Groundwater quality beneath irrigated vegetable fields in a north-central US sand plain

The dramatic expansion of irrigated agriculture since about 1970 in the north-central USA has been accompanied by NO3 and pesticide pollution of groundwater. The expansion has been concentrated in areas with sandy soils and shallow water tables, such as the Wisconsin central sand plain. In some parts of this sand plain, most wells contain detectable pesticide residues and NO3-N concentrations that exceed the 10 mg/L U.S. drinking water maximum contaminant level (MCL). To evaluate the effects on groundwater quality of this agricultural system, we monitored solutes 23 times during a 2-yr period in the upper 3 m of the aquifer beneath and immediately upgradient of four irrigated vegetable fields. Groundwater beneath fields had significantly greater concentrations of most solutes and lower pH than upgradient groundwater. Especially pronounced were Ca, CI, K, Mg, and NO3 differences, with concentrations 5 to 26 times greater under fields. Nitrate N concentrations averaged 21 mg/L under fields, compared with 1 mg/L upgradient. Pesticide residues were ubiquitous beneath fields, and generally persisted for many months after application. Pesticide concentrations often exceeded Wisconsin preventive-action limits (PALs), but seldom exceeded federal MCLs. Even when agricultural management approximated best management practice (BMP) recommendations, the NO3 concentration beneath these fields approached double the MCL, indicating a need for new approaches to control agricultural groundwater pollution.