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Ground Water Hydrology

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Ground Water Hydrology

  1. 1. Ground Water Hydrology
  2. 2. Ground Water Hydrology Syllabus • General Water Balance, Regional groundwater balance, Distribution of subsurface water, different type of aquifers, occurrence of groundwater in hydrological formations, structures and type of wells, component of groundwater studies.
  3. 3. Ground Water Hydrology
  4. 4. Ground Water Hydrology • The ground water is considered a very important natural resource, in arid , semi arid and dry regions, this may be the only source of water supply. Even in humid areas, groundwater is considered a better resource for many economic and hygienic reasons.
  5. 5. Ground Water Hydrology
  6. 6. Ground Water Hydrology
  7. 7. Ground Water Hydrology • Ground Water • Has a suitable composition in most cases and is free from turbidity, objectionable colors, and pathogenic organisms and require not much treatment. • Is relatively much safer from hazards of chemical, radiogenic and biological pollution to which surface water bodies are exposed • Supplies are not quickly affected by drought and other climatic changes and hence are more dependable. • Being available locally in many cases may be tapped and distributed at much lesser cost using very little network of pipes
  8. 8. Ground Water Hydrology
  9. 9. Ground Water Hydrology • Sources of groundwater • Meteoric Water • It is the water derived from precipitation (rain and snow) although bulk of the rain water or melt water from snow and ice reaches the sea through the surface flows or runoffs a considerable part of precipitation gradually infiltrates into ground water. This infiltrated water continuous its downward journey till it reaches the zone of saturation to become the ground water in the aquifer. • Almost entire water obtained from ground water supplies belongs to this category.
  10. 10. Meteoric Water
  11. 11. Ground Water Hydrology Connote Water • This is the water present in the rocks right from the time of their deposition in an aqueous environment. During the process of formation of sedimentary rock in a lake or sea or river, depositions is followed by compaction, which leads to the squeezing out of most of the water present between the sediments. Sometimes however, incomplete compaction may cause retention of some water by these rocks which is known as connote water. And it may be found in rocks like limestone, sandstone and gravels. It is saline in nature and is of no importance as a source for exploitable groundwater.
  12. 12. Ground Water Hydrology
  13. 13. Ground Water Hydrology • Juvenile Water • It is also called magmatic water and is of only theoretical importance as far as water supply scheme is concerned. It is the water found in the cracks or crevices or porous of rocks due to condensation of steam emanating from hot molten masses or magmas existing below the surface of the earth. Some hot springs and geysers are clearly derived from juvenile water.
  14. 14. Ground Water Hydrology • Distribution of Ground Water • The water that goes below the surface of the land may be found to exist in two main zones or environments classified as Vadosa Water and phreatic water or groundwater • In the vadosa water zone itself, three different types of environment are distinguished; soil water, intermediate vadose water and capillary water.
  15. 15. Ground Water Hydrology • The soil water forms a thin layer confined to the near surface depth of the land. It may occur at depth between 1.0 to 9 m and is held up by the root zone of vegetable cover of the globe It is lost to the atmosphere by transpiration and evaporation. • The intermediate vadosa zone occurs immediately below the zone of soil water. It is in fact a zone of non saturation; water in this zone is moving downward under the influence of gravity. It is generally of smaller thickness and may be even absent in many cases. The above zones are sometimes collectively referred as zone of aeration. • The zone of capillary water, also called as capillary fringe. Is present only in soil and rocks of fine particles size underlying the vadosa zone. In the fine particle size zone, groundwater is drawn upward by capillary action, sometimes to height of 2-3 m above saturated zone lying underneath. Growth of vegetation in some desert is very often dependent on presence of capillary fringe.
  16. 16. Ground Water Hydrology
  17. 17. Ground Water Hydrology
  18. 18. Ground Water Hydrology
  19. 19. Distribution of Ground Water
  20. 20. Ground Water Hydrology • The Phreatic Water Zone • Also known as zone of saturation lies below the capillary fringe and is the water held in this zone that is called groundwater in the real sense. The upper surface of water in the zone marks the water table in the area. In this zone the layers or bodies of rocks which are porous and permeable, have all their open spaces such as pores, cavities, cracks etc. completely filled with water. All these openings are interconnected, so that a well dug into this openings are completely filled with water, there is no or very little downward movement of groundwater. In all ground water exploration programmes, the main objective is to locate this zone and determine its extent, geometry and character.
