![]() Although solutions can be found for cases in which the value of the horizontal hydraulic conductivity is different from that of the vertical hydraulic conductivity, it usually assumed that the aquifer has radial symmetry.Īs mentioned above, if pumping is continued for a long time, the groundwater level may reach a state of equilibrium, i.e., no change in drawdown with time. This means that the values of aquifer transmissivity (T) and storage coefficient (S) do not depend on the direction of flow in the aquifer. Besides the above-mentioned basic assumptions, it is also assumed that flow to wells is radially symmetric. Radial flow can be thought of as flow along the spokes of a wagon wheel towards the hub. The flow towards a well is termed radial flow. Groundwater has a constant density and viscosity.The pumping well has an infinitesimal diameter (i.e., negligible storage) and is 100% efficient (i.e., no well losses).The pumping well and the observation wells are fully penetrating.The aquifer is homogeneous and isotropic.All geologic formations are horizontal and of infinite horizontal extent.All changes in the position of the groundwater level are due to the effect of the pumping well alone.The groundwater level of the aquifer is horizontal and not changing with time prior to the start of pumping.The aquifer is bounded on the bottom by a confining layer.The following are the basic assumptions made for analyzing flow to wells: In addition, the concept of partial penetration and the equations for computing steady drawdown in partially penetrating wells installed in confined and unconfined aquifers are presented.ġ0.2 Steady Flow to Fully Penetrating Wellsġ0.2.1 Basic Assumptions for Analyzing Flow to Wells In this lesson, the main equations for analyzing steady flow to fully penetrating wells in confined and unconfined aquifers are derived and their applications are discussed. In contrast, under unsteady-state (transient-flow) conditions, either entire pumped water is assumed to be coming from the aquifer storage within the radius of influence or the pumped water is assumed to be coming partly from the aquifer storage within the radius of influence and partly from external sources beyond the radius of influence depending on field conditions. Note that under steady-state conditions, the entire pumped water is assumed to be coming from external sources beyond the radius of influence. This equilibrium condition changes when the pumping rate is increased or decreased. At this instant of time, a steady flow condition exists in the aquifer and the cone of depression gets stabilized (i.e., it does not change with pumping time). As a result, the radius of influence increases until when the rate of pumping (discharge) becomes equal to the rate of flow into the well from the area around the well. ![]() 10.1) where drawdown is zero.Īs more and more groundwater is pumped through the well, the more water comes from aquifer storage. Thus, the radius of influence (R 0) is the distance from a pumping well to the edge of the cone of depression (Fig. The boundary of the area of influence is called circle of influence and the radius of the circle of influence is called radius of influence. ![]() The outer limit of the cone of depression defines the area of influence of the well. In three dimensions, the drawdown curve takes the shape of an inverted cone centered on the pumping well, which is known as cone of depression. ![]() The faster the well is pumped, the steeper the hydraulic gradient will be in the vicinity of the well.Ī drawdown curve at a given time shows the variation of drawdown with distance from the pumping well. The rate of pumping from an aquifer significantly affects the hydraulic gradient in the aquifer. Drawdown is always maximum at the pumping well and it decreases with an increase in the distance from pumping well (Fig. The difference between the static water level and the pumping water level at any instant is called ‘drawdown’, which is a function of pumping rate, pumping duration and distance from the pumping well. Drawdown pattern in: (a) Confined aquifer (b) Unconfined aquifer.
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