Ground Water and Geology Why can water be found a few feet beneath the ground surface in some places, but not at several hundred feet below the ground surface in others? This question involves underground water, water that geologists call ground water, which fills the pores of soil and rock beneath the ground’s surface. This water comes to the surface as springs, streams, rivers, and wells. Water on the earth’s surface is part of an unending sequence of water movement known as the hydrologic cycle. Water moves from the oceans to the land surface through evaporation and precipitation. It is stored in all three of its natural physical states during its cycle on earth: liquid, water vapor, and ice. 97 percent of the world’s water is held as liquid in oceans, polar ice sheets and glaciers account for about 2 percent, ground water amounts to about 0.6 percent, and water in the form of rivers and lakes amounts to only about 0.1 percent. The remaining fraction of water is held in the atmosphere as water vapor. The total amount of water on the earth’s surface does not change, but is recycled through its three different natural states. Ground water is affected by the hydrologic cycle. When precipitation falls on the land, much of the water runs off the land surface, flowing into rivers, lakes, and eventually the oceans. Some rainwater evaporates directly from the land surface back into the atmosphere and some of the precipitation soaks into the soil and enters the ground water system. A majority of the ground water that soaks into the land is stored in the spaces of soil and rock. This water is stored in an aquifer, which is any geological material that is capable of storing and transmitting appreciable amounts of water. The most important requirement for a body of rock to be classified as an aquifer is that it must contain interconnected openings or pores through which water can move.
Jay Lehr et al., Design and Construction of Water Wells: A Guide for Engineers, (New York, Van Nostrand Reinhold, 1988), 1,4.