Goundwater Essay, Research Paper
Although 70 percent of the Earth’s surface is covered by water, 97 percent of it is contained in the oceans and is too salty to drink (Cunningham and Saigo, 1992). Therefor, fresh water is a precious resource that in some areas of the world is so scarce, that people will fight wars over it. About 25 percent of the world’s fresh water is groundwater. Although groundwater is not as accessible as surface water (lakes, rivers and streams), people are willing to dig wells to obtain it in areas where the climate is too dry to allow much surface water. Many farmers pump water from wells to support irrigation. This is often done with windmills, a very cheap form of energy. In some dry areas like Arizona, large cities use groundwater for their entire water supply. Although groundwater is an excellent water source, there are many complications. For example, it is a resource that is difficult to assess since it is not directly visible. Therefor with out complicated measurements, it is hard to say how much water there is left in your well. It is also difficult to tell where exactly the source of recharge for your well is. Thus if an area near your well becomes polluted, the pollution may seep into the groundwater feeding your well.
The purpose of this project is to explore the subject of groundwater, its place in the water cycle, how it seeps through the ground and how it accumulates in underground pockets called aquifers. The project will also examine the many uses of groundwater in agriculture as well as environmental problems that are presently threatening the quantity and quality of groundwater reserves.
THE WATER CYCLE
The water cycle is a constant movement of water in the air, in the ground and on the surface of the Earth. It starts when water evaporates from plants, the ground, lakes, rivers and oceans. This is called evapotranspiration.
As the vapor rises, the surrounding air gets colder and colder. The dew point is the temperature at which water begins to condense (change into it’s liquid form). At the dew point, it clouds form. Clouds consist of tiny water droplets which remain suspended in the air until they become too large for turbulence to hold up. When this happens, some form of precipitation (rain, hail or snow) falls on the earth and either seeps into the soil or flows across the surface. Most of the time, it does both. Whatever doesn’t evaporate or run off infiltrates into the ground and is called groundwater. When it flows across the surface it is called surface runoff. The surface runoff goes into an ocean, river or stream where it evaporates in to the atmosphere again.
Water infiltrates deeper and deeper through the ground until it eventually reaches a water table, which is the point where water becomes saturated. The water tends to percolate through the ground in such a way to level the water table just as water tends to seek it’s own level in a lake. Water moves very slowly through the ground so the water table never really levels out, but rather, follows the topography of the landscape. The tendency for the water table to level itself results in depressions being filled with water. Thus in a depression, the water table will be above the ground. Since streams and lakes sit in depressions the ground water is constantly feeding them. Thus ground water joins the surface runoff and can evaporate again, repeating the cycle. D’arcy’s Law says that as water infiltrates into the ground and is pulled downward by gravity, it follows curved.
The rock or dirt immediately beneath the earth’s surface is made up of solids and voids. The voids are cracks in the rock, air pockets or sand. All water beneath the surface is in these spaces and is called underground or subsurface water. Subsurface water can be found in two different zones: One zone is immediately underneath the surface and contains water and air in the voids. This is called the unsaturated zone. It is almost always has a second zone called the saturated zone right beneath it. The water in the saturated zone is called groundwater and is the only water used in wells and springs.
AQUIFERS AND ARTESIAN WELLS
As water seeps through the ground, it sometimes becomes trapped in pockets called aquifers. If you dig a well into these, it could be a rich water supply. The previous paragraph shows how water seeps evenly when the ground is homogeneous, aquifers usually indicate that the ground is heterogeneous, that is it consists of layers of materials that have different permeabilities. If a layer of very porous materials like gravel or sand becomes sandwiched in between two impermeable layers of clay or silt this could be the start of an aquifer, a water bearing area under the earth’s surface.
When precipitation infiltrates into the ground, much of it will enter that layer of gravel or sand. Then, since the permeable layer is enclosed by impermeable layers, the water simply collects in the porous layer. Wells dug into the lower parts of aquifers are especially good because as the water is pumped out, new groundwater constantly replenishes them because it seeps through the permeable material very quickly.
There are two types of aquifers: Confined and unconfined. A confined aquifer is usually under pressure from the weight of the water it contains and the ground above it, whereas an unconfined aquifer must be pumped. Because of the fact that the unconfined aquifer is above the confined, it is easier to reach so most small wells are dug into them. But, when a deep well is dug into a confined aquifer, this often results in an artesian well, or spring. This is when a pressurized aquifer is punctured by a pipe and water sprays out with having to be pumped.
Figure 5 Types of aquifers. Modified after Enger and Smith, 1992
Figure 6 A cone of depression in the water table under a heavily pumped well. After Cunningham and Saigo, 1992
Sometimes after an aquifer is heavily pumped, a cone of depression may form around the well. This is a lowering of the water table which results when water is removed faster than it seeps back into the area. This could cause shallow wells nearby to dry up and make pumping more expensive.
