MODELING RECHARGE INTO SHALLOW GLACIAL AND DEEP BEDROCK AQUIFERS RELATED TO THE TROY BEDROCK VALLEY SYSTEM, DEKALB COUNTY, ILLINOIS

DeKalb County is on the western edge of the expanding Chicago metropolitan area and serves as a primary recharge area for the regional sandstone aquifer (Midwest Bedrock Aquifer), which provides 70% of the metropolitan population’s drinking water. Various methods – including field measurements, GIS analyses and shallow groundwater flow modeling – have been used to generate estimates of recharge into the overlying glacial Prairie Aquigroup within DeKalb County but no estimates have been made to quantify subsequent recharge into the Midwest Bedrock Aquifer. This study is using isotopic groundwater dating techniques and results of previous studies within DeKalb County as inputs to a MODFLOW model of the entire DeKalb County system to determine recharge to both the Prairie Aquigroup within the Troy Buried Valley and into the underlying Midwest Bedrock Aquifer.

The model extends from the surface down to the Ancell-St. Peter sandstone, including the glacial units and weathered portions of the Galena-Platteville dolomite formations. It focuses on the Troy Buried Valley system but extends laterally to include the surrounding bedrock, watershed, and natural hydrologic boundaries. In the late 1990’s the City of DeKalb installed new drinking water supply wells into the basal unit of the Prairie Aquigroup within the Troy Bedrock Valley to supplement wells already installed in the Midwest Bedrock Aquifer. Thus the model is being calibrated to 2000 and 2005 conditions to evaluate the effect of this pumping in close proximity to the bedrock subcrop surface along the suspected deeper recharge pathway. This DeKalb County model domain is adjacent to the Illinois State Water Survey’s Kane County model and overlies part of their regional model. The conceptual model hydrostratigraphy and hydraulic inputs and outputs from this model are being compared to those from the Kane County and regional models to ensure consistency with adjacent water supply planning efforts.