  21. 21. Ground Water Hydrology
  22. 22. Ground Water Hydrology
  23. 23. Ground Water Hydrology • Water Balance • In hydrology, a water balance equation can be used to describe the flow of water in and out of a system. • A system can be one of several hydrological domains, such as a column of soil or a drainage basin. • Water balance can also refer to the ways in which an organism maintains water in dry or hot conditions. It is often discussed in reference to plants or arthropods, which have a variety of water retention mechanisms, • general water balance equation is: P= Q+E+∆S • Where, • P= Precipitation • Q= Runoff • E= Evapotranspiration • ∆S = Change in Storage
  24. 24. Ground Water Hydrology • This equation uses the principles of conservation of mass in a closed system, whereby any water entering a system (via precipitation), must be transferred into either evaporation, surface runoff (eventually reaching the channel and leaving in the form of river discharge), or stored in the ground. This equation requires the system to be closed, and where it isn't (for example when surface runoff contributes to a different basin), this must be taken into account. • Extensive water balances are discussed in agricultural hydrology
  25. 25. Water Balance Model
  26. 26. Ground Water Hydrology • A water balance can be used to help manage water supply and predict where there may be water shortages. It is also used in irrigation, runoff assessment (e.g. through the RainOff model ), flood control and pollution control. Further it is used in the design of subsurface drainage systems which may be horizontal (i.e. using pipes, tile drains or ditches) or vertical (drainage by wells).To estimate the drainage requirement, the use of an hydrogeological water balance and a groundwater model may be instrumental.
  27. 27. Ground Water Hydrology
  28. 28. Water Balance Model
  29. 29. Ground Water Hydrology Ground Water • Study of sub surface flow is equally important since about 30 % of the world’s fresh water resources exist in the form of groundwater. Further, the subsurface water forms a critical input for the substance of life and vegetation in arid zones. Due to its importance as a significant source of water supply various aspects of ground water dealing with exploration, development and utilization have been extensively studied by workers from different disciplines, such as geology, geophysics, geochemistry, agricultural engineering, fluid mechanics and civil Engineering.
  30. 30. Ground Water Hydrology • Forms of Subsurface Water • Water in the soil mantle is called subsurface water and is considered in two zones • Saturated Zone • Aeration Zone.
  31. 31. Water table generally below surface, so water can seep in Water can soak into subsurface and become groundwater Where water table intersects surface, water can flow out
  32. 32. Ground Water Hydrology
  33. 33. Ground Water Hydrology Saturated Zone • This Zone is also known as groundwater zone in which all the pores of the soil are filled with water. The water table forms the upper limit and marks a free surface, i.e. a surface having atmospheric pressure.
  34. 34. Ground Water Hydrology Zone of Aeration • In this zone the soil pores are only partially saturated with water. The spaces between the land surface and the water table marks the extent of this zone. The zone of aeration has three subzones.
  35. 35. Ground Water Hydrology
  36. 36. Ground Water Hydrology Soil water zone • This lies close to the ground surface in the major rrot band of the vegetation from which the water is lost to the atmosphere by evapotranspiration. Capillary Fringe • In this the water is held by the capillary action. This zone extends from water table upwards to the limit of the capillary rise. Intermediate Zone • This lies between the soil water zone and the capillary fringe. The soil texture and moisture content and vary from region to region. The soil moisture in the zone of aeration is of importance in agricultural practices and irrigation engineering.
  37. 37. Ground Water Hydrology
  38. 38. Ground Water Hydrology • Saturated Formations • All earth materials from soils to rocks have pore spaces. Although these pores are completely saturated with water table below, from the groundwater utilization aspect only such material through which water moves easily and hence can be extracted with ease are significant. On this basis the saturated formation are classified into four categories. • Aquifer • Aqitard • Aquiclude • Aquifuge
  39. 39. Ground Water Hydrology Aquifer • An aquifer is a saturated formation of earth material which not only stores water but yields it in sufficient quantity. Thus an aquifer transmits water relatively easily due to high permeability. Unconsolidation deposits off sand and gravel form good aquifer.
  40. 40. Ground Water Hydrology
  41. 41. Ground Water Hydrology Aquitard • It is a formation through which only seepage is possible and thus the yield is insignificant compared to an aquifer. It is partly permeable. A sandy clay unit is an example of aquitard. Through an aquitard appreciable quantities of water may leak to an aquifer below it.
  42. 42. Ground Water Hydrology
  43. 43. Ground Water Hydrology Aqiclude It is a geological formation which is essentially impermeable to the flow of water. It may be considered as close to water movement even though it may contain large amount of water due to its high porosity. Clay is an example of an acquiclude.
