How To Understand How Water Is Stored

Transcription

1 Underground water, origin, aquifer, aquicludes, artesian wells, underground provinces of India and its role as geological hazard. The subsurface water is distinguished in two main types Vadose water which occurs from surface downwards upto a variable depth and is in a state of downward movement under the influence of gravity. Its movement is commonly described as infiltration. The thickness of soil and rock through which the vadose water infiltrates is called zone of aeration. Ground water This includes all the subsurface water reaching a depth below which all the pore space, openings and other cavities of the soil and rock are completely filled with water. The thickness, length and width of the saturated strata, the aquifer, constitute the groundwater reservoir in that area. Water table is the name given to the upper surface of the zone of saturation.

2 The following facts are described briefly The water which gets stored in the ground water reservoir through infiltration etc is known as underground water. It has a suitable composition in most cases and is free from turbidity, objectionable colours and pathogenic organisms requiring not much treatment. They are rich in dissolved salts, minerals and gases etc. It is relatively safe from hazards of chemical and biological pollution. Its supplies are not quickly affected by drought and other climatic changes and are more dependable. It is available locally and may be tapped and distributed easily at much lesser costs. Sources of ground water There are following three sources of ground water- Meteoric water - It is water derived from rain and snow i.e. precipitation of water. The precipitation infiltrates into the ground and travel downwards to meet the zone of saturation and become part of aquifer. Water may also be contributed by surface sources like rivers, springs, lakes and seas. In case of influent stream the water table is lower than the water table of stream and water is seeped into the ground. However reverse phenomena can also happen when existing water table is high then river water table and thus ground water nourishes the stream i.e. effluent stream. Connate water This is the water present in the rocks right from the time of their deposition in an aqueous environment. Incomplete compaction of the rock may cause retention of some water and may be encountered in lime stones, sand stone sand gravels. It is commonly saline in nature and is of no importance for use. Juvenile water It is also called magmatic water and is of only theoretical importance. It is the water formed in the cracks or crevices on pores of rocks due to condensation existing below the surface of the earth.

3 Hydrological cycle It is chain of events involving water as it exists in all forms on or within earth. The essential feature of water cycle are : evaporation (from land and oceans), condensation and precipitation and return to land and oceans through interception, run off, infiltration, percolation etc. It is simplified description of the ways in which water moves from one place to another and how much is transported. The water in the atmosphere condenses to form clouds and then falls as rain or snow. The rain water divides itself into four parts Much of the rain soaks into the ground by infiltration and forms groundwater. Another part flows over the surface as runoff and finds its ways into rivers; ultimately it reaches to the sea. Some of the water in the ground returns directly to atmosphere by evaporation. Another part is absorbed by plant roots. It is carried upto leaves and returned to the atmosphere by transpiration. Some of the ground water may come out upon the surface again as springs or through wells but a large part of it reaches the sea through underground circulation. From the sea, the water is again drawn into atmosphere by evaporation thereby completing the hydrological cycle.

4 Types of aquifers- All rocks / sediments strata are neither capable nor suitable for holding and transmitting groundwater. The following terms are described to define following types of aquifers Aquifer It is defined as rock mass layer or formation which is saturated with ground water and which by virtue of its properties is capable of yielding the contained water at economical costs when tapped. Gravels, lime stones and sand stones generally form good aquifers when occurring in suitable geological conditions and geographic situations. The aquifers can be divided into two groups Unconfined aquifers The aquifers in which ground water occurs under atmospheric pressure are called unconfined aquifer. If a well is drilled in an unconfined aquifer, the water level in it will represent the water table. Water occurring in this type of aquifer is called free groundwater. Water table- The depth of upper surface of zone of saturation in free ground water is called water table. Perched aquifer It is a term used for isolated water table in aquifer held by small extension of impervious rock within a large pervious tract. In such cases, the main water table is located much below. Supplies from such isolated reservoirs with perched water table are often unreliable. Confined aquifer A confined aquifer is that in which aquicludes lie both above and below it. The aquicludes restrict the movement of groundwater and as result in confined aquifer the groundwater moves under pressure. Naturally water held in this type of aquifer is not under atmospheric pressure but under a great pressure due to confining medium. The upper surface of water in confined aquifer is called peizometric surface. A region supplying water to a confined aquifer is called recharge area. In the recharge area, the rain water infiltrates underground through the soil. Aquiclude A rocky body or formation which may be porous enough to hold enough quantity of water but which by virtue of its other properties does not allow an easy and quick flow through it. It is to be treated as a practically impermeable rock mass. Compacted clay formations are the best examples of aquicludes. Aquifuge It is an absolutely impermeable rock formation through which there is no possibility of storage or movement of water. Such a formation is almost free from pores and other interstices. Examples are compact interlocking granites and quarzites. Aquetard- It is that geological formation, which does not yield water freely to wells due to its lesser permeability, although seepage is possible through it. The yield from such a formation is thus insignificant.

