Nitrate and Chloride Loading to Groundwater from an Irrigated North‐Central U.S. Sand‐Plain Vegetable Field

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Abstract
Groundwater pollution and associated effects on drinking water have increased with the expansion of irrigated agriculture in north-central U.S. sand plains. Controlling this pollution requires an ability to measure and predict pollutant loading by specific agricultural systems. We measured NO3 and Cl loading to groundwater beneath a Wisconsin central sand plain irrigated vegetable field using both a budget method and a new monitoring-based method. By relying on frequent monitoring of shallow groundwater, the new method overcomes some limitations of other methods. Monitoring-based and budget methods agreed well, and indicated that loading to groundwater was 165 kg ha−1 NO3–N and 111 kg ha−1 Cl for sweet corn (Zea mays L.) in 1992, and 228 kg ha−1 NO3–N and 366 kg ha−1 Cl for potato (Solanum tuberosum L.) in 1993. Nitrate N loading was 56 to 60% of available N, or 66 to 70% of fertilizer N. Sweet corn NO3 loading was about typical for this region, but potato NO3 loading was probably 50% greater than typical because heavy rains provoked extra fertilizer application. Our results imply that typical NO3–N loading would be 119 kg ha−1 for sweet corn and 203 kg ha−1 for potato, even with strict adherence to University Extension fertilizer recommendations. To keep average groundwater NO3–N within the 10 mg L−1 U.S. drinking water standard, each irrigated vegetable field would need to be offset by five to eight times as much land supplying NO3–free groundwater recharge. Copyright © 2001. American Society of Agronomy, Crop Science Society of America, Soil Science Society . Published in J. Environ. Qual.30:1176–1184.