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

A wide variety of methods have been employed to both measure and estimate recharge into near-surface aquifer systems, including “rule-of-thumb” precipitation percentages, stream discharge loss calculations, tensiometer and piezometer measurements, isotope analyses, and GIS and digital modeling. Subsequent recharge rates and pathways to deeper aquifer systems is more difficult to directly measure and requires more comprehensive efforts to digitally model entire systems, in conjunction with limited direct measurements to calibrate the model results. As DeKalb County is on the verge of expansion from the Chicago metropolitan area and serves as a primary recharge location for the regional aquifer system, a combined application of modeling-GIS-field-lab approaches to this ideal critical setting will assist researchers and agencies involved with these problems across the nation.

The goals of this study are to determine the rate of recharge through the surficial deposits to the shallow groundwater system and ultimately the deeper regional aquifer systems in the suburban and rural settings of the county. This will be done through the combination of several specific objectives: 1) description and modeling of the site-specific hydrogeology of DeKalb County, especially the Troy Bedrock Valley setting; 2) a comparison of methods to determine local groundwater recharge (including water budget calculations, calculations from vertical gradients, isotopic groundwater dating and model calibrations); 3) development of recharge estimates (including averages and extreme-condition ranges) to both the shallow glacial aquifers and the underlying regional aquifer; and 4) predictive modeling of the potential future impact of urbanization on local and regional groundwater resources. Recent research has produced recharge estimates between 0.2 and 13 inches per year to the local glacial aquifer system within DeKalb County. These estimates will be compared to and used in conjunction with isotopic groundwater dates as input to a groundwater flow model for the glacial and bedrock aquifer systems down to the regional Ancell-St. Peter Sandstone Formation. In addition, the model's discharge into the regional bedrock aquifer system will be compared to the Illinois State Water Survey's model of the regional groundwater system.