G5-UNDERGROUND WATER

UNDERGROUND WATER

When rain falls on the landmass, some of the water runs off as streams and rivers, some evaporates, and the remaining water percolates into the ground. This groundwater provides water for domestic needs, for irrigating crops, and for industrial use. In some areas, however, overuse of this basic resource has resulted in depletion, land subsidence, saltwater intrusion, and increased pumping cost.

The amount of water that penetrates into the ground is controlled by several factors. They are: a.steepness of slope, b.nature of surface material, c.intensity and duration of rainfall, and d. type and amount of vegetation.

Heavy rain, falling upon steep slopes underlained by hard materials that prevents percolation, will obviously result in a high percentage of runoff. On the otherhand, if rain fall slowly, steadily and upon gentle slopes composed of permiable materials that could be easily penetrated, results in high percentage of percolation. Hence, the percolation and amount of groundwater that is stored depends upon the nature of the subsurface materials and quantity and duration of rainfall.

Nature of subsurface materials: The surface water seeps into the ground because the bedrock, sediments, and soil contain voids or openings. These openings are similar to those of a sponge and are often called pore spaces. The quantity of groundwater that can be stored depends on the number of pore spaces present in the material. The rock that consists of such pore spaces are called porus rock.

Porosity: The ratio between volume of pores to the total volume of rock is known as porosity. Let us understand this with an example. Clay have 50% porosity, whereas gravels have 20 percent porocity. In the case of clay, the ratio between pore spaces to the total volume is 0; that means the pore spaces occupy 50 percent and soild particles occupy the remaing 50 percent. Where as in gravel, the ratio between the pore spaces to the total volume is 20:80; that means the pore spaces occupy 20 percent and soild particles occupy the remaing 80 percent. So, sediments such as clay has high porosity where as sediments such as gravel has low porosity. Where sediments of various sizes are mixed, the porosity is reduced because the finer particles tend to fill the openings between the larger grains.

Most of the igneous and metomorphic rocks, as well as some sedimentary rocks have very low porosity. It is mainly because, they do not have much pore spaces. They become porus, only when there are cracks and fissures in these rocks.

Permiability: Groundwater moves by twisting and turning through the pore spaces that are present in a rock or sediment. A rock or sediment should have the ability to transmit the water through its pore spaces. The smaller the pore spaces, the slower the water moves. The ability or capcity of a rock or sediment that allow the water to perculate through its pore spaces is known as permiability.

Although porosity of clay is high, its pore spaces are so small that water is not allowed to percolate, whereas porosity of gravel is very low but water is allowed to perculate into the ground. Hence, clayey soils are non permiable and latetrite soils are permiable in nature.

Permeable rock strata or sediments such as sand or gravel that transmit groundwater freely are called aquifers. Impermiable layers composed of materials such as clay that hinders or prevents water movement are termed aquicludes.

So, porosity is not always a reliable guide to the amount of ground water that can be produced. The significant factor in determining the availability groundwater is the permiability.

Water Table: When rain falls on the surface, the soil retains a portion of it as moiture. The soil moiture is used as a source of water by plants. When the soil is saturated, the excess water penetrates downward until it reaches a zone, where pore spaces in sediment and rock are compeltly filled with water. This belt is called Saturation Zone. The water held in this zone is called groundwater. The upper limit of groundwater zone is known as the watertable. Extending upward from the watertable is the intermediate zone.

Although we cannot observe the watertable directly, its position can be mapped and studied in detail in areas where wells are numerous. The water level in wells coincides with the watertable of the groundwater. Such maps reveal that the level of water table fluctuates and changes now and then. A number of factors contribute to the fluctuations in the water table. For example, variations in rainfall and varition in permeability lead to uneven infiltration and thus to differences in water table level. The water table level may fluctuate considerably during the course of a year, dropping during dry seasons, and rising following periods of rain.

Erosional work of Groundwater: The primary erosional work carried out by groundwater is that of dissolving rock. Groundwater carries on , its rather unique and important role as an erosional agent in soluable rocks, especially in limestone regions. Although nearly insoluable in pure water, limestone is quiet easily dissolved in water containing carbon dioxide. Rainwater dissolves carbon dioxide from the air and decaying plants and becomes diluted carbomic acid. This infiltrates as groundwater and whenever groundwater comes in contact with limestone, the carbonic acid reacts with the calcite in the rocks to form calcium bicarbonate. It is a soluable material that is carried away in solution.

