impermeable boundary will be more than the drawdown calculated considering infinite areal extend of the aquifer medium. This problem can be solved by considering an imaginary pumping well at a distance equal to the distance between the pumping well and the image pumping well. Fig. 21.7 has shown the equivalent hydraulic system in an aquifer with infinite areal extent. For the equivalent hydraulic system, the time drawdown relationship for the pumping well and also for the imagery recharge well can be obtained separately. The actual drawdown can be obtained using the principle of superposition.

• Fig. 21.7 Equivalent hydraulic system in a aquifer of infinite areal extend

Consider the Fig. 21.8 below. The pumping well is at a distance of x from the impermeable boundary. In order to calculate the actual drawdown at the observation location, an image well is considered at a distance of x on the other side of the line of zero flow. The distance of the observation well from the pumping well is r and from the image well is r' .

For the unsteady condition, the drawdown at a distance r at any time t can be obtained as,

                                                           (21.7)

                                                             (21.8) 

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• Fig. 21.8 Pumping well, observation well and image well