In the broadest sense, groundwater refers to all subsurface water. The term more commonly refers to water beneath the surface of the earth which saturates the pores and fractures of sand, gravel, and rock formations. Groundwater is a major source of water for agricultural and industrial purposes, and is an important source of drinking water for many people around the world.
Some water underlies the Earth's surface almost everywhere, beneath hills, mountains, plains, and deserts. It is not always accessible, or fresh enough for use without treatment, and it's sometimes difficult to locate or to measure and describe. This water may occur close to the land surface, as in a wetland, or it may lie many hundreds of feet below the surface, as in some arid regions. Water at very shallow depths might be just a few hours old; at moderate depth, it may be hundreds of years old; and at great depth or after having flowed long distances from places of entry, water may be several thousands of years old.
Groundwater is stored in, and moves slowly through, moderately to highly permeable rocks called aquifers. The word aquifer comes from the two Latin words, aqua, or water, and ferre, to bear or carry. Aquifers literally carry water underground. An aquifer may be a layer of gravel or sand, a layer of sandstone or cavernous limestone, a rubbly top or base of lava flows, or even a large body of massive rock, such as fractured granite, that has sizable cracks and fissures. In terms of storage at any one instant in time, groundwater is the largest single supply of fresh water available for use by humans.
Groundwater has been known to humans for thousands of years. Scripture (Genesis 7:11) on the Biblical Flood states that "the fountains of the great deep (were) broken up," and Exodus, among its many references to water and to wells, refers (20:4) to "water under the Earth." Many other ancient chronicles show that humans have long known that much water is contained underground, but it is only within recent decades that scientists and engineers have learned to estimate how much groundwater is stored underground and have begun to document its vast potential for use. An estimated one million cubic miles of the world's groundwater is stored within one-half mile of the land surface. Only a fraction of this groundwater, however, can be practicably tapped and made available on a perennial basis through wells and springs. The amount of groundwater in storage is more than 30 times greater than the nearly 30,000 cubic-miles volume in all the fresh-water lakes and more than the 300 cubic miles of water in all the world's streams at any given time (Figure 1).
How Groundwater Occurs
It is difficult to visualize water underground. Some people believe that groundwater collects in underground lakes or flows in underground rivers. In fact, groundwater is simply the subsurface water that fully saturates pores or cracks in soils and rocks. Groundwater is replenished by precipitation and, depending on the local climate and geology, is unevenly distributed in both quantity and quality. When rain falls or snow melts, some of the water evaporates, some is transpired by plants, some flows overland and collects in streams, and some infiltrates into the pores or cracks of the soil and rocks. The first water that enters the soil replaces water that has been evaporated or used by plants during a preceding dry period. Between the land surface and the aquifer water is a zone that hydrologists call the unsaturated zone. In this unsaturated zone, there usually is at least a little water, mostly in smaller openings of the soil and rock; the larger openings usually contain air instead of water. After a significant rain, the zone may be almost saturated; after a long dry spell, it may be almost dry. Some water is held in the unsaturated zone by molecular attraction, and it will not flow toward or enter a well. Similar forces hold enough water in a wet towel to make it feel damp after it has stopped dripping.
After the water requirements for plants and soil are satisfied, any excess water will infiltrate to the water table—the top of the zone below which the openings in soil and rocks are saturated. Below the water table, all the openings in the soil or rocks are full of water that moves through the aquifer to streams, springs, or wells from which water is being withdrawn. Natural refilling of aquifers at depth is a slow process because groundwater moves slowly through the unsaturated zone and the aquifer. The rate of recharge is also an important consideration. It has been estimated, for example, that if the aquifer that underlies the High Plains of Texas and New Mexico—an area of slight precipitation—were emptied, it would take centuries to refill the aquifer at the present small rate of replenishment. In contrast, a shallow aquifer in an area of substantial precipitation may be replenished almost immediately.
Aquifers can be replenished artificially. For example, in some regions large volumes of groundwater used for air conditioning are returned to aquifers through recharge wells. Aquifers may be artificially recharged in two main ways: One way is to spread water over the land in pits, furrows, or ditches, or to erect small dams in stream channels to detain and deflect surface runoff, thereby allowing it to infiltrate to the aquifer; the other way is to construct recharge wells and inject water directly into an aquifer. The latter is a more expensive method but may be justified where the spreading method is not feasible. Aquifer storage and recovery is an artificial recharge method where treated surface water is injected into the aquifer during wet periods and withdrawn from the aquifer during dry periods. Although some artificial-recharge projects have been successful, others have been disappointments; there is still much to be learned about different groundwater environments and their receptivity to artificial-recharge practices.
