Nitrogen from agriculture is known to be a primary source of groundwater NO-N. Research was conducted in a northeastern Missouri watershed to assess the impact of cropping systems on NO-N for a loess and fractured glacial till aquifer underlying claypan soils. Three cropped fields with 10 yr of similar management were each instrumented with 20 to 25 monitoring wells, 3 to 15 m in depth, in 1991 to 1992. Wells were sampled and analyzed for NO-N at least annually from 1991 to 2004. Initial NO-N concentrations were variable, ranging from undetectable to >24 mg L but averaged 7.0 mg L. Groundwater NO-N was significantly higher in Field 3, probably the result of concurrent applications of manure and N fertilizer before 1980. Overall changes in NO-N levels in Fields 1 and 2 were generally small; however, NO-N levels for Field 3 have decreased an average of 0.28 mg L yr. Excessive loading of N into the matrix of the glacial till may have had a long-term impact on NO-N for this field. Despite the presence of dissolved O in the aquifer, evidence of denitrification in some upper-landscape groundwater wells was found. The greatest decreases in NO-N concentration occurred as groundwater moved through an in-field tree line or through a riparian zone. While overall conclusions were complicated by the long-term impact of past management, the capacity of the till to buffer changes, hydrogeologic variability found among wells, and the activity of biological processes, we conclude that cropping practices during this study did not increase glacial till NO-N.
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