HYDROGEOLOGIC INVESTIGATION OF GROUNDWATER FLOW IN TILL CONFINING BEDS OVERLYING GLACIGENIC AQUIFERS IN SOUTH-CENTRAL AND NORTH-CENTRAL MINNESOTA

Buried glacigenic aquifers supply thousands of Minnesotans with drinking water, but data are lacking on aquifer sustainability. In 2014, the USGS Minnesota Water Science Center, Minnesota Department of Health, Minnesota Department of Natural Resources, Minnesota Geological Survey, and Iowa State University, began a joint project to investigate the hydrogeology of till confining units and buried glacigenic aquifers to better understand the sustainability of water resources in the aquifers. Beginning in spring/summer 2017, field sites were established in Olivia in South-Central Minnesota (Des Moines lobe) and at University of Minnesota Hydrogeology Field Camp (HFC) in North-Central Minnesota near Akeley (Wadena lobe). At the Olivia site, a nest of eight piezometers was installed to a maximum depth of 230 feet (ft) – one in the surficial aquifer, six in the till aquitard, and one in the buried aquifer. Four piezometers were installed in the till aquitard at the HFC site – the deepest to 200 ft. Core was described to a depth of 229.5 ft at the Olivia site and to 200 ft at the HFC site to obtain hydrostratigraphic information. Although New Ulm Formation till comprises the top 20 ft, the confining bed at the Olivia site consists primarily of the Good Thunder Formation, a pre-Wisconsinan, silty to clayey loam till, to a depth of 179.5 ft. At the HFC site, sandy loam till of the Hewitt Fm. underlies 105 ft of outwash to a depth of 200 ft. Additional pre-Wisconsinan till units occur below this depth to bedrock at 475 ft. Hydraulic head is monitored continuously using pressure transducers at both sites and the data show a downward hydraulic gradient. Slug tests in the till show horizontal hydraulic conductivity (Kh) values of 3E-08 meters/second (m/s) (7E-03 feet/day (f/d)) for the Good Thunder Formation and 2E-07 m/s (4E-02 ft/d) for the Hewitt Formation. These values and the hydraulic head data will allow us to estimate average linear (groundwater) velocities and the vertical flux through the aquifer. Samples of pore water and groundwater were analyzed for δ¹⁸O and δ²H, enriched ³H, and anions to infer the age of groundwater. Cl concentrations are low at both sites and δ¹⁸O and δ²H data indicate that glacial-age groundwater is absent. Aquifer tests will be conducted at both sites in summer 2018 to estimate vertical K and leakage to the aquifer.