hydrologic Properties of ground water and analysis

Hydrology

1. In hydrogeology, groundwater flow is defined as the "part of streamflow that has infiltrated the ground, entered the phreatic zone, and has been (or is at a particular time) discharged into a stream channel or springs; and seepage water."[1] It is governed by the groundwater flow equation.

2. In hydrogeology, groundwater flow is defined as the "part of streamflow that has infiltrated the ground, entered the phreatic zone, and has been (or is at a particular time) discharged into a stream channel or springs; and seepage water.” It is governed by the groundwater flow equation.

3. Used in hydrogeology, the groundwater flow equation is the mathematical relationship which is used to describe the flow of groundwater through an aquifer. The transient flow of groundwater is described by a form of the diffusion equation, similar to that used in heat transfer to describe the flow of heat in a solid. The steady-state flow of groundwater is described by a form of the Laplace equation, which is a form of potential flow and has analogs in numerous fields.

4. Porosity Porosity is an intrinsic property of every material. It refers to the amount of empty space within a given material. In a soil or rock the porosity (empty space) exists between the grains of minerals. In a material like gravel the grains are large and there is lots of empty space between them since they don’t fit together very well. However, in a material like a gravel, sand and clay mixture the porosity is much less as the smaller grains fill the spaces. The amount of water a material can hold is directly related to the porosity since water will try and fill the empty spaces in a material. We measure porosity by the percentage of empty space that exists within a particular porous media.

5. Permeability Figure 3. Video showing how connected pores have high permeability and can transport water easily. Note that some pores are isolated and cannot transport water trapped within them. Permeability is another intrinsic property of all materials and is closely related to porosity. Permeability refers to how connected pore spaces are to one another. If the material has high permeability than pore spaces are connected to one another allowing water to flow from one to another, however, if there is low permeability then the pore spaces are isolated and water is trapped within them. For example, in a gravel all of the pores well connected one another allowing water to flow through it, however, in a clay most of the pore spaces are blocked, meaning water cannot flow through it easily.

6. Figure 3. Showing how connected pores have high permeability and can transport water easily. Note that some pores are isolated and cannot transport water trapped within them.

7. Aquifer An aquifer is a term for a type of soil or rock that can hold and transfer water that is completely saturated with water. That means that all it is simply a layer of soil or rock that has a reasonably high porosity and permeability that allows it to contain water and transfer it from pore to pore relatively quickly and all of the pore spaces are filled with water. Good examples of aquifers are glacial till or sandy soils which have both high porosity and high permeability. Aquifers allows us to recover groundwater by pumping quickly and easily. However, overpumping can easily reduce the amount of water in an aquifer and cause it to dry up. When a water-bearing rock readily transmits water to wells and springs, it is called an aquifer. Wells can be drilled into the aquifers and water can be pumped out. Precipitation eventually adds water (recharge) into the porous rock of the aquifer. 

8. There might be a confining layer of less porous rock both above and below the porous layer. This is an example of a confined aquifer. In this case, the rocks surrounding the aquifer confines the pressure in the porous rock and its water. If a well is drilled into this “pressurized” aquifer, the internal pressure might (depending on the ability of the rock to transport water) be enough to push the water up the well and up to the surface without the aid of a pump, sometimes completely out of the well. This type of well is called artesian.  Rocks that yield freshwater have been found at depths of more than 6,000 feet, and salty water has come from oil wells at depths of more than 30,000 feet

9. Aquitard The other type is a confined aquifer that has an aquitard above and below it. An aquitard is basically the opposite of an aquifer with one key exception. Aquitards have very low permeability and do not transfer water well at all. In fact, in the ground they often act as a barrier to water flow and separate two aquifers. The one key exception is that aquitards can have high porosity and hold lots of water however, due to the their low permeability they are unable to transmit it from pore to  pore and therefore water cannot flow within an aquitard very well. A good example of an aquitard is a layer of clay. Clay often has high porosity but almost no permeability meaning it is essentially a barrier which water cannot flow through and the water within it is trapped. 

