Hydrogeology (hydro- meaning water, and -geology meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust,commonly in aquifers.The term geohydrology is often used interchangeably.The study of the interaction between groundwater movement and geology can be quite complex. Groundwater does not always flow in the subsurface down-hill following the surface topography. Groundwater follows pressure gradients (flow from high pressure gradient to low) often following fractures and conduits in circuitous paths.Aquifers are broadly classified as being either confined or unconfined (water table aquifers), and either saturated or unsaturated,the type of aquifer affects the properties that control the flow of water in that medium.
What is Ground Water ?
Ground water is the water that is found beneath the earth's surface. It is the water that fills pores and fractures in the ground.
Water table and Vadose Zone
The top of the ground water is called water table. Water table is the boundary between unsaturated zone and saturated zone Between water table and land surface is the unsaturated or vadose zone.In the unsaturated zone/Vadose zone water moves downward to the water table to recharge the ground water .
The vadose zone, also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone i.e. the position at which the groundwater (the water in the soil's pores) is at atmospheric pressure ("vadose" is Latin for "shallow"). Hence the vadose zone extends from the top of the ground surface to the water table. Water in the vadose zone has a pressure head less than atmospheric pressure, and is retained by a combination of adhesion (funiculary groundwater), and capillary action (capillary groundwater). If the vadose zone envelops soil, the water contained therein is termed soil moisture. In fine grained soils, capillary action can cause the pores of the soil to be fully saturated above the water table at a pressure less than atmospheric. In such soils, therefore, the unsaturated zone is the upper section of the vadose zone and not identical to it.
Movement of water within the vadose zone is studied within soil physics and hydrology, particularly hydrogeology, and is of importance to agriculture, contaminant transport, and flood control.
What is an Aquifer ?
Aquifer - "A water-bearing layer of rock, or of unconsolidated sediments, that will yield water in a usable quantity to a well or spring.".Aquifers are typically saturated regions of the subsurface that produce an economically feasible quantity of water to a well or spring .There are two basic types of aquifers: confined and unconfined.
An aquifer is considered unconfined if water only partially fills the aquifer materials and water freely rises and declines along the unsaturated/saturated zone boundary. These unconfined aquifers are often referred to as water-table aquifers and wells that are opened to these unconfined aquifers indicates the position of the water-table. It can also be defined as an aquifer whose upper boundary is defined by the water table (water is at atmospheric pressure). Water usually saturates only part of the geologic unit and there is no upper confining layer. Also called a "water table aquifer".
A confined aquifer is generally defined when water completely fills the aquifer materials and is overlain by a confining bed. A common term for a confined aquifer is an artesian aquifer. The water level from a well that permits water solely from a confined aquifer to enter the well will stand at some point above the top of the confined aquifer but not necessarily above the land surface. The water level in a well open to a specific confined aquifer stands at the level of the potentiometric surface. If the potentiometric surface is above land, the well is often considered as a free-flowing artesian well.It can also be defined as a fully saturated aquifer whose upper and lower boundaries are impervious geologic units. Water is held under pressure and the water level in wells stands above the top of the aquifer.
Confining Bed - "A layer of rock, or of unconsolidated sediments, that retards the movement of water in and out of an aquifer and possesses a very low hydraulic conductivity."
Water is continually recycled through aquifer systems. Groundwater recharge is any water added to the aquifer zone. Processes that contribute to groundwater recharge include precipitation, streamflow, leakage (reservoirs, lakes, aqueducts), and artificial means (injection wells). Groundwater discharge is any process that removes water from an aquifer system. Natural springs and artificial wells are examples of discharge processes.
Pumping water from a well causes a cone of depression to form in the water table at the well site. Overpumping can have two effects. It can cause a change in the groundwater flow direction. It also lowers the water table, making it necessary to dig a deeper well.
Factors influencing groundwater flow :
Many factors influence groundwater movement such as hydraulic head, hydraulic gradient, and velocity which was based on Darcy's Law.Other influencing factors include soil and aquifer properties, aquifer type, geology, and topography. ( Movement of groundwater depends on rock and sediment properties and the groundwater’s flow potential. Porosity, permeability, specific yield and specific retention are important properties of groundwater flow.)
Topography
Ground water general flows away from topographical high spots and toward topographic lows.
Porosity of Earth Materials
Porosity is percentage of rock or soil that is void of solid material.
Porosity is the volume of pore space relative to the total volume (rock and/or sediment + pore space). Primary porosity (% pore space) is the initial void space present (intergranular) when the rock formed. Secondary porosity (% added openings) develops later. It is the result of fracturing, faulting, or dissolution. Grain shape and cementation also affect porosity.
Effective Porosity = porosity available for fluid flow (interconnected and large enough pore throats)
Porosity depends on packing of grains (cubic or rhombohedral), grain size distribution (sorting), and shape of grains. Porosity depends on the range of grain size (sorting) and shape of the subject material, but not on the size. Fine-grained materials tend to be better sorted than coarse-grained materials, thereby exhibiting greater porosities.
Permeability is the capability of a rock to allow the passage of fluids. Permeability is dependent on the size of pore spaces and to what degree the pore spaces are connected. Grain shape, grain packing, and cementation affect permeability.
Aquifer properties :
An aquitard is a zone within the earth that restricts the flow of groundwater from one aquifer to another. An aquitard can sometimes, if completely impermeable, be called an aquiclude or aquifuge. Aquitards comprise layers of either clay or non-porous rock with low hydraulic conductivity.
Storativity
Storativity is the volume of water released from storage per unit decline in hydraulic head in the aquifer, per unit area of the aquifer.Confined aquifers have very low storativity values which means that the aquifer is storing water .. Unconfined aquifers have storativities (typically then called specific yield) greater than , they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water .
Specific Yield
Specific yield is the ratio of the volume of water that drains from a saturated rock owing to the attraction of gravity to the total volume of the rock. (indicates the amount of water released due to drainage from lowering the water table in an unconfined aquifer. ). Grain size has a definite effect on specific yield. Maximum specific yield occurs in medium to coarse sand size sediments. Smaller grains have larger surface areas.Larger surface areas mean more surface tension. Fine-grained sediment will have a lower specific yield than more coarsely-grained sediment. Sorting of material affects groundwater movement.Poorly sorted material is less porous than well-sorted material.
Specific Retention
The ratio of the volume of water a rock can retain against gravity drainage to the total volume of the rock or Specific retention (Sr) is the ratio of the volume of water a rock can retain (in spite of gravity) to the total volume of rock.
Specific yield plus specific retention equals porosity (often designated with the Greek letter phi).
P = Sy + Sr
Porosity, permeability, specific yield, and specific retention are all components of hydraulic conductivity.
Hydraulic Conductivity is the ability of subsurface materials (sand, rock etc.) to allow a fluid (ie water) to flow through it.
Hydraulic Head
Water entering an unconfined or confined well will stand at a particular level. This level is often termed as the hydraulic head and is actually the sum of three components - the pressure head, elevation head and velocity head. The velocity head is often disregarded because ground water movement in most cases is relatively slow.In an aquifer the altitude to which rise in a properly constructed well is the Hydraulic Head.
Hydraulic Gradient
The change in hydraulic head (pressure) per unit distance in a given direction (dimensionless). It is the driving force of fluid flow in a porous medium.
Darcy's Law
Henry Darcy was a French engineer who studied the movement of water through sand in 1856. He found that the rate of water flow through a tube is proportional to the difference in the height of the water between the two ends of the tube, and inversely proportional to the length of the tube. He also discovered that flow was proportional to a coefficient, K, which is called hydraulic conductivity.
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