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

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.