Figure 19—Source, movement, and occurrence of ground water. Arrows in aquifers show direction of flow of water.
EVAPORATION SAND AND GRAVEL WELL Water Table River sand and gravel ARTESIAN WELLS EVAPORATION River WELL COLLECTING AREA FOR ARTESIAN WATER PRECIPITATION Crater WATER TABLE WELL Lake Glacial pebbly clay Glacial sand and aquifer Shale Sandstone aquifer Limestone aquifer Shale GROUND-WATER RESERVOIR
IS WATER FOUND IN UNDERGROUND LAKES AND RIVERS IN ILLINOIS?
No. There are a few large springs that flow from rocks along river bluffs in Illinois, but most ground water occurs in tiny openings within the rocks. Ground water reservoirs may be regarded as similar to sponges rather than as underground lakes or rivers.
WHY DO WELLS SOMETIMES GO DRY?
The water table rises and falls seasonally and from year to year, depending upon the amount of precipitation. Sometimes the water table may fall below the bottom of the well or below the pump. Sometimes overpumping the well or pumping too many wells in a small area lowers the water table sufficiently to check the yield of the well.
WHAT IS AN ARTESIAN WELL?
An artesian well is one in which pressure forces water to rise in the well above the level where it was found. In some artesian wells water flows out at the surface. Conditions for artesian wells are illustrated in [figure 19]. The shale above the sandstone and limestone aquifers in the figure is “tight” and does not permit water to escape upward to the level at which it enters the aquifers in the intake areas. The water is under natural pressure. When a well is drilled through the shale and into the aquifers, water rises, seeking its own level. Most of the deep wells in northern Illinois are artesian wells, though few of them flow at the surface anymore.
WILL YOU ALWAYS FIND WATER IF YOU GO DEEP ENOUGH?
This is generally true. However, in much of the southern two-thirds of Illinois the deeper waters are quite salty. Therefore, only the upper few hundred feet of rocks are worth exploring for water. In the northern third of Illinois, where the deepest water wells are located, fresh water extends in some places to more than 2,000 feet.