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In this article we will discuss about how to construct multiple well systems.
If there are a number of pumping wells in a given well field, the drawdown at any point is the sum of the drawdowns due to each pumping well, for which the distance of the point from each well and the discharge of each well should be known. The drawdowns depend upon the pumping pattern, i.e., the number of wells, their pumping rate and their array. Solutions may be obtained using equilibrium or non-equilibrium equations, as the case may be. Multiple well systems are used for lowering the ground water table in a given area to facilitate excavation for foundation work, etc.
Wells may be closely spaced (resulting in mutual interference) and all the wells may be connected to a common supply pipe to meet the large demand for water supply. For an array of a number of equally spaced (a metres apart) fully penetrating wells, all discharging at the same rate (Q), parallel to a line source (at a distance d), Fig. 5.41, the drawdown at any point (x, y) is given by-
Muskat (1973) developed solutions for well discharges for various well patterns localised near the centre of a well field of radius R such that for each well the head at the external boundary can be taken to be H (i.e. R is the radius of influence for each well), Fig. 5.42. In the following solutions it is assumed that all the wells fully penetrate a confined aquifer, have the same diameter, and drawdown, and discharge for the same period of time. The equations can also be applied to unconfined aquifers by replacing H by H2/2b and hw2/2b.
(a) Two wells spaced at a distance a (a << R), discharging Q1 and Q2 for confined and unconfined aquifers, respectively, are given by-
As the number of wells in the group increase, the mutual interference between wells becomes more, with the result the production capacity per well decreases.