Groundwater-Level Change in the Cambrian-Ordovician Sandstone Aquifers of Northern Illinois, 1980-2014

Groundwater withdrawals from the St. Peter and Ironton-Galesville sandstones in Northern Illinois have led to a decrease in potentiometric heads since pre-development. By 1980, heads in these sandstone aquifers had dropped as much as 800 feet in the Chicagoland area, resulting in a widespread cone of depression. Since 1980 many Chicago suburbs have switched to Lake Michigan water, however, urbanization in the southwest and westernmost suburbs of Chicago has led to an increase in withdrawals. To understand the impact of changing water demand, the Illinois State Water Survey (ISWS) conducted the largest synoptic measurement of water levels in the Cambrian-Ordovician sandstone aquifers of Northern Illinois since 1980. Over 600 water-level measurements were taken in 2014/2015 by ISWS staff using a combination of airline, dropline, and steel tape methods. Water level measurements were also obtained through public water supply records and recent service records from well drilling and service companies. Measurements were used to construct potentiometric surface maps and head change maps, showing the changes in water levels since 1980. Results show a recovery of heads in the Chicagoland region by as much as 200 feet due to the switch to Lake Michigan water, while drawdowns have occurred in the western/southwestern Chicago suburbs due to continued or increased pumping. Drawdowns in the southwestern suburbs are more pronounced in communities located close to the Sandwich Fault Zone, which acts as a flow barrier. Drawdowns have also occurred around the Rockford area and regionally throughout Northwestern and Western Illinois.