Caverns: Among the most spectacular results of groundwater’s erosional handiwork is the creation of limestone Caverns. Although most are relatively small, some have spectacualar dimensions. For example the Mammoth cave in Kentucky, USA is very famous cavern. It is the most extensive in the world, with more than 500 kilometers of interconnected passages.

Most caverns are created at or just below the water table in the zone of saturation. Here the groundwater follows the lines of weakness in the rock, such as joints and bedding planes. As time passes, the dissolving process slowly creates cavities and gradually enlarges them into caverns. The material that is dissolved by the groundwater is carried away and discharged into rivers.

The spectacular features that arouse the greatest curiosity for most cavern visitors are the stone formations and give some caverns a wonderland appearance. These features are created by the seemingly endless dripping of water over great span of time. These cave deposits, however, are also commonly called dripstones.

Perhaps the most familiar dripstones are stalactities. These icicle-like pendents hang from the ceiling of the cavern and form where water seeps through cracks above. When the water come in contact with air in the cave, some of the dissolved carbon dioxide escapes from the drop and calcite begins to precipitate. Deposition occurs as a ring around the edge of the water drop. As drop after drop follows, each leaves an infinitesimal trace of calcite behind, and a hollow limestone tube is created. Water then moves through the tube, remains suspended momentarily at the end, contributes a tiny ring of calcite, and falls to the cavern floor. The stalactite just described is appropriately called a Soda straw. Often the hollow tube of the soda straw becomes plugged or its supply of water increases. In either case, the water is forced to flow, and hence deposit takes place along the outside of the tube. As deposition continues, the stalactite takes on the more common conical shape.

The dripstones that form on the floor of a cavern and reach upward toward the ceiling are called stalagmites. The water supplying the calcite for stalagmite growth falls from the ceiling and splatters over the surface. As a result, stalagmites do not have a central tube and are usually more massive in appearence and rounded on their upper ends. Various dripstone features such as stalactite and stalgmites found in caverns are collectively called speleotherms.

Karst Topography: Some areas in the world, to a large extent, have been shaped by the dissolving power of groundwater. Such areas are said to exhibit karst topography. This term is derived from a plateau region located along the northeastern shore of the Adriatic Sea in the part of Yugoslavia called Slovenia where such topography is strikingly developed. The most common geologic setting for karst development is an area where limestone is present near the surface beneath a layer of soil. Karst areas characteristically exhibit an irregular terrain punctuated with many depressions, called sinkholes or sinks.

Sinkholes: Sinkholes commonly form in one or two ways. Some develop gradually over many years without any physical disturbance to the rock. The limestone immediately below the soil is dissolved by downward-seeping rain water that is freshly charged with carbon dioxide. With time, the bedrock surface is lowered and the fractures into which the water seeps are enlarged. grow in size, soil subsides into the widening voids, from which it is removed by groundwater flowing in the passages below. These depressions are usually deep and are characterized by relatively gentle slopes.

By contrast, sinkholes can also form suddenly and without warning when the roof of a cavern collapses under its own weight. Typically, the depressions created in this manner are steep-sided and deep. When they form in populous areas, they may represent a serious geologic hazard.

As many of our valuable natural resources, groundwater is beingexploited at an increasing rate. Thus, problems like saltwater intrution and groundwater contamination occurs at many places. Let us study in detail about how the underground water get contaminated.

Saltwater intrusion: In many coastal areas the groundwater resource is being threatened by encroachment of salt water . In order to understand this problem, we must examine the relationship between fresh groundwater and salt groundwater. Figure 7.6 is a diagrammatic cross section that illustrates this relationship in a coastal area underline by permeable homogeneous materials. Since fresh water is less dense than salt water, it floats on the salt water. Itforms a large, lens-shaped body that may extend to considerable depths below sea level. In such a situation, if the water table is 1 meter above sea level, the base of the fresh water body will extend to a depth of about 40 meters below sea level. Thus when excessive pumping lowers the water table by a certain amount, the bottom of the freshwater zone will rise by 40 times that amount. Therefore, if groundwater withdrawal continues to exceed recharge, the salt water intrudes into the fresh water zone. Deep wells and wells near the shore are usually the first to be affected.

