![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Ground Water Basics
Ground Water Basics
Where is Ground Water Located?
Subsurface structural conditions and the various controls which determine the presence and movement of water within geologic formations are the keys to understanding ground water.
Formations or strata within the saturated zone from which ground water can be obtained for beneficial use are called aquifers. An aquifer is a water-saturated geologic unit that will yield water to wells or springs at a sufficient rate so that the wells or springs can serve as practical sources of water supply.
Water-bearing formation and ground water reservoir are other terms often used in place of the word aquifer, a geologic formation must contain pores or open spaces which are filled with water, and these openings must be large enough to permit water to move through them toward wells and springs at a perceptible rate.
Both the size of the pores and the total amount of all pores in a formation can be small or large, depending on the type of material. Pores in a fine-grained material like clay are extremely small but the combined volume of the pores in such a formation is usually large. While a clay formation has large water-holding capacity, water can not move readily through the tiny open spaces. This means that a clay formation will not yield water to wells, and therefore it is not an aquifer, even though it may be water-saturated.
A coarser material such as sand contains larger open spaces through which water can move fairly easily. A saturated sand formation is an aquifer, because it can can hold water and it can transmit water at a perceptible rate when pressure differences occur.
The upper surface of the zone of saturation is called the water table. The shape of the water table is controlled partly by the topography of the land and tends to follow, in a general way, the shape of the land surface.
How does Ground Water move?
Water that infiltrates the soil is called subsurface water, but not all of it becomes ground water. Basically, three things happen: 1. it may be pulled back to the surface by capillary force and be evaporated into the atmosphere; 2. it may be absorbed by plant roots growing in the soil and then re-enter the atmosphere by transpiration; or 3. water that has infiltrated the soil deeply enough may be pulled on downward by gravity until it reaches the level of the zone of saturation - the ground water reservoir that supplies water to wells.
Upon joining the body of ground water, the percolating water moves through the pores of saturated subsurface materials and may reappear at the surface in areas at lower elevations than the level where it entered the ground water reservoir. Ground water discharges naturally at such places in the form of springs and seeps which maintain the flow of streams in dry periods. The streams carrying both surface runoff and natural ground water discharge, eventually lead back to the oceans.
What is the Distribution of Ground Water?
Geologists call the earth's crust the lithosphere. When they speak of the lithology of a cut or section through the crust, they mean the kinds of rocks that occur in a succession of layers or strata below the surface that make up any part of the lithosphere. Geologists refer to all materials of the earth's crust as rocks, whether they be unconsolidated materials such as sand or clay or consolidated materials such as granite or sandstone.
The outer part of the earth's crust is normally porous to a greater or lesser depth. This part is called the zone of rock fracture. The pores or openings in this portion of the lithosphere may be partially or completely filled with water.
The upper strata, where the openings are only partly filled with water, is called the vadose zone. Immediately below this, where all the openings are completely filled with water, is the zone of saturation.
Why is Ground Water Important?
Because we see surface waters and because such tremendous amounts of money have been spent in building dams, levees, reservoirs, etc., involving surface water, it is only natural that we tend to think of that water as the major source of the world's needs. Actually, less than 3 percent of the fluid fresh water available at any given moment on our planet Earth occurs in streams and lakes. The other more than 97 percent - an estimated 8 trillion acre-feet - is underground.
The fluid fresh water in lakes and streams represents water which is in transit, while the subsurface sources represent water in storage. The ground water has been accumulating over a period of many centuries, with rainfall each year adding only slightly to its volume. During an average year, the water in streams is replaced an average of 31 times.
In addition, not all the water beneath the earth's surface can be recovered from the water-bearing formations in which it is found. Some lies in rock formations so deep that pumping costs prohibit its recovery. Some lies in water-bearing formations that resist recovery to varying degrees and may even defy pumping efforts.
While comparative figures of the amounts of water available from the surface and ground sources can not be used as true indicators of actual water resources, they do point out that the available supply of ground water is many times greater than the available supply of surface water and that too little emphasis has been placed on development and use of the vast fresh water supply that lies beneath the earth's surface.
Where does Ground Water Begin?
The Earth's water cycle, or hydrologic cycle, is the continuous circulation of moisture and water on our planet. The cycle has neither a beginning nor an end, but the concept of the hydrologic cycle commonly begins with the waters of the oceans, since they cover about three-fourths of the earth's surface.
Radiation from the sun evaporates water from the oceans into the atmosphere. The water vapor rises, then collects to form clouds. Under certain conditions, the cloud moisture condenses and falls back to earth as rain, hail, sleet, or snow - the various forms of precipitation. Hydrologists and others call this "water of meteoric origin."
Precipitation that falls upon land areas is the source of essentially all our fresh water supply. We depend upon it to replenish the quantity taken from lakes, streams, and wells for man's numerous uses.
Contact: Rinaldo.Lawrence@epa.gov