You'll notice that the aquifer extends to about 2,200 feet below the surface.
The Maquoketa shale layer (red layer in the diagram) above the sandstone aquifer is considered impermeable and acts as a barrier between the surface and the deeper sandstone aquifer.

The aquifer supplies southeast Wisconsin with 33.3 million gallons of water per day.
It supplies Illinois with approximately 65 million gallons of water per day.
Keep in mind that we are drawing out ten times more water than is being recharged into the aquifer.

To date, I have quoted only one textbook in this website:
Fetter, C.W. Applied Hydrogeology. Upper Saddle River, New Jersey: Prentice Hall, 2001.

This website has been compiled by Peter G. Greenan.  The site may eventually become incorporated into an on-line earth science course offered by Northeastern Illinois University some time in the future.  

This website is not complete as of yet.