Groundwater - Mathematics and Statistics for the Environment - page 579

21 Groundwater
… Anyone who has learned the language of running water will see its character in the dark.
—John Muir (1872)
Once polluted, groundwater is difficult, if not impossible, to clean up, since it contains few decompos-
ing microbes and is not exposed to sunlight, strong water flow, or any of the other natural purification
processes that cleanse surface water.
— Odum (1993)
21.1 GROUNDWATER AND AQUIFERS
Part of the precipitation that falls on land may infiltrate the surface, percolate downward through the
soil under the force of gravity, and become what is known as groundwater . Groundwater, like sur-
face water, is an extremely important part of the hydrologic cycle. Almost half of the people in the
United States obtain their public water supply from groundwater. Overall, there is more groundwa-
ter than surface water in the United States, including the water in the Great Lakes, but sometimes it
is not economical to pump it to the surface for use, and in recent years the pollution of groundwater
supplies from improper disposal has become a significant problem (Spellman, 1996).
Groundwater is found in saturated layers under the Earth's surface called aquifers that lie under
the Earth's surface. Aquifers are made up of a combination of solid material, such as rock and
gravel, and open spaces called pores . Regardless of the type of aquifer, the groundwater in the
aquifer is always in motion. The aquifer that lies just under the Earth's surface is the zone of satura-
tion, an unconined aquifer (see Figure 21.1). The top of the zone of saturation is the water table. An
unconfined aquifer is not contained, except on the bottom, and is dependent on local precipitation
for recharge. This type of aquifer is often referred to as a water table aquifer .
The actual amount of water in an aquifer is dependent upon the amount of space available
between the various grains of material that make up the aquifer. The amount of space available is
called porosity . The ease of movement through an aquifer is dependent upon how well the pores
are connected. The ability of an aquifer to pass water is its permeability . Types of aquifers include
the following:
1. Unconfined aquifers are a primary source of shallow well water (see Figure 21.1). However,
because these wells are shallow they are subject to local contamination from hazardous
and toxic material such as fuel and oil, agricultural runoff containing nitrates and micro-
organisms, and septic tanks that provide increased levels of nitrates and microorganisms.
This type of well may be classified as groundwater under the direct influence of surface
water (GUDISW) and may, therefore, require treatment for control of microorganisms
(disinfection).
2. Conined aquifers are sandwiched between two impermeable layers that block the flow of
water. The water in a confined aquifer is under hydrostatic pressure. It does not have a free
water table (see Figure 21.2).
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