Flow to a well in a two-aquifer system

An approximate solution for flow to a well in an aquifer overlain by both an aquitard and a second aquifer containing a free surface is obtained from numerical inversions of exact analytical solutions for Laplace transforms. This study extends previous work on this topic in three ways: (1) it improves an eigenvalue decomposition solution method used earlier by Hemker and Maas in 1987 for unsteady flow in multiaquifer systems; (2) it obtains an approximate solution that is simpler to evaluate than previously known solutions; and (3) it utilizes a much more general solution from a MODFLOW finite-difference model to explore both limitations of the approximate solution and the physical behavior of the two-aquifer system. In addition, criteria are obtained that show when the two-aquifer solution can be replaced with the simpler Boulton solution when analyzing pumping test field data.