Groundwater

Water exists in many forms including water vapor, liquid, and ice, all of which are found on Earth including atmospheric moisture, precipitation, soil moisture, groundwater, ice, snow, oceans, and seas; within plants and animals; in lakes, streams, and rivers; and in man-made reservoirs. Water cycles between these forms both visibly, as when atmospheric [Page 217]moisture falls as rain, and invisibly, as when precipitation seeps into the soil to become groundwater. Water makes up 70 percent of the Earth's surface. Most of the water is contained in the oceans, with only a small percentage available as freshwater, most of which is frozen in glaciers.

Groundwater reaches the surface through springs like this one in the Westcave Preserve in Texas. Groundwater is the biggest source of freshwater for human consumption.

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Freshwater constitutes approximately 2.5 percent of all terrestrial water sources, with only 0.75 percent being groundwater. This amount of water sounds minuscule, but a comparison between available surface and groundwater resources reveals quite a difference. At any given time, the Earth contains roughly 30,300 cubic miles of surface water resources contained in lakes and streams compared with millions of cubic miles of groundwater available within a half-mile of the land surface. With that comparison in mind, it comes as no surprise that groundwater is the largest single supply of freshwater available for human consumption. Groundwater resides below the surface of the Earth in porous layers of rock and sand called aquifers. Soil and rock strata differ in porosity, in the amount of water that can be held in the soil, and in permeability—the rate at which water flows through the soil. An aquifer that holds groundwater between layers of relatively impermeable rock or soil (e.g., clay or shale) is called a confined aquifer. Some confined aquifers are under pressure, such that when a natural opening, such as an artesian spring, or a man-made opening, such as a well, taps into the confined aquifer, water emerges from the surface under pressure. Unconfined aquifers are characterized by relative permeability between the ground surface and the water table below the surface where groundwater is stored.

Groundwater can be found almost everywhere on Earth at varying depths, some very near the surface, as in the case of wetlands, or at some depth below the surface, as in some dry areas in the western United States. Groundwater close to the surface may be just a few hours old, whereas groundwater located far beneath the surface of the Earth might be thousands of years old. Often, the older the groundwater, the more difficult it is to replenish by natural means.