COMPARISON OF POTENTIOMETRIC SURFACES FOR THE CAMBRIAN-ORDOVICIAN SANDSTONE AQUIFERS IN NORTHERN ILLINOIS AND SOUTHERN WISCONSIN, 1980-2014

The St. Peter and Ironton-Galesville sandstones, the principle stratigraphic units that comprise the Cambrian-Ordovician aquifers in Illinois, are a primary source of water for northern Illinois and southern Wisconsin, especially where uncontaminated shallow groundwater or surface water is not available. By 1980, heads in these sandstone units had dropped as much as 800 feet in the Chicagoland area since pre-development, resulting in a widespread cone of depression. By the mid-1990’s, many Chicago suburbs had switched to Lake Michigan water. However, urbanization in the southwest and westernmost suburbs of Chicago has led to an increase in withdrawals from the sandstone aquifers.

To understand the impact of changing water demand, the Illinois State Water Survey (ISWS) and the Wisconsin Geological and Natural History Survey (WGNHS), University of Wisconsin-Extension has just concluded the largest synoptic measurement of water levels in the Cambrian-Ordovician sandstone aquifers of northern Illinois and southern Wisconsin since 1980. Over 700 measurements in Illinois and Wisconsin were taken in 2014/2015 and used to construct potentiometric surface maps and head change maps, showing the changes in water levels since 1980. Water levels in public water supply and industrial facility wells were measured by ISWS and WGNHS staff using a combination of airline, dropline, acoustic, 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.

Results of the 2014 mass measurement show a recovery of heads in the Chicagoland region by as much as 200 feet since 1980 due to the switch to Lake Michigan water. While drawdowns have occurred in the western suburbs, they are largely muted (less than 100 feet) due to the proximity to the recharge zones of the sandstone aquifers. The exception is where groundwater withdrawals are in close proximity to the Sandwich Fault zone, which serves as a barrier to flow. As a result, heads in this area have decreased by as much as 300 feet since 1980. As the southwestern and western suburbs of Chicago continue to expand, monitoring of water levels will be essential in management of groundwater resources and development of regional groundwater flow models.