complete unit under ground water hydrology.pptx

unit 5 ground water hydrology

1. UNIT-5 GROUND WATER HYDROLOGY

2. Sources of Groundwater Groundwater is derived from precipitation and recharge from surface water . It is the water that has infiltrated into the earth directly from precipitation, recharge from streams and other natural water bodies and artificial recharge due to action of man . Infiltration and further downward percolation from sources like rain, melting of snow and ice, rivers and streams, lakes, reservoirs, canals and other watercourses are the usual main sources that contribute to the groundwater of a region.

3. CLASSIFICATION OF SUBSURFACE WATER

4. Forms of Subsurface Water: Water in the soil mantle is called subsurface water and is considered in two zones: 1. Saturated zone/ Phreatic zone 2. Aeration zone/Vadose zone Saturated Zone This zone, also known as groundwater zone, is the space in which all the pores of the soil are filled with water . The water table forms its upper limit and marks a free surface, i.e. a surface having atmospheric pressure. Zone of Aeration In this zone, the soil pores are only partially filled with water . The space between the land surface and the water table marks the extent of this zone. Any flow of water in this zone is in unsaturated soil matrix condition . The zone of aeration has three subzones: Soil Water Zone : This lies close to the ground surface in the major root band of the vegetation from which the water is transported to the atmosphere by evapotranspiration. Intermediate Zone : This subzone lies between the soil water zone and the capillary fringe. The thickness of the zone of aeration and its constituent subzones depend upon 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 practice and irrigation engineering. Capillary Fringe: In this subzone, the water is held by capillary action . This subzone extends from the water table upwards to the limit of capillary rise.

5. Infiltration and Percolation: Infiltration and percolation are both processes that move water in the pores of the soil. The main difference between these two processes is that i nfiltration occurs closer to the surface of the soil. Infiltration denotes the entry of rainwater or ponded water in to the earth surface. The interface is the essence of infiltration process. The descending motion of the infiltered water through soil and rock is called percolation . Infiltration delivers water from the surface into the soil and plant rooting zone while percolation moves it through the soil profile to replenish groundwater supplies or become part of the sub-surface runoff process . Thus, the percolation process represents the flow of water from unsaturated zone to the saturated zone. Saturated Formation: All earth materials, from soils to rocks have pore spaces . Although these pores are completely saturated with water below the water table, 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 formations are classified into four categories: 1. Aquifer 2. Aquitard, 3. Aquiclude 4. Aquifuge 1. 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 its high permeability. Unconsolidated deposits of sand and gravel form good aquifers. 2. 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.

6. 3. Aquiclude It is a geological formation which is essentially impermeable to the flow of water . It may be considered as closed to water movement even though it may contain large amounts of water due to its high porosity. Clay is an example of an aquiclude. 4. Aquifuge It is a geological formation which is neither porous nor permeable . There are no interconnected openings and hence it cannot transmit water. Massive compact rock without any fractures is an aquifuge.

7. The availability of groundwater from an aquifer at a place depends upon the rates of withdrawal and replenishment (recharge). Aquifers play the roles of both a transmission conduit and a storage. Aquifers are classified as unconfined aquifers and confined aquifers on the basis of their occurrence and field situation. An unconfined aquifer (also known as water table aquifer) is one in which a free water surface, i.e. a water table exists. Only the saturated zone of this aquifer is of importance in groundwater studies. Recharge of this aquifer takes place through infiltration of precipitation from the ground surface . A well driven into an unconfined aquifer will indicate a static water level corresponding to the water table level at that location. A confined aquifer , also known as artesian aquifer , is an aquifer which is confined between two impervious beds such as aquicludes or aquifuges . Recharge of this aquifer takes place only in the area where it is exposed at the ground surface. The water in the confined aquifer will be under pressure and hence the piezometric level will be much higher than the top level of the aquifer.

