SEDIMENTOLOGIC MODELING AND TRANSMISSIVITY MAPPING TO SUPPORT GROUNDWATER FLOW AND CONTAMINANT TRANSPORT MODELING IN GLACIAL SEDIMENTS

Over the past decade, geologists and hydrogeologists at the Illinois State Geological Survey (ISGS) have been trying to provide users with maps and accompanying information that better support decisions about groundwater quantity and quality problems. Recent advances within the ISGS into new methods in 3-D geologic and hydrogeologic mapping have provided opportunities to push out new map products that further benefit hydrogeologic problem solving.

A strategy is evolving at the ISGS where sedimentologic models of varying complexity are used with 3-D maps of sand/gravel aquifer distribution and thickness to produce sets of transmissivity maps that provide insight on the predicted ranges and distributions of hydraulic conductivity. The complexity of the sedimentologic models are based on the quantity and quality of available data, confidence in the conceptual models governing the overall glacial framework, and the objectives being addressed by the mapping efforts. On one end, transmissivity maps reflect general sedimentologic shifts, such as proximal to distal relationships based on distance from ice margins, together with broad ranges in hydraulic conductivity from published table-based sources. This approach results in generalized maps reflecting broad zonation of estimated maximum and minimum transmissivity values. At the other end of the complexity spectrum, geostatistical methods are used with the traditionally-developed 3-D maps to simulate a number of possible distributions of sediment lithotypes or facies within aquifers. Then these lithotype distributions are populated with geostatistically-simulated values of porosity and permeability to create a range of possible 3-D models of the aquifer hydraulic properties. These can be ranked and upscaled as appropriate for the groundwater problem at hand.