Many people assume that water is an unlimited natural resource. Is this true? Although water appears abundant, recent circumstances indicate that it is seriously vulnerable to pollution and depletion. Throughout this essay we will examine what ground water is, how it is important in the water cycle, how its contamination can affect us, and what we can do to protect our ground water from contamination.
Ground water is the water found in spaces between soil particles and rocks, within cracks of the bedrock. Ground water constitutes approximately 4% of all water in the hydrologic cycle. (Averett, McKnight, 1986) Some ground water can be found beneath the land surface in most of the United States. Because of its availability and general good quality, ground water is widely used for household needs and other purposes.
Water can be found beneath the ground almost everywhere. About 97% of the worlds fresh liquid water is ground water. The quality and amount of ground water that is available varies from place to place. Major reservoirs of ground water are referred to as aquifers. Aquifers in some of the provinces extended underground far beyond the areas where they are mapped at or near the land surface. (http://capp.water.usgs.gov/gwa/ch_1/L-text1.html) These aquifers occur in two types of geologic formations. Consolidated formations are those composed of solid rock with ground water found in the cracks. Unconsolidated formations are composed of sand and gravel, cobblestones, or loose earth or soil material. The amount of ground water in an unconsolidated formation varies depending on how closely packed the solid materials are and how fine-grained they are. Sand and gravel, and cobblestone formations are generally high-yield aquifers, whereas, finer-grained earth materials may have low yields.
Aquifers and aquifer systems can be grouped into three categories, depending on the degree of consolidation of the rocks and deposits that compose the aquifers. Rocks of Precambrian, Paleozoic, and early Mesozoic ages generally are consolidated; rocks of Cretaceous and Tertiary ages generally are semi consolidated; and deposits of Quaternary age generally are unconsolidated. (http://capp.water.usgs.gov/gwa/ch_1/L-text1.html)
Most people are more familiar with surface water than ground water. Surface water bodies such as lakes, streams and oceans can be seen all around, but not ground water bodies. One important difference between ground water and surface water bodies is that ground water moves much slower than surface water. Water in a stream may move several meters per minute, but water in an aquifer may move only a few meters per month. This is because ground water must overcome more friction, or resistance, to move through small spaces between rocks and soil.
The exchange of water between surface water bodies and aquifers is important. Rivers usually start as small streams and get larger as they flow downstream. The water they gain is often ground water. Such a stream is called a gaining stream. It is also possible for streams to lose water to the ground at some points. In these cases, aquifers are replenished or recharged by water from the losing stream. A stream that flows near the surface of an aquifer will lose water to the aquifer if the water surface in the stream is higher than the water table of the aquifer. A stream will gain water if the water surface of the stream is lower than the water table in the adjacent land. The water in transit through ground water systems may also be viewed as water in storage. (Speidel, Ruedisili, 1988)
How is Ground Water Important to the Water Cycle:
Ground water is an integral part of the water cycle. The cycle starts with precipitation falling on the surface. Runoff from precipitation goes directly into lakes and streams. Some of the precipitation return to the atmosphere by evapotranspiration (evaporation plus transpiration by plants), but much of it either flows overland into streams as direct runoff or enters streams as base flow (discharge from one or more aquifers). (http://capp.water.usgs.gov/gwa/ch_1/L-text1.html) The remaining water, called recharge water, drains down through the soil to the saturated zone, where water fills all the spaces between soil particles and rocks. Upon percolating downward below the water table, soil waters become ground waters. (Berner, Berner, 1987) The top of the saturated zone is the water table, which is usually the level where water stands in a well, if the local geology is not complicated. Water continues to move within the saturated zone from areas where the water table is higher toward areas where the water table is lower. When ground water comes to a lake, stream or ocean it discharges from the ground and becomes surface water. This water then evaporates into the atmosphere, condenses, and becomes precipitation, thus completing the water cycle.
Contaminated Ground Water and its Effects:
Ground water is often taken for granted. In many locations pollution is beginning to change the quality of the water. Contaminants which threaten peoples health have been found in several important ground water reservoirs. Some of the contaminants may be so expensive to remove that they make the water virtually unusable for years. Dissolved solids in ground water primarily result from chemical interaction between the water and the rocks or unconsolidated deposits through which the water moves. (http://capp.water.usgs.gov/gwa/ch_1/L-text1.html)
Ground water becomes polluted when undesirable substances become dissolved in water at the lands surface and are carried down, or leached, to the aquifer with the percolating water. To determine whether a particular substance will pollute a particular aquifer, the properties of the unwanted substance and the soil above the aquifer need to be considered along with the amount of the substance available for leaching.
Sometimes ground water contamination occurs naturally. Acid rain is form of contaminated precipitation that will seep into the ground water. Fortunately, most acid rain is neutralized (buffered) by soil and rocks so ground water is protected from the impact of acid rain. (http://www.agwt.org/trust_giles/info/acid_rain.html) The most serious contamination is usually the result of human activities on the land surface. An aquifer provides a plentiful water supply that often attracts a multitude of people to the overlying land. The water is used for such activities as drinking, personal hygiene, residential maintenance, and industrial and agricultural purposes. Many of these activities involve the use and disposal of chemicals, which are potential pollutants. When these chemicals are used or disposed of incorrectly unacceptable amounts can get into the ground water and contaminate it. Several valuable aquifers have been polluted by the people living and working above them.