8. What is an artesian well or spring ? The water rises under pressure from a permeable stratum overlaid by impermeable rock Artesian well, deep drilled well through which water is forced upward under pressure. The water in an artesian well flows from an aquifer, which is a layer of very porous rock or sediment, usually sandstone, c apable of holding and transmitting large quantities of water. The geologic conditions necessary for an artesian well are an inclined aquifer sandwiched between impervious rock layers above and below that trap water in it. Water enters the exposed edge of the aquifer at a high elevation and percolates downward through interconnected pore spaces. The water held in these spaces is under pressure because of the weight of water in the portion of the aquifer above it .  If a well is drilled from the land surface through the overlying impervious layer into the aquifer, this pressure will cause the water to rise in the well . 

9. Potentiometric or piezometric surface " which is an imaginary  surface  that defines the level to which water in a confined aquifer would rise were it completely pierced with  wells . If the  potentiometric surface  lies above the ground  surface , a  flowing artesian well  results. Flowing artesian well : A flowing artesian well is one that has been drilled into an aquifer where the pressure within the aquifer forces the groundwater to rise above the land surface naturally without using a pump . Flowing artesian wells can flow on an intermittent or continuous basis and originate from aquifers occurring in either unconsolidated materials such as sand and gravels or bedrock, at depths ranging from a few meters to several thousand meters. All flowing wells are artesian, but not all artesian wells are flowing wells. Eg: A well drilled in a valley where the surrounding highlands have a higher elevation, allowing the water to flow from the highlands into the valley and up the well.  Non Flowing artesian well : A non-flowing artesian well is a well that taps into a confined aquifer and with pressure the water rises above the top of the aquifer but does no necessarily reach the land surface as the potentiometric (piezometric) surface is lies below ground level. Eg:A well drilled on a hill where the aquifer's pressure is not enough to overcome the difference in elevation between the wellhead and the water source in the higher ground. 

10. A  perched aquifer  is separated from another water-bearing stratum by an impermeable layer. Since  this type of aquifer occurs above the regional (original) water table, in the unsaturated zone, the aquifer is called a perched aquifer . One main difference between perched and unconfined aquifers is their size (a perched aquifer is smaller). In fact, a perched aquifer is a very small, unconfined aquifer that doesn’t contain much water and is only recharged by local precipitation.  A well drilled into a perched aquifer usually yields only enough water for a single household. Water table (zone of saturation) Zone of unsaturation

11. The position of the water table relative to the water level in a stream determines whether the stream contributes water to the groundwater storage or the other way about. If the bed of the stream is below the groundwater table , during periods of low flows in the stream, the water surface may go down below the general water table elevation and the groundwater contributes to the flow in the stream. Such streams which receive groundwater flow are called effluent streams (gaining streams) . Perennial rivers and streams are of this kind. If, however, the water table is below the bed of the stream, the stream-water percolates to the groundwater storage and a hump is formed in the groundwater table . Such streams which contribute to the groundwater are known as influent streams (losing streams). Intermittent rivers and streams which go dry during long periods of dry spell (i.e. no rain periods) are of this kind.

12. Aquifer Properties The important 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 depend upon the composition of the aquifer. Porosity The amount of pore space per unit volume of the aquifer material is called porosity. It is expressed as: where n = porosity, V v = volume of voids and V = volume of the porous medium. 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 on the nature of solution channels . In qualitative terms porosity greater than 20% is considered as large , between 5 and 20% as medium and less than 5% as small. The openings responsible for porosity in rocks may have diverse origin. Pore spaces may be left between individual grains and particles during the process of deposition in sedimentary rocks. Openings may also be caused in the rocks due to development of cracks crevices joints, fissures solution channels and gas holes in different categories of rocks . Sedimentary rocks are relatively highly porous because there is a great variation in degree of packing and arrangement of grains. Limestones and dolomites are susceptible to the development of the solution channels and caverns whereas igneous rocks may become porous due to escape of gases . All types of rocks when subjected to tectonic forces often develop joints and cracks of varied geometry that make them porous to some extent. Porosity is a broad measure of the Storage Capacity of an aquifer. Porosity = Volume of interstices/ Total Volume of rock * 100

13. Some reservoirs have distinct  primary and secondary porosity .  Primary porosity  is the original porosity of the rock when it formed , and  secondary porosity  is the pore space created by subsequent processes such as fracturing . Igneous  or  metamorphic rocks  have the lowest primary porosity because they commonly form at depth and have interlocking crystals.

