The occurrence, circulation, distribution, and properties of any liquid water residing beneath the surface of the earth. Generally ground water is that fraction of precipitation which infiltrates the land surface and subsequently moves, in response to various hydrodynamic forces, to reappear once again as seeps or in a more obvious fashion as springs. Most of ground-water discharge is not evident because it occurs through the bottoms of surface water bodies.

Ground water can be found, at least in theory, in any geological horizon containing interconnected pore space. Thus a ground-water reservoir (an analogy to an oil reservoir) can be a classical porous medium, such as sand or sandstone; a fractured, relatively impermeable rock, such as granite; or a cavernous geologic horizon, such as certain limestone beds. Ground-water reservoirs which readily yield water to wells are known as aquifers; in contrast, aquitards are formations which do not normally provide adequate water supplies, and aquicludes are considered, for all practical purposes, to be impermeable. These terms are, of course, subjective descriptions; the flow of water which constitutes an economically viable supply depends upon the intended use and the availability of alternative sources. Aquifer

To effectively utilize ground water as a natural resource, it is necessary to be able to forecast the impact of exploitation on water availability. When ground water is used for water supply, a concern is the potential energy in the aquifer as reflected in the water levels in the producing well or neighboring wells. When a ground-water reservoir which does not readily transmit water is tapped, the energy loss associated with flow to the well can be such that the well must be drilled to prohibitively great depths to provide adequate supplies. On the other hand, in a formation able to transmit fluid easily, water levels may drop because the reservoir is being depleted of water. This is generally encountered in reservoirs of limited areal extent or those in which natural infiltration has been reduced either naturally or through human activities.

Problems involving ground-water quantity were once the primary concern of hydrologists; interest is now focused on ground-water quality. Ground-water contamination is a serious problem, particularly in the highly urbanized areas of the United States. Hydrology Water pollution