We determined the impacts of irrigated vegetable agriculture on ground water quality in a 29 km(2) portion of the Wisconsin central sand plain (WCSP). Vegetable fields cover 22% of the study area and are used for growing potatoes, sweet corn, snap beans, peas, field corn, and soybeans, We found that contaminant plumes from fields underlay about 54% of the area. Plumes were 3 to >16 m thick and in places occupied the entire aquifer thickness. Impacted ground water retained the chemical signature observed under fields, and compared with unimpacted ground water, had elevated NO3-N (median 13.7 mg L-1 impacted vs. 0.5 for unimpacted) and Cl (median 23 vs. 2.0 mg L-1) and 1.4 to 2.5 times the Ca, Mg, K, Na, and SO4. NO3-N detections exceeded 10 mg L-1 in 58% of impacted monitoring wells compared with none of the unimpacted wells. Evidence of denitrification was observed, but rarely, Residues of the pesticides alachlor, atrazine, carbofuran, metolachlor, and metribuzin were detected, but usually at summed concentrations of <1 mu g L-1 outside of fields. The maximum observed sum of pesticide residues was 8 mu g L-1. Not all potential pesticide residues were detectable by the analytical method. Agricultural impacts limit the aquifer's value as a source of potable water, because wells are unable to tap unimpacted ground water through much of the area. Results of this study are generally applicable to other humid sandy areas in the north-central United States and elsewhere, Impacts might be more severe elsewhere because the agricultural Land use is frequently denser and ground water flowpaths are longer.
