Slug tests in a confined aquifer: experimental results in a large soil tank and numerical modeling

Abstract

Variable-head (slug) tests in a confined aquifer can be interpreted using different methods that either consider or neglect the influence of the instantaneous deformation of an elastic solid matrix. This paper defines a unified interpretation for slug tests: it is based on the velocity graph describing the conservation equation underlying all methods. If the storativity S has no influence, the velocity graph is a straight line. If S has an influence, the theory considering this influence predicts the graph should be a curve. Numerous slug tests were performed in a large tank containing a confined aquifer. Other tests were used to determine independently the transmissivity T and S values of the confined aquifer which are compared with those obtained from slug tests. The velocity graphs of the slug tests provided straight lines instead of the smooth curves as predicted by the theory. A numerical analysis of these tests in the sand tank was performed using a finite element method. The analysis gave straight lines instead of curves for any S value and therefore confirmed the experimental observation (in velocity graphs) that slug test results do not depend of S and thus cannot be used to determine the S value.Key words: slug test, hydraulic conductivity, storativity, numerical modeling.