Characteristics of unsteady flow to wells in unconfined and semi-confined aquifers

An approximate solution by Zhan and Zlotnik for flow to a horizontal or slanted well in an unconfined aquifer is shown to be general enough to include the earlier solutions of Theis, Hantush and Jacob, Boulton and Neuman and is used to investigate the behavior of flow to a well in unconfined and semi-confined aquifers. The semi-confined aquifer is bounded on top by one or more aquitard layers containing a free surface, which is allowed to draw down with time as flow is abstracted from the well. Drawdown curves for unconfined and semi-confined aquifers are observed to have similar shapes but greatly different time scales, drawdown magnitudes and drawdown distributions along vertical lines in the pumped aquifer. This emphasizes the importance in applications of choosing mathematical models that correctly fit aquifer geologies. The Zhan and Zlotnik solution utilizes several approximations, and comparing solutions for flow to vertical wells in unconfined and semi-confined aquifers with the exact Neuman and Boulton solutions tests the accuracy of these approximations. Then the solution is used to plot drawdown contours around a horizontal well in an unconfined aquifer, and a number of reasons are given for the relative efficiency of using a horizontal well to abstract water from an aquifer.