was detected almost twice as frequently as cyanazine (2-chloro-4-ethylamino-6- methylpropionitrileamino-s-triazine). Results showthat information on pesticide metabolites is necessary to understand the environmental fate of pesticides. Consequently, if pesticide metabolites are not quantified, the effects of chemical use on groundwater qualitywould be substantially underestimated. Thus, continued research is needed to identify major degradation pathways for all pesticides and to develop analytical methods to determine their concentrations in water and other environmental media.