5 Artesian wells/aquifers These are wells that to water confined groundwater under pressure in an aquifer. When the well is dug or drilled through the confining layer (aquicludes) into an aquifer, water rises up in the well and flows at the surface. The conditions necessary for artesian well are as follows- Presence of an aquifer which must be confined between two impermeable beds. These beds prevent the water from escaping. The aquifer must be inclined so that end of it is exposed at the surface from where it can receive water. Absence of leakages The artesian aquifer is a confined type of aquifer in which the contained water is under such a hydrostatic head that when tapped, the water will rush to the surface and may rise above ground to certain height.

6 Wells Wells are defined as openings or holes dug or drilled into an aquifer (water bearing formation) with a view to of withdrawing water for drinking, agricultural or other uses. Water may flow through these wells either due to natural hydrostatic pressure or may have to be pumped out. Depending upon the methods of construction the wells may be classified as dug wells, driven wells, bored wells, jetted wells and drilled wells. Dug wells These wells are excavated by means of picks and shovels and their diameter is usually more that 1 m. Dug wells seldom exceed a depth of 20 m. Driven wells The wells in the unconsolidated materials may be constructed by driving a pipe at the end of which there is a drive point. The diameter of these wells seldom exceeds seven centimeters. Bored wells The bored wells are constructed in the unconsolidated materials by means of hand or power augers. Jetted Wells These wells are excavated in the looses earth materials by the force of the jet or water which is produced by pumping water through hollow drill rods. Drilled Wells The water from unconsolidated aquifers is extracted by drilling deep wells. These wells may be constructed either by churn drilling or by hydraulic rotary drill methods. The drilled wells may attain a depth of 70m or more.

7 Basic concepts in well hydraulics Static water level When a well is dug into an unconfined aquifer, water enters into it and after some time acquires a stable level. This level, when expressed as distance from the surface is termed as static water table. Pumping water level The level of water in the well as determined during pumping is called pumping water level. Discharge or yield Each well yield only a specific quantity of water per unit time depending upon the characteristic of the aquifer and design of the well. The volume of water discharged through the well per unit of time is called its yield and expressed in cum / day. Drawdown This term gives the difference between static water level and the pumping water level. At any time during pumping the level of water at different points from well may be actually at different heights with respect to the level in the well. It is best represented by a curve and this curves known as drawdown curve. Cone of depression The cone made after drawing water from the well. The lowering down of water is maximum nearest to the well and it decreases with the increase in distance from the well. When plotted around the well it is formed as cone. Radius of influence It is the horizontal distance from the centre of a well to the extreme of the cone of depression and is indicator of the nature of aquifer. Thus, a very large radius of influence and a relative shallower cone indicates a highly permeable formation and conversely a smaller radius of influence with a deeper cone of depression is found in less transmissive aquifers. Recharge This means water added back into the aquifer naturally or by artificial methods.

8 Hydrological properties of rocks Porosity - Porosity is the measure of the pore spaces in rocks. It is defined as the percentage of pore spaces existing in a given volume of rock. It is measure of the actual space in soil or rock in which water may be held. Porosity of rock mass depends upon the size, shape and pattern of arrangement of its component mineral grains. Clay has porosity %, sand %, granite %. Permeability - Permeability is the capacity of the rocks to transmit water. It is measured by the quantity of water passing through a unit cross section of the aquifer. Transmissibility is another term which represents same physical meaning as the capability of the entire soil of full width and depth (i.e. B X D) is represented by permeability where as that of the soil of unit width and full depth is known as transmissibility. Example sand 300 cm / day, clay 3-5 cm / day. Specific retention The ratio of the volume of water retained to the volume of aquifer material expressed as percentage. Specific yield The ratio of the volume of water which a saturated aquifer will yield by gravity flow to its total expressed as a percentage. Specific retention + specific yield = Porosity.

