LESSONS FROM A BOREHOLE: USING HEAD DIFFERENCES AND GEOPHYSICAL LOGGING IN A NESTED WELL TO CONCEPTUALIZE GROUNDWATER FLOW ACROSS THE SANDWICH FAULT ZONE IN NORTHERN ILLINOIS

The Illinois State Water Survey (ISWS) and the U.S. Geological Survey (USGS) recently completed drilling of a new deep sandstone monitoring well in Kendall County, IL. The 1,180 feet deep borehole has two nested wells, one which is open to the St. Peter Sandstone and one that is open to the deeper Ironton-Galesville Sandstone. Groundwater withdrawals from the St. Peter and Ironton-Galesville Sandstones in northeastern Kendall and western Will Counties have resulted in head declines of over 800 feet since pre-development. The monitoring well is also located south of the Sandwich Fault Zone, which is a high-angle fault system that extends across Northern Illinois for about 85 miles and offsets bedrock by as much as 800 feet. The fault zone is conceptualized as a flow barrier in ISWS groundwater flow models, however little is known about the effects of varying displacement and heterogeneity on regional and local groundwater flow. The monitoring well, therefore, is an important component to understanding head decline and groundwater flow across the fault zone to the large cone of depression north of the fault.

After development of the wells, a head difference of 70 feet was observed between the sandstone aquifers. We discuss the implications of this head difference, geophysical logging of the borehole, and recent slug tests in conceptualizing regional hydrogeology and the Sandwich Fault Zone in ISWS groundwater flow models. A series of cross sections parallel to the fault were also used to map zones of lower horizontal hydraulic conductivity where the St. Peter or Ironton-Galesville Sandstones meet shale units across the fault.