Variability of Nitrate Loading and Determination of Monitoring Frequency for a Shallow Sandy Aquifer, Arena, Wisconsin

Project Background: Nitrate contamination has long been known to be a problem in Wisconsin’s sandy aquifers with shallow depth to groundwater (Jackson and others, 1987). Recent data suggest that 10-20 percent of domestic wells in agricultural counties across the state exceed the drinking water standard of 10 mg/l NO₃ -N (Kraft, 1994); in Dane County 24 percent of rural wells exceed this standard (Bridson and others, 1994). Shallow, sandy aquifers are especially vulnerable to nitrate contamination due to rapid infiltration and the limited attenuation capability of these coarsetextured soils. In areas where sandy soils are used for crop production, nitrate contamination is common. For example, 34 percent of wells in the Lower Wisconsin River Valley exceed the 10 mg/l NO₃-N standard (Cates and Madison, 1992). Management of agricultural nitrogen inputs is becoming a growing concern; the Wisconsin Department of Agriculture, Trade, and Consumer Protection (DATCP) is considering new regulations aimed at managing nitrogen applications. In order to determine how various agricultural management practices impact groundwater, we first need to identify the temporal variability in nitrate concentrations reaching the saturated zone. Most groundwater monitoring projects assume that monthly or quarterly samples are adequate to characterize nitrate concentrations in groundwater, yet this assumption has not been tested.

Purpose and Scope: The objectives of this project were 1) to determine the temporal variability in nitrate concentrations in shallow, sandy aquifers, 2) to define the relationship between recharge and groundwater nitrate concentrations, and 3) to develop recommendations concerning monitoring frequency for such aquifers. Specifically we hoped to examine the distsribution of nitrate and cloride across hydrographs generated by rainfall or other climatic events.