Science of Water : Concepts and Applications

32 The Science of Water: Concepts and Applications
AQUIFERS
As mentioned, part of the precipitation that falls on land infi ltrates the land surface, percolates
downward through the soil under the force of gravity, and becomes groundwater. Groundwater,
like surface water, is extremely important to the hydrologic cycle and to our water supplies. Almost
half of the people in the United States drink public water from groundwater supplies. Overall, more
water exists as groundwater than surface water in the United States, including the water in the Great
Lakes. But sometimes, pumping it to the surface is not economical, and in recent years, pollution of
groundwater supplies from improper disposal has become a signifi cant problem.
We fi nd groundwater in saturated layers called aquifers under the Earth’s surface. Three types
of aquifers exist: unconfi ned, confi ned, and springs.
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 in a constant
state of motion. This motion is caused by gravity or by pumping.
The actual amount of water in an aquifer depends 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.
For example, clay can hold a lot of water and has high porosity, but the pores are not connected,
so water moves through the clay with diffi culty. The ability of an aquifer to allow water to
infi ltrate is called permeability.
The aquifer that lies just under the Earth’s surface is called the zone of saturation, an unconfi
ned aquifer (see Figure 2.4). The top of the zone of saturation is the water table. An unconfi ned
aquifer is only contained on the bottom and is dependent on local precipitation for recharge. This
type of aquifer is often called a water table aquifer.
Unconfi ned aquifers are the primary source of shallow well water (see Figure 2.4). These wells
are shallow (and not desirable as a public drinking water source). They are subject to local contamination
from hazardous and toxic materials—fuel and oil, septic tanks, and agricultural runoff providing
increased levels of nitrates and microorganisms. These wells may be classifi ed as GUDISW,
and therefore require treatment for control of microorganisms.
Unconfined aquifer
Ground
surface
Water table well
Percolation
Rain
Water table
Infiltration
FIGURE 2.4 Unconfi ned aquifer. (From Spellman, F.R. 1996. Stream Ecology and Self-Purifi cation: An
Introduction for Wastewater and Water Specialists, Lancaster, PA, Technomic Publishing Company.)