  44. 44. Ground Water Hydrology
  45. 45. Ground Water Hydrology Aquifuge • It is a geological formation which neither porous nor permeable. There are no interconnected openings and hence it cannot transmit water. Massive compact rock without any fracture is an acquifuge.
  46. 46. Ground Water Hydrology Aquifer • Formation of ground which contain water and may transmit water in usable quantity are known as aquifer. Thus these are the geological formations in which groundwater occurs. (i.e. Sands, gravels).
  47. 47. Confined aquifer overlain by less permeable materials Unconfined aquifer open to Earth’s surface and to infiltration Perched aquifer underlain by low-permeability unit Artesian aquifer: water rises in pipe (maybe to surface)
  48. 48. Ground Water Hydrology Aquifer are mainly of two types Unconfined Aquifer • An unconfined aquifer is the one in which water table forms the upper surface of the zone of saturation. An aquifer where the water table is the upper surface limit and extends below till the impermeable rock strata is called the unconfined aquifer. Confined Aquifer • When an aquifer is sandwiched between two impermeable layers, it is known as a confined aquifer. It is also known as a pressure aquifer, or an artesian aquifer. Confined aquifers are completely filled with water and they do not have a free water table and the aquifer will be under pressure.
  49. 49. Ground Water Hydrology
  50. 50. Ground Water Hydrology Leaky Aquifer • An aquifer bound by one or two aquitards is known as a leaky aquifer. It is also known as semi-confined aquifer. Perched Aquifer Perched Aquifer is a special type of an unconfined aquifer. An impermeable saucer-shaped stratum of a small aerial extent occurring in the zone of aeration may retain and hold some amount of water is called perched aquifer.
  51. 51. Ground Water Hydrology
  52. 52. Ground Water Hydrology
  53. 53. Ground Water Hydrology Water Table A water table is the free water surface in an unconfined aquifer indicating the level of the water table at that point. The water table is constantly in motion adjusting its surface to achieve a balance between the recharge and outflow from the surface storage.
  54. 54. Ground Water Hydrology
  55. 55. Water Table • Fluctuations in the water level in a dug well during various seasons of the year, lowering of the groundwater table in a region due to heavy pumping of the wells and the rise in the water table of an irrigated area with poor drainage, are some common examples of the fluctuation of the water table. In a general sense, the water table follows the topographic features of the surface. If the water table intersects the land surface the ground water comes out to the surface in the form of springs or seepage.
  56. 56. Ground Water Hydrology
  57. 57. Aquifer Functions • As said earlier aquifer serves as a underground reservoir and distribution system or conduct at the same time. • The storage capacity of rocks depends on the porosity of the rock on the one hand and the nature and inter-connections of the pores.
  58. 58. Aquifer Properties • The importance properties of an aquifer are its capacity to release the water held in its pores and its ability to transmit the flow easily. These properties essentially depends upon the composition of the aquifer.
  59. 59. Aquifer Properties
  60. 60. Aquifer Properties Porosity • The amount of pore space per unit volume of the aquifer material is called porosity. It is expressed as n = Vv Vo Where, n= porosity, Vv= Volume of voids Vo= Volume of porous medium
  61. 61. Porosity
  62. 62. Porosity
  63. 63. Aquifer Properties • In an unconsolidated material the size distribution, packing and shape of particles determine the porosity. In hard rocks the porosity is dependent on the extent, spacing and the pattern of fracturing or the nature of solution channels.
  64. 64. Aquifer Properties Specific Yield • While the porosity give a measure of the water storage capability of a formation, not all the water held in the pores is available for extraction by pumping or drainage by gravity. The pores hold back some water by molecular attraction and surface tension. The actual volume of water that can be extracted by the force of gravity from a unit volume of aquifer material is known as specific yield Sy, the fraction of water held back in the aquifer is known as Specific retention Sr, thus porosity • n= Sy + Sr
  65. 65. Aquifer Properties Thus, • Specific Yield: is the ratio of the volume of water that, after saturation, can be drained by gravity to its own volume. It is usually expressed as percentage, Thus • Sy= Wy x 100 V
  66. 66. Aquifer Properties Specific Retention: Sr of a soil or rock is the ratio of the volume of water it will retain after saturation against the force of gravity to its own volume. It is also expressed as percentage. • Thus, Sr = W r x 100 V Where, Wr is the volume of the retained water and V is the bulk volume of the soil or rock Since Wy and Wr constitutes the total volume of water in a saturated material it is apparent that porosity will be equal to sum of specific yield and the specific retention, i.e. n= Sy + Sr
  67. 67. Aquifer Properties
  68. 68. Flow in Aquifer • Movement of water through the aquifer is in general a function of three forms of energy contained in groundwater; pressure, velocity and elevation head. According to Bernoulli's equation • The sum of energy potential of these forms, H is • H= p + V2 + Z r 2g
  69. 69. Flow in Aquifer • Where, • p= pressure, • r= specific weight of water, • V= is the velocity, • g= acceleration due to gravity, • Z= the elevation head • The velocity factor in great many situations in groundwater is almost negligible.