14. Specific Yield Porosity is a property of primary importance in determining the water bearing qualities of a rock. Only porous rock can be aquifers but high porosity itself is not sufficient to ensure water yielding capacity of a rock. Thus, clays which are generally characterized with a high porosity, 50-60% may be practically useless as aquifers . It is because the pores or voids in between clay particles are so minute that water is held up within these pores very firmly. The main interest is always to determine how much water can be actually obtained per unit area of the aquifer . This water is released by the aquifer under the force of gravity and depends both on the quantity and quality of the pores and other openings present in the rock. N is the total porosity of an aquifer.The actual volume of water that can be extracted by the force of gravity from a unit volume of aquifer material is known as the specific yield, S y . The fraction of water held back in the aquifer is known as specific retention, S r. The term effective porosity is sometimes used to express the volume of pores which is effective in the release of water from an aquifer. It is also called kinematic porosity and is smaller than the total porosity. Factors that are commonly responsible to adversely effect the porosity of rock are : Adhesion to Grains : Some groundwater is attached to the surface of grains of rocks due to molecular attraction and is not easily available for circulation. Unconnected Pores : Though present in good abundance and even containing some water, such pores are separated from each other . Hence the liquid in them cannot circulate . Broadly, similar condition may exist in crystalline rocks where all fractures are not thoroughly inter-connected.

15. Specific yield, is therefore, broadly a function of grain size and shape and also continuity of the interstices . Specific retention is the term use to express the amount of water retained by a unit volume of aquifer after allowing gravity drainage through it. Thus specific yield and specific retention together sum up to the total porosity of an aquifer. The representative values of porosity and specific yield of some common earth materials are given in Table below: It is seen from Table above that although both clay and sand have high porosity the specific yield of clay is very small compared to that of sand.

16. 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 potentials of these forms . H is : H = p/r + V 2 /2g + Z where, p is fluid pressure; r is specific weight (fluid density) of water, V is fluid velocity, g is the acceleration due to gravity and Z is the elevation head . Permeability, Hydraulic Conductivity and Transmissivity are important terms related to various aspects of movement of water in an aquifer. Permeability : When used in general sense, permeability is the capacity of a rock (or any other solid medium) to transmit fluids through it. Permeability is essentially related to the quality of pores and other interstices in a rock . Accordingly, rocks which are made up of well assorted grains and which are profusely traversed by interconnected joints, crevices and cavities allow the water to pass through them easily . On the other hand, rocks with closed or dead end pores do not allow water transmission . There is observed a great variation in permeability of rocks. In general, rocks are sometimes classified into three group on the basis of coefficient of permeability, K (m/d). Class K (m/d) Examples Extremely permeable >10 Coarse sandstone, limestone pebbles, gravels Semi permeable 10-0.1 Fine grained sands, loams, slightly jointed crystalline rocks Impermeable <0.1 Clays, compact igneous rocks

17. Hydraulic Conductivity : In groundwater geology or hydrogeology, the quantitative measurement of flow of water is generally expressed by the term Hydraulic Conductivity rather than permeability. The hydraulic conductivity K, may be defined as the flow velocity per unit hydraulic gradient( is the slope of the water table or potentiometric surface, that is, the change in water level per unit of distance ) , It is expressed as meters/ day or meters/second. Tranmissivity : This term was introduced by Theiss in 1935 to express the capability of an entire aquifer to transmit water. The coefficient of transmissivity T, (transmissibility) was defined by Theiss as : “ Rate of flow in gallons/minute through a vertical section of an aquifer Consider an aquifer of unit width and thickness B , (i.e. depth of a fully saturated zone). The discharge through this aquifer under a unit hydraulic gradient (K ) is T = KB This discharge is termed transmissibility, T its units are m 2 /s . Problem: What is the transmissivity of an aquifer that has a thickness of 20 m and hydraulic conductivity of 15 m/d T = 20 * 15 = 300 m 2 /d Type of Flow : In general, groundwater flow through the interstices is of a laminar type although the turbulent flow also occurs in certain situations . In the laminar flow , the water droplets and drops move very slowly in a continuous, steady ribbon like fashion through pores and other openings. In such a case, water moving laterally through the thickness of an aquifer flows in layers, there being no intermixing water from overlaying layers. The turbulent flow is characteristic of subsurface streams and involves complete intermixing of water molecules, which move in an irregular fashion.

