| Glacial processes have produced regional-scale, borehole-scale, and smaller-scale heterogeneity and variability in the Oak Creek Formation. Six advances of Lake Michigan Lobe ice between 14.5 and 13.5 kA B.P. deposited fine-grained diamicton units (glacial till) that contain interbedded sand and silt units deposited in small lakes, multiple Glenwood I stages of Glacial Lake Chicago, and meltwater streams. Surging ice in Valparaiso and Tinley advances, development of an ice shelf in the Lake Border advances, and the decrease in the transport distance with each successive advance, has produced borehole-scale heterogeneity that increases eastward. Supraglacial sediment with high variability occupies the upper few metres of the diamicton.; Glacial regime, distance of glacial transport, and the type and location of sediment entrained probably control the smaller-scale variability in diamicton properties. Physical properties, such as magnetic susceptibility, calcite and dolomite percentage in the coarse silt fraction, and clay mineralogy, exhibit no significant differences among the basal till units. Grain size, sand lithology, total sulfur and sulfate, and percent organic matter differ significantly among most of the basal till units. Grain size controls geotechnical properties, particularly Atterberg limits and dry unit weight. Grain size and fractures control hydrogeological properties and produce a zone of higher and more variable field-measured hydraulic conductivity in the upper 33 ft (10 m) than in diamicton below this depth. Oxidation of inorganic sulfide minerals, dolomite dissolution, and ion exchange control the geochemical properties of the diamicton and the groundwater; differences in groundwater flux probably produce variability in sulfate concentration.; The three scales of heterogeneity and variability produce spatial variability in vertical groundwater flux and average linear velocity that are verified by environmental isotope data. Sites in the Lake Border moraines show the highest flux and velocity values and sites in the Tinley moraine exhibit the lowest values. Thus, the investigation of subsurface heterogeneity and variability in complex glacial terrain benefits the assessment of groundwater contamination potential. |