10. Aquifuge Aquifuge is a geological formation that is not porous nor permeable. There is an absence of interconnected openings so it cannot transmit water. Any massive compact rock without any fractures is called an aquifuge. Solid rocks are a type of Aquifuge. What is Aquiclude? Aquiclude is a geological formation that is impermeable which means it does not allow the passage of water through it. But it is highly porous so it contains a large amount of water in it. The aquiclude is formed when an aquifer is overlaid by a confined bed of impervious material.  One example of aquiclude is clay.

11. Unconfined Aquifer   It is the topmost water-bearing stratum having no aquiclude lying over Unconfined aquifers are also known as water table aquifers or non-artesian aquifer This is a free water surface aquifer which means there exist a water table Through the infiltration of precipitation from the ground surface, the recharging of these aquifers takes place. If a well is driven into an unconfined aquifer it will indicate a static water level corresponding to the water table level at that location.

12. Confined Aquifer Confined aquifers are also known as an artesian aquifer The aquifers which are embedded between two impervious beds of Aquiclude or Aquifuge are called a confined aquifer These aquifers are recharged from the places which are opened or exposed to the ground surface The piezometric level will be much higher in this aquifer than the top level of the aquifer due to the water under pressure If both of the confining beds of a Confined aquifer are aquitards then it is also called a leaky aquifer.

13. Aquifer Aquiclude Aquitard Aquifuge These are permeable These are impermeable These are partly permeable These are impermeable There is a yield of water These do not yield water There is a yield of water but the yielding will be so slow These do not yield water This can store water This can store water This can store water This cannot store water Sand and gravel are some of the examples of aquifer Clay is an example of an aquiclude Sandy clay is an example of aquitard Compact rocks like basalt and granite are some of the examples of Aquifuge

14. Specific yield The capacity of a formation to contain water is measured by porosity. However, a high porosity does not indicate that an aquifer will yield large volume of water to a well. The only water which can be obtained from the aquifer is that which will flow by gravity. The specific yield of an aquifer is defined as the ratio expressed as a percentage, of the volume of water which after being saturated, can be drained by gravity to its own volume. Sy = Volume of water drained by gravity Total volume Sy=Wy/V×100 Where Wy = volume of water drained by gravity. Specific Retention: - Specific yield is always less than porosity since some water will be retained in the aquifer by molecular and surface tension. The specific retention of an aquifer is the ratio, expressed as a percentage, of the volume of water it will retain after saturation against the force of gravity to its own volume. Sr = Volume of water retained Total volume Sr= Wr /V×100 Where Wr = volume of water retained.

15. (S) storage coefficient or storativity The amount of water stored or released per unit area of aquifer given unit head change Ss specific storage: The amount of water stored or released per unit volume of aquifer given unit head change S= Ss b, where b is the aquifer thickness Storage change is accomplished via compression of the aquifer matrix and the fluid. Fluid compressibility 10-10 Pa-1 while typical (sand) aquifer compressibility is 10-8 Pa-1 Typical values of S (dimensionless) are 10-5 10-3 Measuring storativity derived from observations of multi-well tests

16. In an unconfined aquifer (sometimes also called a water table aquifer), the water pumped from a well is obtained by lowering the water table and draining the volume of rock or sediment just above the water table. Pumping water from the well creates a lower-than-normal water level surface in the vicinity of the well; this is known as the cone of depression (Figure 14). The volume of water pumped from the well is equal to the specific yield of the sediments (or rocks) times the volume of the cone of depression (the total volume of sediments that have been drained) most of the water contained in an aquifer becomes part of the the specific yield, when the water level is lowered and sediments (or rock fractures) are drained

