Water Resources  Elmhurst College
Ground Water  Chemistry Department
   Virtual ChemBook

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Groundwater System


Water is continuously collected, purified, and distributed in the hydrologic cycle.
As water precipitates in the form of rain or snow, some of the water infiltrates the ground and fills the spaces and pores in soil and rock - this is called groundwater. The water table is the upper surface of the zone of saturation where all of the pore spaces are filled. The ability of the soil or rock layers to absorb and hold water depends on how porous and permeable it is. Porous, well-saturated layers of sand, gravel, or bed rock are called aquifers. A confined aquifer is sandwiched between layers of rock that have a very low permeability to water. An unconfined aquifer collects on top of a impermeable layer of rock. An artesian well may flow without pumping because the confined aquifer is under pressure from water many miles away at a higher elevation. Most other wells need a pump of some type to bring the water to the surface.

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Water Table Draw Down or Groundwater "Mining":

Most aquifers are replenished naturally by infiltration of water from precipitation of rain and snow in the recharge area which may be many miles from the point of withdrawal from wells. If water is pumped from many wells at a withdrawal rate in an aquifer that exceeds the natural recharge rate, the water table drops. As shown in the diagram, a cone of depression may form around a well. Depending upon the depth, other wells in the area may go dry. If this situation prevails for any significant amount of time, this is called water "mining". This may happen from rapid withdrawals for irrigation purposes from so called "fossil aquifers" which get very little if any recharge.
Such "fossil aquifers" underlie the Sahara and Kalahari deserts in Africa, the Great Artesian Basin in Australia, the central Asia basins, and the Ogallala Aquifer in the western part the Midwest of the United States.

Circular Irrigation Plots in the Plains States

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 Salt Water Intrusion:

When fresh water is withdrawn at a faster rate than it can be replenished, a draw down of the water table occurs with a a resulting decrease in the overall hydrostatic pressure. When this happens near an ocean coastal area, salt water from the ocean intrudes into the fresh water aquifer as shown in the diagram. The result is that fresh water supplies become contaminated with salt water as is happening to communities along the Atlantic and Gulf coasts.