DELINEATING HYDROSTRATIGRAPHIC UNITS IN GLACIAL TERRAIN: EXPECT VARIABILITY IN HYDRAULIC RESPONSES AND PROPERTIES

Multiple glaciation events have formed complex sequences of aquifers (e.g., outwash sand and gravel) and aquitards/confining units (e.g., till, lake sediment, and loess) in the Upper Midwest. Vertical flux of water (leakage) through the aquitards/confining units is poorly documented, impeding evaluation of anthropogenic contamination and sustainability in confined aquifers. With funding from the Legislative-Citizen Commission on Minnesota Resources in 2014, the U.S. Geological Survey, Iowa State University, Minnesota Geological Survey and Minnesota Department of Health investigated the variability of vertical groundwater flow through till aquitards/confining units and till hydraulic properties at five sites in Minnesota. The sites contain till stratigraphic units of clay loam to loamy sand texture and lie within the late Wisconsin Des Moines (Litchfield sites - 2; New Ulm Fm.), Superior (Cromwell site; Aitkin and Cromwell Fms.), and Wadena (HFC site; Hewitt Fm.) glacial lobes, as well as a Pre-Illinoian unit (Olivia site; Good Thunder Fm.). Till thickness ranged from 60 to 166 ft and core was obtained and described at each site. Thirty-one small-diameter piezometers were installed in five nests, with screens set in surficial aquifers, till aquitards/confining units, and underlying confined aquifers. Downward hydraulic gradients of 0.04 to 0.56 were observed in till at four sites; one site showed a slight upward gradient. Hydraulic conductivity of the till was estimated from slug tests (horizontal K_h) and aquifer tests in the confined aquifer (vertical K_v). Geometric mean K_h ranged from 0.0002 to 0.07 ft/d, both in the New Ulm Fm till at the Litchfield sites, ½-mile apart. Values of K_v from aquifer tests were within an order of magnitude of K_h. At three sites, hydraulic head in the till responded to pumping in the underlying aquifer, suggesting vertical connectivity into the aquitard; however, the vertical extent varied among the sites. Reverse water-level fluctuation, a temporary water-level rise in an aquitard/confining unit when pumping an underlying aquifer, dominated at one site. Overall, our results suggest that variability in hydraulic gradient, K_h, K_v, and vertical connectivity occurs between and within till stratigraphic units, both horizontally and vertically. Extrapolation of site-specific data from a till stratigraphic unit to an entire hydrostratigraphic unit may result in significant overestimation or underestimation of hydraulic response and properties.