Many people think of groundwater as underground streams, rivers, or lakes. For the most part, however, groundwater exists in saturated layers of sand, soil, or rock. The water completely fills the pores, or spaces, in the soil at some depth beneath the earth's surface forming a saturated zone.
Those saturated zones capable of yielding significant amounts of water are referred to as aquifers ("water carriers"). Aquifers can range in thickness from several feet to hundreds of feet and can cover an area of a few acres to hundreds or thousands of square miles. They can be shallow or hundreds of feet deep, although most U.S. aquifers occur within 2,500 feet of the earth's surface.
Two basic types of aquifers exist.
Unconfined, or water table, aquifers are bounded on the bottom by an impermeable layer of clay or rock, but hove no defined upper boundary. They are replenished by precipitation that filters down from the surface, forming the saturated zone above the restricting clay or rock.
A confined, or artesian, aquifer is bounded both above and below by a confining layer of clay or rock. Rain and surface waters replenish the aquifer from a recharge zone that may occur many miles from the point where the aquifer is tapped for use.
The amount of groundwater an aquifer can hold and how rapidly water moves through the aquifer depend on its porosity and permeability.
Porosity is a measure of the total amount of empty space (openings or pores) within the underground layer, and determines the aquifer's storage capacity. It can range from less than I percent for solid rock (less than .01 cubic feet of empty space in a cubic foot of rock), to 40 percent in sand and gravel, to 55 -percent in cloy.
Permeability serves as a measure of the relative ease with which groundwater flows. A permeable aquifer has many interconnecting pore spaces, cracks, or crevices that allow relatively free movement of water.
Sand aquifers, for example, are both highly porous and highly permeable, allowing large volumes of groundwater to move fairly rapidly.
On the other hand, clay soils tend to have high porosity, but low permeability. That is, while clays have a lot of pore space, the spaces are very small so that water movement is very slow.
Back to Groundwater ContaminationDate created: 95.10.16 Hal Horning hhorning@pond.com