18. Darcy’s Experiments and Darcy’s Law In 1855, Henri Darcy, a French hydraulic engineer, oversaw a series of experiments aimed to understand the rates of water flow through sand layers, and their relationship to pressure loss along the flow paths. Darcy’s experiments consisted of a vertical steel column, with a water inlet at one end and an outlet at the other. The water pressure was controlled at the inlet and outlet ends of the column using reservoirs with constant water levels (denoted  h 1  and  h 2 ) . The experiments included a series of tests with different packings of river sand, and a suite of tests using the same sand pack and column , but for which the inlet and outlet pressures were varied. Schematic of Darcy’s original experimental apparatus Darcy’s findings laid the foundation for the modern science of hydrogeology by quantifying the relationships between volumetric groundwater flow rate, driving forces, and aquifer properties . Specifically, Darcy’s experiments revealed proportionalities between the flux of water(movement of water),  Q , through the laboratory “aquifer” and different characteristics of the experimental system: (1) Q was directly proportional to the difference in water levels from inlet to outlet, h 1  - h 2  =  Δh :                Q   ∝  Δ h (2) Q was directly proportional to the cross sectional area of the tube:                Q  ∝  A

19. 3) Q was inversely proportional to the length of the column:                Q  ∝ 1/Δ L Combining these proportionalities leads to Darcy’s Law, the empirical law that describes groundwater flow:                Q  =  KA ( Δ h /Δ L ) where K is a constant of proportionality that defines the water flux for a given hydraulic gradient ( Δh /ΔL). The above equation can also be recast in terms of the water volume flux per unit area, Q/A (also called "Darcy flux" or "Darcy velocity" with units of length per time):                Q /A =  K ( Δ h /Δ L )

20. Darcy’s Law In 1856, Henry Darcy, a French hydraulic engineer, on the basis of his experimental findings proposed a law relating the velocity of flow in a porous medium . This law, known as Darcy’s law, can be expressed as: V = Ki where V = apparent velocity of seepage = Q/A in which Q = discharge and A = cross-sectional area of the porous medium. V is sometimes also known as discharge velocity. i = -dh/ds = hydraulic gradient, in which h = piezometric head and s = distance measured in the general flow direction ; the negative sign emphasizes that the piezometric head drops in the direction of flow. K = a coefficient, called coefficient of permeability , having the units of velocity. The discharge Q can be expressed as : Q = KiA where (–ΔH) is the drop in the hydraulic grade line in a length Δs of the porous medium. Darcy’s law is a particular case of the general viscous fluid flow. It has been shown valid for laminar flows only. For practical purposes, the limit of the validity of Darcy’s law can be taken as Reynolds number of value unity, i.e.

21. where Re = Reynolds number d a = representative particle size , usually d a = d 10 where d 10 represents a size such that 10% of the aquifer material is of smaller size. Often, the mean grain size is used as d a . v = kinematic viscosity of water Except for flow in fissures and caverns , to a large extent groundwater flow in nature obeys Darcy’s law. Further, there is no known lower limit for the applicability of Darcy’s law. It may be noted that the apparent velocity V used in Darcy’s law is not the actual velocity of flow through the pores. Owing to irregular pore geometry the actual velocity of flow varies from point to point and the bulk pore velocity ( v a ) which represents the actual speed of travel of water in the porous media is expressed as: where n = porosity. The bulk pore velocity v a is the velocity that is obtained by tracking a tracer added to the groundwater. Reynold number : The  Reynolds number  is the ratio of inertial forces to viscous forces within a fluid which is subjected to relative internal movement due to different fluid velocities. A region where these forces change behavior is known as a boundary layer, such as the bounding surface in the interior of a pipe.