A well, in simple concept, may be regarded as nothing more than an extra large pore in the rock. A well dug, bored or drilled into saturated rocks will fill with water approximately to the level of the water table. If water is pumped from a well, gravity will force water to move from the saturated rocks into the well to replace the pumped water. This leads to the question: Will water be forced in fast enough under a pumping stress to assure a continuing water supply? Some rock, such as shale or solid granite, may have only a few hairline cracks through which water can move. Obviously, such rocks transmit only small quantities of water and are poor aquifers. By comparison, rocks such as fractured sandstones and cavernous limestone have large connected openings that permit water to move more freely; such rocks transmit larger quantities of water and are good aquifers. The amount of water that an aquifer will yield to a well may range from a few hundred gallons a day to as much as several million gallons a day.
Quality of Groundwater
For a region as a whole, the chemical and biological character of groundwater is acceptable for most non-potable uses. The quality of groundwater in some regions, particularly shallow groundwater, may be changing as a result of human activities. Groundwater is less susceptible to bacterial pollution than surface water because the soil and rocks through which groundwater flows screen out most of the bacteria. Bacteria, however, occasionally find their way into groundwater, sometimes in dangerously high concentrations. But freedom from bacterial pollution alone does not mean that the water is fit to drink. Many unseen dissolved mineral and organic constituents are present in groundwater in various concentrations. Most are harmless or even beneficial; though occurring infrequently, others are harmful, and a few may be highly toxic.
Water is a solvent and dissolves minerals from the rocks with which it comes in contact. Groundwater may contain dissolved minerals and gases that give it the tangy taste enjoyed by many people. Without these minerals and gases, the water would taste flat. The most common dissolved mineral substances are sodium , calcium, magnesium, potassium, chloride, bicarbonate, nitrate, phosphate and sulfate. In water chemistry, these substances are called common constituents.
Water typically is not considered desirable for drinking if the quantity of dissolved minerals exceeds 1,000 mg/L (milligrams per liter). Water with a few thousand mg/L of dissolved minerals is classed as slightly saline, but it is sometimes used in areas where less-mineralized water is not available. Water from some wells and springs contains very large concentrations of dissolved minerals and cannot be tolerated by humans and other animals or plants. Many regions of the world are underlain at depth by highly saline groundwater that has only very limited uses.
Dissolved mineral constituents can be hazardous in large concentrations to animals and plants; for example, too much sodium in the water may be harmful to people who have heart trouble. Boron is a mineral that is good for plants in small amounts, but is toxic to some plants in only slightly larger concentrations.
Water that contains a lot of calcium and magnesium is said to be hard. The hardness of water is expressed in terms of the amount of calcium carbonate—the principal constituent of limestone—or equivalent minerals that would be formed if the water were evaporated. Water is considered soft if it contains 0 to 60 mg/L of hardness, moderately hard from 61 to 120 mg/L, hard between 121 and 180 mg/L, and very hard if more than 180 mg/L. Very hard water is not desirable for many domestic uses; it will leave a scaly deposit on the inside of pipes, boilers, and tanks. Hard water can be softened at a fairly reasonable cost, but it is not always desirable to remove all the minerals that make water hard. Extremely soft water is likely to corrode metals, although it is preferred for laundering, dishwashing, and bathing.
Groundwater, especially if the water is acidic, in many places contains excessive amounts of iron. Iron causes reddish stains on plumbing fixtures and clothing. Like hardness, excessive iron content can be reduced by treatment. A test of the acidity of water is pH, which is a measure of the hydrogen-ion concentration. The pH scale ranges from 0 to 14. A pH of 7 indicates neutral water; greater than 7, the water is basic; less than 7, it is acidic. A one unit change in pH represents a 10-fold difference in hydrogen-ion concentration. For example, water with a pH of 6 has 10 times more hydrogen-ions than water with a pH of 7. Water that is basic can form scale; acidic water can corrode. Water for domestic use generally should have a pH between 5.5 and 9.
In recent years, the growth of industry, technology, population, and water use has increased the stress upon both our land and water resources. Locally, the quality of groundwater may have been degraded. Municipal and industrial wastes and chemical fertilizers, herbicides, and pesticides not properly contained may have entered the soil, infiltrated some aquifers, and degraded the groundwater quality. Other pollution problems include sewer leakage, faulty septic tank operation, and landfill leachates. In some coastal areas, intensive pumping of fresh groundwater has caused salt water to intrude into freshwater aquifers, a situtation known as saltwater intrusion (Figure 3).
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