17. Example 1: To illustrate the meaning of specific yield, let’s assume we have an “aquifer box” that measures 1 acre in area by 100 feet deep. Let’s further assume that the box is entirely enclosed except at the surface. If the specific yield is 15% (0.15), how much water do we have to pump to lower the water table in the entire aquifer box by 1 foot? The answer is: Water pumped = Volume to be drained x Specific yield = (1 acre area x 1 foot drawdown) x 0.15 = 0.15 acre-feet (~ 50,000 gallons) (1 acre foot=325851.429 us gallons)

18. In a confined aquifer, water and the sediments or rocks containing the water are under pressure and therefore slightly compressed. When pumping from a well in a confined aquifer, the water table inside the well is lowered, resulting in lower water pressure in the deep

19. DARCY’S LAW

20. Flow of viscous fluid Flow type: 1. Turbulent flow 2. Laminar flow Turbulent Flow: h/L directly propotional to V 2 where L= distance wherein the drop of head is h Laminar Flow: h/L directly propotional to V If velocity gradually increased, a stage arrives when laminar flow is converted into turbulent flow.

21. 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.

22. 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 law of conservation of mass states that {“The mass in an isolated system can neither be created nor be destroyed but can be transformed from one form to another”. According to the law of conservation of mass, the mass of the reactants must be equal to the mass of the products for a low energy thermodynamic process.} 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.

23. Darcy’s Law Equation To understand the mathematical aspect behind liquid flow in the substance, Darcy’s law can be described as: Darcy’s law describes the relationship among the instantaneous rate of discharge through porous medium and pressure drop at a distance . Using the specific sign convention, Darcy’s law is expressed as: Q = -KA dh/dl Wherein: Q is the rate of water flow K is the hydraulic conductivity A is the column cross-section area dh/dl indicates a hydraulic gradient.

24. Permeability and hydraulic conductivity usually vary over spaces within natural porous media. This is termed heterogeneity. Homogeneou : independent of space They may also vary depending on direction of measurement at any given point which is termed as anisotropy. If hydraulic conductivity dependent of position, the porous medium is homogeneous otherwise heterogeneous. If hydraulic conductivity is independent of direction, the porous medium is isotropic otherwise anisotropic

25. Stratified formations are anisotropic homogeneous

26. Limitations of Darcy’s Law Darcy’s law can be applied to many situations but does not correspond to these assumptions. Unsaturated and Saturated flow. Flow in fractured rocks and granular media. Transient flow and steady-state flow. Flow in aquitards and aquifers. Flow in Homogeneous and heterogeneous systems.

27. Wellhead Delineation - Homogeneous, Isotropic Aquifer   The site under consideration consists of a sand and gravel aquifer sandwiched between two rivers. The aquifer is about 2000 m long and 1300 m wide. The average land elevation is about 10 m. The head gradient between the two rivers causes a general flow towards the North. A well pumps at a distance of 200 m from the Northern river. The resulting Well Head Protection Area is sub-divided into 2, 5 and 10 year WHPAs. The RED zone denotes an area around the well, in which the water would reach the well within 2 years. Similarly, the YELLOW and BLUE zones denotes 5 and 10 year zones respectively. The classification of these zones is very important. Usually, it is not desirable to have possible sources of contamination, such as gas stations, industries etc within the 2 and 5 Year Zones. The zones are delineated using a process of reverse particle tracking. In this process, several particles are added around the well and then traced backwards in time. Notice that the time displayed has a negative sign. This signifies that particles are being tracked backwards in time.

28. Aquifer type Confined Thickness (m) 40 Conductivity (m/day) 5 Pumping rate (cu.m/day) 5000 River Head - South (m) 10 River Head - North (m) Grid 201 x 151 Grid size (m) 10 D t, 0-2 yrs (days) 2.5 D t, 2-5 yrs (days) 5 D t, 5-10 yrs (days) 10     2 Yr WHPA 5 Yr WHPA 10 Yr WHPA

29. phreatic zone The  phreatic zone ,  saturated zone , or  zone of saturation , is the part of an aquifer, below the water table, in which relatively all pores and fractures are saturated with water. Above the water table is the unsaturated or vadose zone.

