The information presented in this chapter provides the reader with an understanding of ground water’s origin, environment, movement, and interaction with surface water. For all practical purposes, ground water is all water beneath the surface of the earth. Because ground water is hidden from view, many people think of its occurrence in the form of underground lakes, streams, and veins. While such features exist in the limited settings of cavernous limestone and the lavas of some volcanic flows, most ground water occurs as water filling pore spaces between rock grains in sedimentary rocks or in crevices such as fractures and faults in crystalline rocks (Figure 2.1).
Figure 2.1 Porosity and permeability of common geologic units that form aquifers. Water-filled voids are represented in blue.
The ultimate source of ground water is precipitation (in the form of rain, snow, or hail). The precipitation that does not evaporate or immediately flow to rivers, streams, or lakes percolates into the ground, where some of it eventually reaches the water table. The concept of the hydrologic cycle is central to understanding the occurrence of ground water. The hydrologic cycle, as the name implies, is an endless dynamic process of the circulation of water between the atmosphere, the oceans, and the land. The basic components of the hydrologic cycle are shown schematically in Figure 2.2. Seventy-one percent of all U.S. precipitation originates from land surface evaporation, whereas the remaining 29 percent is produced by evaporation from the oceans. The integrated nature of the hydrologic cycle makes ground water vulnerable to pollution sources in the atmosphere, on or within land surfaces, or in surface waters.
Figure 2.2 The hydrologic cycle describes the circulation of water between
the atmosphere, land, and open water bodies.
The degree and rate of infiltration (recharge) will vary widely depending upon the land use, soil physical properties and moisture content, and the intensity and duration of precipitation. When rainfall is intense, exceeding the rate of infiltration, water accumulates on the surface and runs off downhill as overland flow. Water that infiltrates the ground surface becomes soil moisture, which may evaporate or be taken up by vegetation as nourishment. Excess soil moisture is pulled down by gravity and percolates through the ground to some depth where all the openings within the soil or rock are saturated with water. The top of that zone of saturation is called the water table.