SPATIAL VARIABILITY IN THE VERTICAL CONNECTIVITY OF TILL CONFINING BEDS: EXAMPLES FROM THE NEW ULM FORMATION IN CENTRAL MINNESOTA

Buried glacigenic aquifers of finite extent comprise an important water supply for Minnesota. Knowing the source of water for these confined aquifers and their contamination susceptibility is essential to evaluating sustainability. In 2014, the USGS Upper Midwest Water Science Center, in cooperation with Iowa State University and three State of Minnesota agencies, initiated a project to characterize buried glacigenic aquifers and properties of till confining beds (aquitards). Two piezometer nests were installed in till and an underlying glacigenic aquifer in the Des Moines lobe at Litchfield. Eleven piezometers were installed at LFO1 and LFO2 at depths between 20 and 162 ft. Nest LFO1 has 40 ft of glaciolacustrine sediment overlying 151 ft of unoxidized, loamy till of the New Ulm Formation, Villard Member (NVT). At the LFO2 nest located in the municipal well field, 16 ft of oxidized, loamy NVT overlies 97 ft of unoxidized NVT. Fractures were observed in till core at both nests. The nests are only 2200 ft apart, yet the properties and hydraulic response of the confining bed vary between them. During daily pumping of city wells, hydraulic heads in piezometers in till at LFO2 were unresponsive, whereas hydraulic heads at LFO1 in the bottom 25 ft of the till declined. During the aquifer test, hydraulic heads in the till piezometers at LFO2 were also unresponsive, but they declined in all three till piezometers at LFO1, allowing estimation of vertical hydraulic conductivity, Kv (4E-09 m/s;1E-03 ft/d) and leakage factor, L (10,800 ft). Horizontal hydraulic conductivity (Kh) from slug tests is two orders of magnitude lower at LFO2 (Kh=2E-09 m/s; 6E-04 ft/d) than at LFO1 (Kh=8E-07 m/s; 0.23 ft/d). Steeper hydraulic gradients (downward) also occur at LFO2, as might be expected. Cl concentrations and Cl/Br ratios in the till suggest anthropogenic contamination at depth at both nests. An enriched ³H value at LFO1 of 16.08 tritium units (TU) at 95 ft suggests young (1970s) groundwater, which is at odds with the estimated travel time to that depth. Glacial-age water is absent at both nests, based on δ¹⁸O and δ²H data. In total, multiple lines of evidence at this site suggest a greater vertical connectivity in the till confining bed at LFO1, likely due to better-connected fractures. Fracture-flow modeling is underway to test that hypothesis.