Sometimes when a cone of depression forms and an aquifer becomes entirely dried up, the ground can get too lightly packed and the surface may cave in. A sink hole or large surface crater is caused by the collapse of an underground channel or cavern.
Some aquifers can be very small, but others can span miles. The Ogallala aquifer, the largest aquifer in the world and the biggest body of fresh water is in the United States. It lies beneath eight different states and is as deep as 1200 feet in some places. Each year, mostly agricultural sources withdraw more than the total annual flow
of the Colorado River, and the water table has dropped 150 feet (Cunningham and Saigo, 1992). Almost all the states Ogallala touches are dry places with huge demands for water.
USES OF GROUNDWATER
Groundwater plays an important role in agriculture across the world. Farms and crops in dry areas use irrigation as a means of obtaining water. This allows crops like cotton and wheat to be grown in areas where the moisture regime would only be sufficient to allow cattle or sheep ranching. Some time ago, irrigation could only be accomplished in plains and flat areas but due to the innovation of a sprinkler-like system called center pivot irrigation, hills and elevated terrain can be irrigated as well.
The use of irrigation water from the Ogallala Aquifer has allowed farmers in the mid-western United States to increase agricultural productivity by up to 3 fold. The heavy use of the aquifer since these pivot pump systems were invented in the sixties resulted in a major decline in the water level. Many people are worried that US food production will decrease significantly over the next twenty years due to the drying of this aquifer. Many farmers are crying out to have water pumped south from lake Michigan, but the impact would be devastating to the water levels of the Great Lakes and to the amount of water flowing down the St Lawrence River. In Canada, irrigation is used in the Okanagan valley for growing fruits in orchards.
Besides declining water levels, there are other problems including groundwater pollution. The most common causes of groundwater pollution are waste and chemical dumps, graveyards, out houses or other sewage disposals. Although a dump may be placed many miles away from any populated area, groundwater seeps through the ground in odd ways.
The best documented case of this was in New York near Niagara Falls, a place called Love Canal. In the 1890s, a man named William Love began the construction of a canal and it was never finished, leaving a practically useless hole in the ground. But in the 1930s, when industries starting coming to the area it was sure that they would find a use for the hole. Forty years after William T. Love began construction, companies in the surrounding area began dumping chemical waste into the canal. Especially a company called Hooker Chemicals and Plastics Corporation. It bought the land in 1942 and they dumped approximately 352 million pounds into the canal until 1952 (www.ethics 101.org). Worried about their public opinion, Hooker just covered the canal over.
Later, the Education Board built a playground and elementary school there, and through the next twenty years, the waste that had been buried in the Love Canal began to seep through the soil and enter peoples’ basements, contaminate underground pipes and wells and enter the Niagara River poisoning the fish Lake Ontario so much that the fishery had to be closed. It was not until the 1970s that the danger of these wastes were understood. In 1977, the elementary
school was closed and the families were relocated, but the fish from Lake Ontario are still too contaminated with PCBs and Mirex to be sold commercially (Chiras, 1988).
Groundwater is a precious resource which needs to be managed carefully. Due to the fact that it is not visible it can be easily overused. Besides being used for irrigation and drinking water in dry areas, groundwater feeds streams, lakes, rivers, swamps, marshes, ponds and other types of aquatic ecosystems and is therefor an important component in our natural world. Many plants and animals rely on these aquatic habitats, which would all dry out if they were not constantly being fed through groundwater. The water cycle is very complex and difficult for us to understand and we often use the land in a way that upsets the water cycle because we cannot see the movement of groundwater and the water table. Water seeps through the ground in accordance to the laws of physics. When the ground is permeable, the water table tends to be more level then the surface topography. Thus the tops of hills tend to be dry whereas valleys tend to collect water. How water will move through the ground and where exactly it will collect is difficult to predict unless you know the exact composition of the ground that it is moving through and how permeable each layer is. Because of this pollution may wind up mile from where it was put and many people may end up drinking contaminated water.
Allan, J. D. 1995. Stream Ecology: Structure and Function of Running Waters, Chapman and Hall. Pages 1-4.
Chiras, D. D. 1988. Environmental Science: A framework for decision making. Benjamin Cummings. Pages 415-417.
Cunningham, W. P. and B. W. Saigo. 1992. Environmental Science: A Global Concern, WC Brown Publishing Group. Pages 304-316.
Enger, E. D. and B. F. Smith. 1992. Environmental Science: A Study of Interelationships, WC Brown Publishing Group. Pages 342-363.
Freedman, B. 1998. Environmental Science: A Canadian perspective, Prentice Hall. Pages 32-39.
Smith, R. L. 1996. Ecology and Field Biology: Fifth Edition, Harper Collins Publishers Inc. Page 66.
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