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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
GROUND WATER
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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Update: June 2012
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