GROUNDWATER FLOW AND GEOCHEMISTRY OF TILL CONFINING UNITS OVERLYING BURIED GLACIAL AQUIFERS: EXAMPLES FROM THE DES MOINES AND WADENA LOBES IN MINNESOTA

Thousands of Minnesotans rely on buried glacial aquifers for drinking water, but data are lacking on aquifer sustainability and susceptibility. The USGS, in cooperation with Iowa State University, the MN Geological Survey, and MN Departments of Health and Natural Resources, initiated a 5-yr study in 2014 to characterize the flux (leakage) of groundwater through till confining units and their geochemistry at four sites in Minnesota. In the study discussed here, till of the Hewitt Formation (sandy loam) of the Wadena lobe (late Wisconsin) at the University of Minnesota’s Hydrogeology field camp (HFC) and till of the Good Thunder Formation (clay loam; pre-Illinoian and pre-Des Moines lobe) in Olivia were investigated. Data gathered included horizontal hydraulic conductivity (Kh) from slug tests, hydraulic gradients estimated from continuous hydraulic head data in the till units and underlying aquifers, and geochemistry (δ¹⁸O and δ²H, enriched ³H, and major anions) from groundwater and pore water samples. The HFC site lies in a forested area and consists of four piezometers in the till to depths of 200 feet (ft). Mean Kh is 2E-07 meters/second (m/s), the gradient is downward, and the estimated residence time in the till (based on Darcy’s Law) is 25 years. A ³H peak occurs 10 ft into the till at 115 ft, Cl/Br ratios are low in both pore water and groundwater, and Br, Cl, and NO₃-N concentrations are generally consistent in the profile. In contrast, the Olivia site lies in a small town (pop. 2,353) surrounded by agriculture, with five piezometers in the till to depths of 175 ft. Mean Kh is 3E-08 m/s, the gradient is downward, and the estimated residence time in the till is 130 years. Modern ³H is present to 60 ft (with pre-bomb concentrations below) and Cl/Br ratios and Cl concentrations for groundwater samples suggest anthropogenic influence. Groundwater was not recharged under glacial conditions at either site (based on values of δ¹⁸O and δ²H). In conclusion, the data suggest that our understanding of the flux of water and anthropogenic chemicals through till confining units must consider the till properties as well as land use above them. Ongoing work characterizing major ion species will help better define the extent of anthropogenic influence in the confining units and the underlying aquifers.