ESTIMATING RECHARGE TO BURIED-VALLEY AQUIFERS IN MINNESOTA: EARLY HYDROLOGIC AND GEOCHEMICAL SURPRISES IN OVERLYING TILL AQUITARDS

Buried-valley aquifers set within glacigenic sequences provide drinking water to thousands of Minnesota residents. Because the aquifers are finite in extent, not knowing the source and amount of recharge to these aquifers hinders long-term water-resource planning. Since 2014, the USGS Minnesota Water Science Center has been investigating buried-valley aquifers overlain by till aquitards within the Des Moines and Superior lobes in central and northeastern Minnesota. In summer 2015, we obtained and described till core, installed 19 piezometers to depths of 104 m, and measured hydraulic heads and estimated till K with slug tests, ultimately to calculate vertical recharge flux to the underlying aquifers. We also analyzed pore water and groundwater for anions and stable isotopes that might suggest anthropogenic impacts. Surprisingly, the sites differ hydrologically and the geochemistry data appear to be at odds with the hydrology. At the Des Moines Lobe site in central Minnesota, the aquitard consists of a silty, sandy till aquitard of the Villard Member of the New Ulm Formation to a depth of 36 m. Downward-directed, vertical hydraulic gradients occur in the aquitard (geometric mean K=1 x 10^-9 ms^-1) suggesting recharge to the aquifer. At the Superior Lobe site, 13 m of outwash is underlain by the sandy, stony till - both of the Cromwell Formation (till geometric mean K=3 x 10^-7 ms^-1) to a depth of 52 m. Hydraulic gradients here suggest an upward component of flow and no vertical recharge to the aquifer. Geochemical data are at odds with hydrology at both sites. At a Des Moines Lobe site, stable isotopes in porewater and groundwater show modern isotopic values with depth, with mean δ¹⁸O and δ²H values of -9.31‰ and -61.15‰, respectively. Concentrations of Cl and Cl:Br mass ratios in the aquitard suggest penetration of anthropogenic impacts into the aquitard despite its low K value. We suspect fractures may be present. At the Superior Lobe site, isotope values are consistently 2‰ more negative; however Cl, NO₃-N concentrations, and Cl:Br mass ratios decline from the surface, suggesting downward, not upward, groundwater flow in the aquitard. We hope to solve these apparent contradictions with continued hydraulic head monitoring, regional hydrogeologic analysis, groundwater modeling, and age dating of groundwater in the aquitard.