22. Viscous force tends to keep the layers moving smoothly one over the other .   Inertia forces tend to move the particles away from the layer. 

23. What is Darcy’s Law? Darcy’s law states the principle which governs the movement of fluid in the given substance. Darcy’s law equation that describes the capability of the liquid to flow via any porous media like a rock. The law is based on the fact according to which, the flow between two points is directly proportional to the pressure differences between the points, the distance, and the connectivity of flow within rocks between the points . Measuring the inter-connectivity is known as permeability. Darcy’s Law Application One application of Darcy’s law is to flow water through an aquifer . Darcy’s law with the conservation of mass equation is equivalent to the groundwater flow equation, being one of the basic relationships of hydrogeology. Darcy’s law is also applied to describe oil, gas, and water flows through petroleum reservoirs. The liquid flow within the rock is governed by the permeability of the rock. Permeability has to be determined in horizontal and vertical directions. For instance, shale consists of improbabilities which are less vertically. This indicates that it is not easy for liquid to flow up and down via shale bed but easier to flow side to side.

24. DIFFERENT ROCKS AS AQUIFERS Rocks show great variation in their water bearing properties. Sedimentary Rocks : Gravels :These are classed among the best kind of formations to yield groundwater. But these too show a great variation in their water bearing properties depending on degree of assortment, packing and cementing of the grains. Generally, the ideal gravel is coarse, clean and well sorted material which has a very high effective porosity and permeability . Hydraulic conductivity K of pebble gravels free from interstitial packing may range anywhere between 100-1000 m/day. Their porosity may range between 25-40 % and specific yield between 15-30 %. Sands: Sand beds or loosely packed sandstones are rated next to gravels as water yielding materials. The sand layers are characterized with a porosity of 25-40 percent and specific yield of 15-25 % . Their hydraulic conductivity is also quite high ranging between 10-100 m/day. In many cases they prove even better than gravels because of their extensive continuity. The beach sands are example of best water bearing formations in many regions. Sandstones : These sedimentary rocks show great variation in their water yielding capacity, which is chiefly controlled by their texture and nature of the cementing material. Coarse grained sandstones with rather imperfect cement are classed among the excellent aquifers, the fine grained varieties which are thoroughly compacted or cemented may prove useless as water yielding rocks . In permeability sandstones may vary from semipermeable to highly permeable, with porosity ranging from 5-30 percent, specific yield from 5-15 percent and hydraulic conductivity between 1-100 m/day.

25. 4. Limestone : Some of the best known productive aquifers of the worlds are of limestone rock. At the same time, these rocks have proved totally unproductive in many other regions. This great variation in behavior of limestones as water bearing rocks is explained by the fact that in limestones and dolomites and similar calcium rich rocks , the availability of water is controlled by development of secondary fracturing, solution channels, cavities and cervices and similar openings rather than by primary porosity. It is therefore, not surprising that the porosity of limestones varies from 1 to 20 percent. Hydraulic conductivity in highly karsted (cavernous) limestones may be as high as 100 m/day or even more, to be classed among the extremely permeable rocks. Igneous Rock : Igneous rocks are either intrusive or extrusive in nature. The instrusive rocks are dense in texture with all the component minerals very closely crystallined so that there are available practically no openings or pores. Such rocks therefore, are nonporous, impermeable and without any scope for holding any groundwater supplies . The intrusive rocks are sometimes traversed by extensive cracks and joints systems and fissures due to folding, faulting and other forces. When presence of such fissures-and-joint systems is established the rock in this zone may prove to be holding groundwater supplies.

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