INCORPORATING ANTHROPOGENIC STRESSES INTO GROUNDWATER FLOW MODELS TO IMPROVE CONCEPTUALIZATION OF GEOLOGY

Groundwater withdrawals in northeastern Illinois have changed rapidly over the past 40 years, with demand increasing where surface water is not available. These groundwater demands are often satisfied by withdrawals from Cambrian-Ordovician sandstone units. Pumping from these “deep” sandstone aquifers has led to a large cone of depression that is expected to expand in the coming decades. To understand the impact of changing groundwater demands, the Illinois State Water Survey has developed a regional groundwater flow model of northeastern Illinois. This model was calibrated to historic observations, but a few recent stresses due to urbanization have altered water levels in ways that the model did not predict. This has led the ISWS to re-evaluate the influence of certain stratigraphic controls on groundwater flow in two sandstone units, the upper St. Peter and the lower Ironton-Galesville.

The St. Peter and Ironton-Galesville sandstone units are separated by a thick aquitard. However, in the early 1980’s, the majority of wells constructed in northeastern Illinois were open to both sandstone units, bypassing this aquitard. As a result, the sandstone aquifers in the presence of these interformational boreholes have the same head. In the last decade, however, many newly constructed sandstone wells are open to only the deeper Ironton-Galesville. In the absence of neighboring interformational boreholes, high-capacity Ironton-Galesville wells appear to have lower heads than the overlying St. Peter sandstone, which impacts the simulated quantity of water in each unit.

Another recent stress is that many new sandstone wells have been installed in the last decade in the vicinity of the Sandwich Fault zone. Although displacement of sandstone units in this zone can exceed 200 ft, in recently urbanized areas, the observed displacement is much less. Even in areas of low displacement, however, drawdowns that have developed over the last decade are greater than initially predicted by the groundwater flow model. To simulate such drawdowns near the Sandwich Fault zone, the permeability of this area had to be decreased. Moving forward, it is important to incorporate new stresses on the deep sandstone aquifers into the groundwater flow model of northeastern Illinois to improve our understanding of the local and regional geology.