The aquifer underlies 1.26 million acres (5,100 km²) of land in east-central Illinois and spans 15 counties (Panno and Korab, 2000). The Mahomet ranges from four to fifteen miles (24 km) wide (6 to 24 km) and 50 to 200 feet (15 to 60 m) thick, although the average thickness is 100 feet (30 m). The aquifer is confined except in Mason, Menard, and Tazewell Counties in the west near the Illinois River.