30. Water flows from areas where the  water table  is higher to areas where it is lower. This flow can be either  surface runoff  in rivers and streams, or subsurface runoff infiltrating rocks and soil. The amount of runoff reaching surface and groundwater can vary significantly, depending on rainfall, soil moisture,  permeability , groundwater storage, evaporation, upstream use, and whether or not the ground is frozen.

31. A  drainage equation  is an equation describing the relation between depth and spacing of parallel subsurface  drains , depth of the  watertable , depth and  hydraulic conductivity  of the soils. It is used in  drainage  design Parameters in Hooghoudt's drainage equation A well known  steady-state  drainage equation is the Hooghoudt drain spacing equation. 

32. In isotropic aquifers or aquifer layers the hydraulic conductivity (K) is equal for flow in all directions, while in anisotropic conditions it differs, notably in horizontal ( Kh ) and vertical ( Kv ) sense.

33. ISOTROPIC AQUIFER In isotropic aquifers or aquifer layers the hydraulic conductivity (K) is equal for flow in all directions, while in anisotropic conditions it differs, notably in horizontal ( Kh ) and vertical ( Kv ) sense.

34. Water in porous aquifers slowly seeps through pore spaces between sand grains

35. Sandy deposits formed in shallow marine environments and in windblown sand dune environments have moderate to high permeability while sandy deposits formed in river environments have low to moderate permeability.   Rainfall and snowmelt enter the groundwater where the aquifer is near the surface. Groundwater flow directions can be determined from potentiometric surface maps of water levels in wells and springs. Aquifer tests and well tests can be used with Darcy's law flow equations to determine the ability of a porous aquifer to convey

36. Karst  aquifers typically develop in  limestone . Surface water containing natural  carbonic acid  moves down into small fissures in limestone. This carbonic acid gradually dissolves limestone thereby enlarging the fissures. The enlarged fissures allow a larger quantity of water to enter which leads to a progressive enlargement of openings. Abundant small openings store a large quantity of water. The larger openings create a conduit system that drains the aquifer to springs. [13] Characterization of karst aquifers requires field exploration to locate  sinkholes, swallets ,  sinking streams , and  springs  in addition to studying  geologic maps . Water in karst aquifers flows through open conduits where water flows as underground streams

37. Transboundary aquifer When an aquifer transcends international boundaries, the term  transboundary aquifer  applies. [19] Transboundariness is a concept, a measure and an approach first introduced in 2017. [20]  The relevance of this approach is that the physical features of the aquifers become just additional variables among the broad spectrum of considerations of the transboundary nature of an aquifer: social (population); economic (groundwater productivity); political (as transboundary); available research or data; water quality and quantity; other issues governing the agenda (security, trade, immigration and so on).

38. A  drainage equation  is an equation describing the relation between depth and spacing of parallel subsurface  drains , depth of the  watertable , depth and  hydraulic conductivity  of the soils. It is used in  drainage  design. Hooghoudt's equation Hooghoudt's equation can be written as:.[3] Q L2 = 8 Kb d (Dd - Dw ) + 4 Ka (Dd - Dw )2 where: Q = steady state drainage discharge rate (m/day) Ka = hydraulic conductivity of the soil above drain level (m/day) Kb = hydraulic conductivity of the soil below drain level (m/day) Di = depth of the impermeable layer below drain level (m) Dd = depth of the drains (m) Dw = steady state depth of the watertable midway between the drains (m) L = spacing between the drains (m) d = equivalent depth, a function of L, (Di-Dd), and r r = drain radius (m) Steady (equilibrium) state condition In steady state, the level of the water table remains constant and the discharge rate (Q) equals the rate of groundwater recharge (R), i.e. the amount of water entering the groundwater through the watertable per unit of time. By considering a long-term (e.g. seasonal) average depth of the water table ( Dw ) in combination with the long-term average recharge rate (R), the net storage of water in that period of time is negligibly small and the steady state condition is satisfied: one obtains a dynamic equilibrium.