Groundwater contamination: The pollution of groundwater is a serious matter, particularly in areas where aquifers supply a large quantity of water. A very common type of groundwater pollution is sewage. Its sources include an ever-increasing number of septic tanks as well as inadequate or broken sewer systems and barnyard wastes.

If groundwater is contaminated with bacteria from sewage, groundwater may become purified through natural processes. The harmful bacteria may be mechanically filtered out by the sediment through which the water perculates, destoyed by chemical oxidation, and/or assimilated by other organisms. In order for purification to occur, however, the aquifer must be of the correct composition. For example, extremely permeable aquifers such as highly fractured rocks, coarse gravel, or cavernous limestone have such large openings that contaminated groundwater may travel long distances without being cleansed.

Here , the water flows too rapidly and is not in contact with the surrounding material long enough for purification to occur. Although the contaminated water has travelled a long distance before reaching the well 1, the water moves too rapidly through the limestone to be purified. On the other hand, when water moves through sand or permiable sandstone, it can sometimes be purified within distances as short as a few tens of meters. The openings between sand grains are large enough to permit water movement, yet the movement of water is slow enough to allow ample time for its purification. As illustrated in figure no.7.8 the discharge from the septic tank percolates through the permeable sandstone, it is purified in a relatively short distance, before reaching the well 2.

Other sources of contamination: Other sources and types of contamination also threaten groundwater supplies. These include widely used substances such as fertilizers that are spread across the land surface, and pesticides. In addition, a wide array of chemicals and industrial materials may leak from pipelines, storage tanks and landfills. Some of these pollutants are classified as hazardous, meaning that they are highly toxic. As rainwater oozes through the refuse, it may dissolve a variety of organic and inorganic materials. If the leached material reaches the water table, it will mix with the groundwater and contaminate the supply. Since groundwater movement is usually slow, polluted water may go undetected for a considerable time.

Most contamination is discovered only after drinking water has been affected. By this time, the volume of polluted water may be very large, and even if the source of contamination is removed immediatly, the problem is not solved. Although the sources of groundwater contamination are numerous, the solutions are relatively few. Once the source of the problem has been identified and eliminated, the most common practice in dealing with contaminated aquifers is simply to abandon the water supply and allow the pollutants to be flushed away gradually. But the most effective solution to groundwater contamination is prevention.

Earlier, we have learnt that running water is a powerful agent of erosion. Infact, many of the present-day landscapes were modified by the widespread glaciers of the most recent ice age and still strongly reflect the handiwork of ice. Glaciers, of course, are not just a phenomenon of the geological past. As we shall see in the next lesson, they are still sculpturing and depositing in many regions today.

llPoints to Remember:

1 The surface water seeps into the ground because the bedrock, sediments, and soil contain voids or openings. These openings are similar to those of a sponge and are often called pore spaces

2. The quantity of groundwater that can be stored depends on the number of pore spaces present in the material. The rock that consists of such pore spaces are called porus rock.

3. The ratio between volume of pores to the total volume of rock is known as porosity. The ability or capcity of a rock or sediment that allow the water to perculate through its pore spaces is known as permiability.

4. Permeable rock strata or sediments such as sand or gravel that transmit groundwater freely are called aquifers. Impermiable layers composed of materials such as clay that hinders or prevent water movement are termed aquicludes.

5. Water that penetrates downward until it reaches a zone, where pore spaces in sediment and rock are completely filled with water is called Saturation Zone. The water held in this zone is called groundwater. The upper limit of groundwater zone is known as the watertable.

6. The groundwater follows the lines of weakness in the rock, such as joints and bedding planes. As time passes, the dissolving process slowly creates cavities and gradually enlarges them into caverns.

7. The features that are created by the seemingly endless dripping of water over great span of time. These cave deposits, however, are also commonly called dripstones.

8. Stalactities are icicle-like pendents hang from the ceiling of cavern and form where water seeps through cracks above. When the water is forced to flow, and hence deposit, along the outside of the tube and more common conical shape stalactites are created.

9. The term karst topography is derived from a plateau region called Slovenia where such topography is developed. It is located along the northeastern shore of the Adriatic Sea, Yugoslavia

10. Karst areas characteristically exhibit an irregular terrain punctuated with many depressions, called sinkholes or sinks. sinkholes are formed when the limestone immediately below the soil is dissolved by rain water and forms depressions. 1 dvering contaminated water