GROUNDWATER TRANSPORT IN THE WISCONSIN CENTRAL SANDS REGION

The Central Sands region of Wisconsin is an important agricultural area, with more than 30% of the land surface devoted to agricultural practices and over 1,200 high-capacity irrigation wells. The intensive agricultural nature of the region, combined with the high hydraulic conductivity of the surficial aquifer system, make the system highly vulnerable to nitrate contamination. Data from the University of Wisconsin-Stevens Point Center for Watershed Science and Education show that about 15% of potable wells in the area exceed the maximum contaminant level for nitrate as nitrogen. This is a major concern since drinking water in the region is fully sourced from groundwater.

To assess which areas are most vulnerable to nitrate contamination and to determine how changes in nitrate leaching will affect nitrate concentrations in groundwater, a groundwater modeling study is being conducted. In the study, we use the MODFLOW-NWT groundwater flow model of the Wisconsin Central Sands region, published by the USGS (Fienen et al., 2021), to study groundwater transport in central Wisconsin. MODPATH is then implemented to assess nitrate flow paths, estimate the potential residence time of nitrate within the aquifer, and identify zones where nitrate leaching may have long lasting consequences. MT3D is then implemented in the regional flow model and the results from the two methods (MODPATH & MT3D) are compared to assess the strengths and weaknesses of each approach when using a repurposed groundwater flow model to predict nitrate transport.

Our results show that while many modeled groundwater particles have short residence times (median age 15 years), a large proportion of these particles have longer residence times (25% with residence times longer than 50 years). Therefore, while changes in agricultural practices can help improve groundwater nitrate concentrations, nitrate leached to the system in the past will continue to influence nitrate concentrations in groundwater into the future. Use of the two transport methods (MODPATH & MT3D) provides a useful comparison of the information that can be provided by each method, while also revealing the advantages and disadvantages of repurposing regional scale flow models to investigate new questions.