Most human activities at the land surface cause some change in the quality of water in the aquifer beneath them. The importance of the effect of a particular activity is related to the amounts and types of contaminants released. The severity of an occurrence is also related to the ability of the soil and ground water system to degrade or dilute the contaminants, and the degree to which the contamination will interfere with uses of the water. Contamination is usually more serious in a drinking water supply than in water for other uses. Except where contaminated water is injected directly into an aquifer, essentially all ground water pollutants enter the aquifer through recharge water from the land surface. Some examples of the contaminants are pesticides, certain petroleum products, mercury, nickel, lead, nitrate nitrogen, bacteria and viruses, and petroleum residues and combustion products from automobiles along roadways. All of these are considered harmful if ingested in sufficiently high amounts, and in some instance may be carried into surface water bodies by ground water.
Each human activity has a particular impact on ground water. Some agricultural activities add nitrate, nitrogen and pesticides to the ground water. Residential areas add nitrate, nitrogen, and pesticides from landscaping activities. Those with septic systems usually add nitrate, nitrogen, bacteria, viruses, and synthetic organics used in household cleaning products and septic tank cleaners. Industrial activities tend to add organic chemicals and metals, through in widely varying amounts. Gasoline storage area (including service stations) may have leaks and spills of petroleum products. Roadways contribute petroleum pollutants leaked from vehicles and metals from exhaust fumes. The most concentrated impact comes from older sanitary landfills, whose leach ate may contain many different chemicals at relatively high concentrations.
Since ground water moves slowly, many years may pass before a pollutant released on the land surface above the aquifer is detected in water taken from the aquifer some distance away. Unfortunately, this means that contamination is often widespread before being detected. Even if release of the contaminant is stopped, it may take many years for an aquifer to purify itself naturally. Although water can be treated to remove contaminants, this can be very costly. The best protection against water pollution is prevention.
Ground water becomes depleted in areas where more water is being drawn out of an aquifer and consumptively used than is entering or recharging the aquifer. This usually causes a lowering of the water table, making the ground water more difficult and expensive to obtain. Deserts have been expanding rapidly in China because of drought and sinking underground water tables depleted by industrial and agricultural use and population growth. (http://www.worldwaterconservation.com/china.html). Ground water depletion is a major problem in the dry western part of the United States. In the eastern United States, precipitation continuously replenishes ground water supplies, and so depletion is most likely to be a problem in certain localized situations, or during droughts. A situation may involve someone pumping a large amount of water from a small aquifer and causing a neighbors well to go dry (lowering the water table below the well screen). Rapidly expanding urban areas often impose an extra burden on ground water supplies in the form of depletion and pollution. In coastal area, chronic over pumping can cause saltwater intrusion. Salt-water intrusion takes place in coastal areas where fresh water removal from the aquifer permits saline water from the ocean to intrude into the aquifer.
Protecting our ground water from contamination will require thoughtful management and cooperation on the part of citizens and the various levels of government. In many cases, land use planning is the best instrument available for protecting aquifers still containing good quality water. If potential contamination sources are prevented from locating over critical recharge areas, the risk of contamination can be greatly reduced. Careful use and proper disposal of the chemicals causing contamination is also necessary. Industries, farmers, and homeowners located above ground water supplies need to practice good management with respect to the use and disposal of chemicals. Regulations, which govern the use and disposal of hazardous wastes, need to be enforced. An equally important step is to make people aware of their potential impact on ground water. Action is needed to protect our valuable ground water resources in many parts of the United States.
Is water an unlimited natural resource? No! Although it is still plentiful now it is important that we understand and respect the fact that water can become contaminated and no longer be useful to us. Consider life with no water. We cannot survive without it, so be more observant and careful of your water usage and contamination. With everyones cooperation we can help reverse the effects of ground water pollution and continue to keep our water useable.
1)The American Well Owner, 2000, Number 1 Web site (http://www.agwt.org/trust_giles/info /acid_rain.html)
2)Kurtenbach, E. (4/6/??) Dust Storm Blasts Beijing. China News 2 pages. Retrieved 10/15/00 from the world wide web: http://www.worldwaterconservation.com/china.html
3)The Ground Water Atlas of the Untied States. Web site (http://capp.water.usgs.gov/gwa/ch_1/L-text1.html)
4)Averett, R. & McKnight, D. (1986). Chemical Quality of Water and the Hydrologic Cycle. Chelsea: Lewis Publishers, Inc.
5)Berner, E. & Berner, R. (1987). The Global Water Cycle. Englewood Cliffs: Prentice Hall
6)Speidel, D. & Ruedisili, L. & Agnew, A. (1988). Perspectives on Water Uses and Abuses. New York: Oxford University Press.