9 Springs A spring is natural outflow of ground water. There are two types of springs - Descending type They are also called non pressure type and include all those type of springs in which ground water pours out from a water bearing formation due to one or other of following geological conditions A part of water bearing formation is exposed on the surface due to erosion of the covering rock. A ground water reservoir starts overflowing An aquifer forms a natural wedge due to faulting etc A fissure or system of fissures connects the surface with saturated formation. Ascending Type The water is always some hydrostatic head. Where such a layer is intercepted by a natural fissure or geological continuity water gushes out with force. Springs can be further divided into three types as follows- Gravity Springs When the ground water table rises high and water overflows through sides of a natural valley or depression. Surface spring Impervious obstruction supporting the underground water or storage become causing the water table go up and get exposed to the ground surface. Artesian spring When the storage of water is under pressure, by which the spring is formed, is known as artesian spring. These type of springs are able to provide almost uniform quantity of water.

10 Groundwater in engineering projects / role of ground water as geological hazard The economy, safety, design and construction of all major engineering projects like dams and reservoirs, tunnels and highways etc are intimately related to the groundwater regimes of the area in which projects are located. Dams and reservoirs A dam is built across a river primarily to store water in the form of reservoir. The whole idea of dam would become irrelevant if the foundations on which it is built are made of porous rocks or if a stretch of a reservoir rock is permeable. The leaking water will affect inventory and may damage abutments and even fail the dam. Tunnels All tunnels are underground passages for traffic or water passage. Depending upon the level of water table, the design considerations require full knowledge of ground water parameters. Highway and cuts - Groundwater present very complex problems in laying out of highways and airfields. If the water table is high and is near just surface, then same may pose a serious problem. The stability of slopes is affected, thus study of groundwater is regime is very important. Irrigation projects The rise in water table to the root zone of plants causes water logging, and this may affect crops. The salinity of the soil rises and the land become unsuitable for agriculture. A thorough study of groundwater regime in irrigated areas and water logged areas form important step in reclaiming the land for cultivation.

11 Underground provinces of India - A groundwater province encompasses regions with similarities of groundwater. As long as the similarity exists, these regions need not be adjacent to each other. Also the rock types in a ground water province can be of different types with varying compositions. Steps adopted for exploration, development of groundwater are also more or less similar within the same province. India is made of three prominent provinces as follows The northern mountainous belt covered by Himalaya system mountains from Laddakh and Kashmir in the north west and extending upto Assam in the north east covering Himachal, Punjab and Uttaranchal. In these regions, runoff is the most characteristic features, infiltration being the least on the hill slopes. Hence the potential of groundwater in these areas is limited to the inter mountain valleys and plains. The Indo Gangetic plains extending from Punjab in the west to Bihar and beyond the east. The large alluvial tract constitutes one of the largest and most potential ground water reservoirs of the world. This tract is made of large number of aquifers. Peninsular India including Maharashtra and the entire Southern states present varying situations with respect to groundwater potential depending primarily on the geological make up. Peninsular India is made up of hard, massive, quite deep and thick rock but variously fissured formations. The weathered rocks are good receptacle of rain water and water infiltrates to the underlined aquifers. These are area of good water development down to a depth of 100 m or so. The coastal and deltaic areas provide sufficiently large and suitable groundwater regimes for economic consumptions. It may be said with generalization that groundwater quality as available in greater parts of India is quite suitable for all the tree types of uses i.e. drinking, irrigation and industrial.

12 Fresh water crisis It has been widely accepted that inspite of vast ground water reserves, India is fats heading towards a fresh water crisis. Large population is facing great shortage of water, especially in dry spells of year. The shortage of freshwater is due to two main causes Improper management of available water resources. Increasing environmental degradation leading to pollution of water resources, both of surface and under ground. Following are main reasons for increased water demand- Increase in population Fast growing urbanization. Fast growing industrialization. Expansion of irrigated areas Water is getting polluted due to discharge of huge volumes of untreated water in these water bodies. Ground water has been reported a steep fall in large areas of the country due to fats withdrawal of water. Increasing deforestation and resulting soil degradation also contributes in decreasing the replenishment. Clouds coming down through industrially polluted air can not be expected to bring fresh water. Salinity of soil is also contributing crisis for fresh water as salt content affects the aquifers.