Equilibrium hydraulics of wellsThe fgure shows a well in a homogeneous aquifer of infnite extent with an initially horizontal water table. The corresponding steady state piezometric draw down surface is also shown for the assumed constant pumped discharge Q. The well is assumed to have a radius rw and the radius of infuence is thought to bewhere the potentiometric surface is nearly equal to the original undisturbed value !" measured from a datum. #t the well" the depth of water is designated by hw" which is also measured from a common datum. $n general" at a certain radius r measured from the center of the well"the potentiometric surface stands at a height %h& measured above the datum.The yield from the well Q may be expressed in terms of 'arcy&s law as"()*+here K is the coe,cient of permeability of the formation" i is the hydraulic gradient that is" the slope of the potentiometric surface at the well bound and A is the surface area of the well through which the fow is converging into the well from the aquifer. Thus"-(.*$n the above equation" b is the Thic/ness of the aquifer. 0aturally" the same amount of water also travels through the aquifer at a radial distance r from the center of the well. Thus" yield would also be (1*The above expression is true if the aquifer thic/ness b is assumed to be constant throughout. The above equations give us a value of the yield" Q" of the well but for that a measure of the gradient of the potentiometric surface is essential. This may be done by inserting a piezometer penetrating into theaquifer and noting the water level there.. (0ext fgure*earranging and integration (2*+hich gives"(3*-This is called the Thiem Equation and can be used to estimate the transmissibility.(4*0onequilibrium !ydraulics of +ells'uring the initial period of pumping from a new well" much of the discharge is derived from storage in the portion of the aquifer unwatered as the cone ofdepression develops.$n )513" 6harles 7. Theis published a brief articlethat was the frst transient solution for groundwater fow toward a well. !e did so by understanding the vital analogy between groundwater fow and heat transfer. The Theis legacy is not limited to the transient theory. !e was one of the frst scientists to emphasize the importance of geological heterogeneity.$n )513 Theis presented a formula based on the heat8fow analogy which accounts for the e9ect of time and the storage characteristics of the aquifer. !is formula is:-#nd is given by this equation:..;or the water8table aquifers it is essentially the specifc yield. The integral in the )st formula is commonly written +(u* and called the well function of u" can be evaluated from the series..-+hen u is small"the terms of the last Equation following ln(u* are small and maybe neglected. The Equation1 indicates that u will be small when t is large" in this case modifed solution of the Theis method is possible by writing..Q< $s the fow in cubic meter.h< is the change in drawdown between time=oundary E9ects-The assumption of a symmetrical cone of depression implies a homogeneousaquifer of great extent#quifer analysisTechniques of the previous sections are suitable for analysis of single wells ora small well feld" but study of a large aquifer generally requires more e,cient computational systems" or simply aquifer analysis is an e9ective means of determining the capabilities and limitation of a well to deliver waterto the surface. > # Hele-Shaw apparatus is one instrument use for analyzing aquifer" but itis use in two8dimensional groundwater8fow problem> # Analog Computer is use when the problem is in three8dimensional?otential of a @roundwater eservoirA basic problem in engineering groundwater studies is the question of the permissible rate of withdrawal from a groundwater basin. Thisstudy identifed a set of equations that are based on an analytical solution to the ground8water8fow equation for the case of one8dimensional fow in a fnite8width aquifer bounded by a linear surface8water feature such as a stream. This quantity" commonly called the safe yield" is defned by Aeinzer.Bafe yieldThe safe yield of a groundwater basin is governed by many factors" one of the most important being the quantity of water available. #nd Bafe yield is generally considered equal to the average replenishment rate of the aquifer from natural and artifcial recharge. Evaporation" transpiration and basin outfow are also factored in to replenishment rates. -Beawater $ntrusion$s the movement of salt water into freshwater aquifers" which can lead to contamination of drin/ing water sources and other consequences. Baltwater intrusion occurs naturally to some degree in most coastal aquifers" owing to the hydraulic connection between groundwater and seawater. =ecause saltwater has a higher mineral content than freshwater" it is denser and has a higher water pressure. #s a result" saltwater can push inland beneath the freshwater. ;resh water is lighter than salt water. Therefore" fresh water CfoatsC on top ofsalt water. This principle becomes extremely important when considering thedrilling of a well in order to tap into the ground water of any island. The weight of the rain water that percolates into the ground depresses the salt water beneath it forming a profle that has the appearance of a lens. This is called the Ghyben-Herzberg lens. The principle of this relationship was discovered independently by a 'utch scientist named Baden-Ghyben and a @erman scientist named Herzberg.#rtesian aquifer#n artesian aquifer is an underground layer which holds groundwater under pressure. This causes the water level in the well to rise to a point where the pressure is equal to the weight of water putting it under pressure. This type of well is called an artesian well. +ater may evenreach the ground surface if the natural pressure is high enough" in which case the well is called a fowing artesian well.-#n aquifer is a geologic layer which can hold water such as sand and gravel" limestone" or sandstone" through which water fows and is stored. #n artesian aquifer is trapped between roc/s or clay which causes the pressure. +ater returns to the aquifers when the water table at its recharge zone is at a higher elevation than the head of the well.-