REVISITING ONE OF THE LARGEST AQUIFER TESTS IN HISTORY: IMPLICATIONS FOR FAULT ZONE HYDROGEOLOGY AND THE DECLINING GROUNDWATER SUPPLY IN NORTHEASTERN ILLINOIS

Groundwater withdrawals from the St. Peter and Ironton-Galesville Sandstones in northeastern Illinois have resulted in head declines up to 900 feet since pre-development. Municipal and industrial demands continue to stress the aquifer, creating a large cone of depression centered in Will County, IL. The Sandwich Fault Zone, a high-angle fault system that extends across northern Illinois, acts as a regional flow barrier and exacerbates drawdown. However, little is known about the effects of varying displacement and heterogeneity of the fault zone on regional and local groundwater flow. A large-scale aquifer test­—perhaps the largest in North American history— was conducted by the Illinois State Water Survey in 1942 at a WWII munitions plant adjacent to the Sandwich Fault Zone. Over 9 million gallons per day was pumped from 9 sandstone wells for two months. Water level measurements during drawdown and subsequent recovery, as well as withdrawal rates from each well, were recorded daily. We modeled the aquifer test with varying conceptualizations of the fault to better understand the hydrogeology of the fault zone, which has important implications for the future supply of groundwater at the local and regional scale.