  70. 70. Flow in Aquifer • Permeability, hydraulic conductivity and transmissibility are important terms related to various aspects of movements of water in aquifer.
  71. 71. Flow in Aquifer • Permeability: • When used in general sense, permeability is the capacity of a rock to transmit fluids through it. It is often expressed as Intrinsic Permeability, of which darcy d is the unit • Permebility is essentially related to the quantity of pores and other interices in a rock.
  72. 72. Flow in Aquifer
  73. 73. Flow in Aquifer
  74. 74. Flow in Aquifer • Transmissibility • The terms was introduced by theiss in 1935 to express the capacity of an entire aquifer to transmit water. The coefficient of transmissibility is defined as: • Rate of flow in gallons/ minute through a vertical section of an aquifer 1 foot wide extending to the full saturated length of the aquifer under a unit hydraulic head.
  75. 75. Flow in Aquifer • Hydraulic Conductivity • In groundwater geology or hydrology, the quantitative measurement of flow or water is generally expressed by the terms of hydraulic Conductivity rather than permeability. • The hydraulic conductivity K, may be defined as the flow velocity per unit hydraulic gradient. It is expressed as meter / day or meter / sec
  76. 76. Flow in Aquifer
  77. 77. Flow in Aquifer • Darcy’s Law As stated earlier groundwater is not static but it slowly moving through aquifers. The rate of flow of groundwater through the aquifers most of which are natural porous media, can be expressed by Darcy’s Law
  78. 78. Darcy’s Law • Henry Darcy a French hydraulic engineer investigated the flow of water through the horizontal beds of sand to be used for water filtration. On the basis of his experimentation Darcy in 1856 indicated that for laminar flow conditions the velocity of flow of water through saturated porous media is proportional to the hydraulic gradient. It is universally known as Darcy’s law may be expressed as V= Ki Where, V= Velocity of flow K= coefficient of permeability i= Hydraulic gradient= = Δh / L; Δh = head loss in a length L of flow path
  79. 79. Darcy’s Law • Further since • V= Q/A • Q= KiA • Where, • Q= rate of flow (or Discharge) • A= total c/s area of the porous medium perpendicular to the direction of flow.
  80. 80. Darcy’s Law
  81. 81. Wells • Wells as defined as opening or hole dug or drilled into an aquifer with the view of withdrawing water for drinking, agriculture, or other uses. Mostly these are vertical holes drilled or dug into the ground upto the aquifer. Water may flow through these wells either due to natural hydrostatic pressure or may have to be pumped out. These may be quite shallow or deep depending on the depth at which the water bearing strata are encountered.
  82. 82. Wells
  83. 83. Wells
  84. 84. Types of Wells Gravity Well • Gravity well, it is also called a water table well and is a vertical or nearly vertical hole penetrating the zone of saturation below the ground. The essential character of such a well which is at atmospheric pressure, and represents, when at rest, the water table of the area around the well. Water will not normally flow out of such well on its own; it has to be pumped out or taken out. Most wells driven in the aquifer for withdrawal of water are actually gravity well. When water has to be pumped out of such wells it forms the location of the Tube wells.
  85. 85. Wells
  86. 86. Types of Wells Galleries • These are horizontal tunnels or open ditches that are dug out through a water bearing layer formation to intercept water. These are dug generally perpendicular to the direction of flow of water in the aquifer.
  87. 87. Galleries
  88. 88. Galleries
  89. 89. Types of Wells • Artesian Wells • These are the holes drilled through the confined or artesian aquifer. In such wells water generally flows out at the ground surface, and even may gush out to some height.
  90. 90. Artesian Wells
  91. 91. References • “Irrigation Water Resource and Water Power Engineering” By Prof:- P.N. Modi Standard Book House • “Hydrology and Water Resource Engineering” By Prof:- R.B.Khasia • Publications of C. P. Kumar • • MIT Open-CourseWare • hydrology-fall-2005/lecture-notes/
  92. 92. Thanks! Lake Powell