Forest Preserve District of Cook County, Illinois
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Ground Water
Nature Bulletin No. 408-A   February 27, 1971
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

We take for granted and are familiar with our surface waters -- ponds, marshes and lakes; brooks, creeks and rivers on their routes to the sea. Maps usually show them in blue. Hidden beneath the land, however, and somewhat mysterious to most of us, are large reservoirs of Ground Water. We know it is there because of the water supplies we get from wells. We encounter it in basements, tunnels, mines, and in caverns such as Mammoth Cave. It is a vital resource but except in times of drought, or in problem areas where the water tables are falling and wells are going dry, too little attention has been paid to ground water, its movements, and how to conserve it.

Ground water is the supply which saturates and permanently fills all the spaces in the subsoil, or the glacial drift, or the bedrock, or the deeper layers of porous rocks such as sandstone. Its upper surface is called the Water Table, which may be anywhere from a few inches to hundreds of feet underground. Above the water table the earth is saturated only after a long spell of wet weather .

Ground water is replenished by the downward seepage of rain and melting snow; also by seepage from ponds and lakes, or from stream channels. Ordinarily it moves downward and onward very slowly, percolating rather than flowing through the minute spaces it finds. The direction and speed of travel depend upon what kinds of rocks lie underground, how their layers slope, and what kinds of surface deposits lie above them.

Ground water may reappear as springs and seeps on hillsides, or along water courses where it helps to maintain stream flow in dry weather. It may find its way to a subterranean stream which is carving channels and caverns through a limestone formation. Or it may not reappear until, perhaps years later and hundreds of miles away, it is pumped up from a well drilled into a porous rock far beneath the surface.

In Chicago area the bedrock is Niagara limestone which, because of erosion by glaciers, varies greatly in thickness. It outcrops or lies very near the surface in some places but usually is overlain with glacial drift or ground-up rock materials which may be a hundred or more feet thick. This drift and the seams, crevices and other openings in the Niagara limestone provide a large storage capacity for ground water which supplies thousands of shallow wells.

Beneath the Niagara lies a thick stratum of shale. Beneath that are layers of limestones and water-bearing sandstones. About 2000 feet down, we encounter a very thick bed of sandstone -- the Mt. Simon -- which lies above the dense primeval granite. These layers of rock slope upward so that, as we travel north and west to the Rock River and on through Wisconsin, we find them outcropping on the surface where they absorb the rainfall which filters downward through them.

Many towns, and hundreds of industries which require cooling water, have drilled wells into the deeper sandstones. Before the beginning of this century, most of those around Chicago were artesian or flowing wells. Now, however, the total pumpage exceeds 65 million gallons per day and is so concentrated in some places that the water table has dropped as much as 400 feet and the effects are noticed in other wells 30 or 40 miles away.

Fortunately, we have an inexhaustible supply in Lake Michigan if it can be piped to us.

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