From abstract: Sedimentary rocks of Cambrian and Ordovician age form a major aquifer system in most of Wisconsin and Iowa, northern Illinois, northwestern Indiana, southeastern Minnesota, and northern Missouri. Many metropolitan areas depend on the aquifer for all or part of their water supplies. Declines in potentiometric head have been large in the most heavily pumped areas, most notably Chicago, Milwaukee-Waukesha, Minneapolis-St. Paul, and Des Moines.