POURING WATER DOWN OUR MAWS: DESATURATION OF AQUIFERS IN AN ARTIFICIALLY CONNECTED SYSTEM

The Cambrian-Ordovician sandstone aquifers of northeastern Illinois have an extensive record of use. Historically, most high-capacity wells in the region were multi-aquifer wells (MAWs) primarily open to the St. Peter and the Ironton-Galesville Sandstones (upper and middle aquifers, respectively), with a subset also open to the Mt. Simon Sandstone (deepest aquifer); each unit is otherwise confined by at least 400ft of low permeable material separating the aquifers. Though communities nearest to Chicago switched to Lake Michigan water in the 1980s and 1990s, many MAWs are still in place as emergency backup wells, with others having been improperly sealed or abandoned. Sandstone withdrawals continue to be a primary source of water for the southern and western suburbs.

Past synoptic measurements have shown minimal head difference between the St. Peter and the Ironton-Galesville Sandstones; however, in 2014 a zone of significantly lower heads was observed in the Ironton-Galesville Sandstone in Kendall and Will counties. Changes to well construction practices have largely driven the propagation of this cone of depression, with many new high capacity wells being open to only the Ironton-Galesville Sandstone. Low heads in this unit have caused desaturation of the St. Peter Sandstone where MAWs are present, sometimes several miles removed from the source of pumping. Additionally, remnant MAWs open to the Mt. Simon Sandstone have the potential to be a source of saline contamination as the Ironton-Galesville cone of depression expands.

The sustainability of the sandstone aquifers of northeast Illinois has become a precarious balancing act as impacts propagate across confining units via MAWs. The St. Peter Sandstone is at greatest risk of desaturation where an extensive network of MAWs is in place; in contrast the Ironton-Galesville Sandstone is at risk of desaturation where few MAWs exist. All of this points to a greater need to monitor well construction practices and impacts not just to neighboring wells, but the system as a whole.