Analysis of a Pumping Test in an Anisotropic Aquifer by Use of an Inverse Numerical Model

Abstract

An interpretation method based on an inverse numerical axi-symmetric model is further generalized so that pumping tests in laterally anisotropic aquifers can also be interpreted. This method was originally developed for the interpretation of simple or multiple pumping tests in layered laterally isotropic aquifers. It is applied to one of the six tests made in the fractured upper part of the Brabant Massif (Flanders, Belgium). All observed drawdowns and residual drawdowns were simultaneously interpreted. The introduction of the apparent distance of the observation wells permitted a search for the laterally anisotropic characteristics of the pumped aquifer. The results of the inverse model are the optimal values of the hydraulic parameters and their joint confidence region. Cross sections through this exact or approximate region provide insights regarding the location of the bounds of the joint confidence region, and they also serve as an additional tool in the collinear diagnostic. The contours of the sums of the squared residuals for different combinations of drawdown measured in different wells provide a basis for judging the consistency of the data. These contours also permit an assessment of how the observations of the different wells complement each other to reach a unique solution. On the basis of the consistency of the data, it is concluded that the fractured aquifer behaves as an equivalent laterally anisotropic medium.