Therefore, the following steps are followed to calculate the composite drawdown due to well interference in unconfined aquifers (Kasenow, 2001): (15.1) is always an estimate-in-error of the actual drawdown. Thus, when two or more wells are discharging groundwater from an unconfined aquifer with intersecting cones of depressions, the composite drawdown predicted by Eqn. As a decrease in the saturated thickness of an unconfined aquifer reduces the aquifer transmissivity, the multiple-well system in this aquifer will result in a composite hydraulic gradient greater than that of an equivalent confined system in order to compensate for a reduced value of aquifer transmissivity. In unconfined (water-table) aquifer systems, if the drawdown is significant compared to its initial saturated thickness, the use of linear superposition will result in a predicted composite drawdown that is less than the actual composite drawdown.

The linear superposition principle is valid only for confined aquifer systems, in which the value of aquifer transmissivity does not change with drawdown. Where, s = total drawdown at a given point due to the pumping of multiple wells, and s a, s b, s c, …., s n are individual drawdowns at the point caused by the pumping of wells a, b, c, …., n, respectively.ġ5.1.2 Unconfined Aquifer and Well Interference (Modified from Todd, 1980)ġ5.1.1 Well Interference in Confined Aquifer Systemsįrom the principle of superposition, the drawdown at any point in the area of influence caused by the pumping of several wells is equal to the sum of the individual drawdowns caused by each pumping well, which is mathematically expressed as follows (Todd, 1980): Well interference in a well field having three pumping wells. For a group of wells forming a well field, the drawdown can be determined at any point in the area of influence if well discharges are known, or vice versa (Todd, 1980).įig. 15.1, the well interference for a three-well system is presented graphically in which the individual drawdown curves are shown as dotted lines, while the composite drawdown due to the simultaneous pumping of three wells is shown as solid lines. Since the Laplace equation (i.e., steady-state groundwater flow in homogeneous and isotropic aquifer systems) is linear, the superposition of drawdown effects is found by simple addition. At any given point in a confined aquifer, the total drawdown due to simultaneous pumping of multiple wells is calculated as a sum of the drawdowns caused by individual wells. Well interference increases drawdown, and hence pumping lift is increased. When the cones of depression of two or more nearby pumping wells overlap, the well is said to interfere with another well. However, there are often several wells tapping the same aquifer and located within the radii of influence of the wells, which result in intersecting cones of depression. The cases of well hydraulics considered so far have involved only one well pumping from an aquifer system.

Well drawdown cone