Water, too, is a mineral resource, and our most necessary one. The
source of nearly all water is rain and snow, which collect in lakes and
streams or seep into the ground to form our two types of water
supplies—surface water and ground water.
Ground water in Illinois collects underground in deposits of sand and
gravel or in the porous sandstone or limestone layers of bedrock. These
rock materials are called “aquifers” because of their ability to hold
water and to allow it to flow into wells that are drilled into them.
The aquifers are not equally distributed throughout Illinois, so
ground-water supplies are abundant in some parts of the state but are
scarce in others. Some Illinois cities depend wholly or partly on
surface supplies from lakes and rivers.
The sand and gravel aquifers yield more ground water than any others in
Illinois. The major sand and gravel aquifers were deposited by streams
and lie along river valleys. Some of these river valleys are no longer
visible to us because they were buried by glacial drift. They are
recognized, however, from well records or drilling records.
The northern third of Illinois has the most abundant supplies of ground
water. Bedrock formations of Devonian through Cambrian age are favorable
aquifers and supply drinkable (potable) water at depths of from 200 to
2,000 feet. These formations, however, lie much deeper in the Illinois
Basin, where the water they furnish is too salty for most uses.
West of the Illinois River and at the southern tip of the state, the
Mississippian, Devonian, and Silurian limestones are the chief aquifers.
Their yield, however, is small.
In the Illinois Basin all these formations are overlain by Pennsylvanian
rocks which offer only small, scattered supplies of ground water.