INVESTIGATING GROUNDWATER RECHARGE TO BURIED VALLEY AQUIFERS IN MINNESOTA USING PORE WATER GEOCHEMISTRY IN TILL AQUITARDS

Buried-valley aquifers in glacial sediments provide drinking water to thousands of Minnesota residents. However, long-term water-resource planning is impeded by the lack of groundwater recharge data through overlying aquitards. Since 2014, the USGS Minnesota Water Science Center (MNWSC) has been characterizing hydraulic and geochemical properties of till aquitards at two sites in central and northeastern Minnesota to estimate recharge to buried aquifers. One site is located in, and considered representative of the hydrogeologic conditions present in, the Des Moines Lobe and the second in the Superior Lobe. Nineteen piezometers were installed in four nests, with mostly 3-ft-screened intervals in aquitards both above and below the buried aquifer to a maximum depth of 340 feet. We hypothesized initially that water moves slowly through the aquitard and sought evidence that stable isotopes in pore water would show remnant post-glacial meltwater and that Cl and Br in pore water would show no evidence of surface contamination. Pore water was extracted from continuous core by a hydraulic press and stainless steel capsule system at the California WSC in San Diego. Visual inspection of the loamy Des Moines Lobe till core at the L-1/L-2 sites suggested it was suitable for squeezing pore water, whereas the sandy loam Superior Lobe till at the C-1/C-2 sites was too coarse-grained for squeezing. Sixteen depth-specific samples (6-inch-long sections of 3-inch diameter core) underwent pressures of 5,200–6,500 psi, netting from 6 to 12.8 mL of fluid per sample. Isotope results did not show older glacial pore water with depth, but instead showed values more similar to modern precipitation. The L-1 site showed uniform isotope values from 66 to 120.5 ft., with mean δ¹⁸O and δ²H values of -10.43‰ (S.D.=0.13) and -70.12‰ (S.D.=0.85), respectively. Site L-2 also showed uniform isotope values from 29 to 114 ft., with mean δ¹⁸O and δ²H values of -9.31‰ (S.D.=0.24) and -61.15‰ (S.D.=1.55), respectively. Cl concentrations ranged from 33 to 294 mg/L at L-1 and 24 to 43 mg/L at L-2 and Cl/Br mass ratios ranged from 65 to 1362 at both sites, suggesting inputs from sewage, agriculture, or road salt. Thus, our hypothesis is rejected. Water from the ground surface has recently travelled to the bottom of the aquitard and is likely recharging the aquifer.