Traditional Aquifer Tests: Comparing Apples to Oranges?

Abstract

Traditional analysis of aquifer tests uses the observed hydrograph at one well caused by pumping at another well for estimating transmissivity and storage coefficient of an aquifer. The analysis relies on Theis' or Jacob's approximate solution, which assumes aquifer homogeneity. Aquifers are inherently heterogeneous at different scales. If the observation well taps into a low permeability zone while the pumping well is located in a high permeable zone, the resulting situation contradicts the homogeneity assumption embedded in the traditional analysis. As a result, a practical but important question we ask: What do we derive from the traditional analysis? Using numerical experiments in synthetic aquifers, we answer this question. Results of the experiments indicate that the effective transmissivity, Teff , and storage coefficient, Seff , values vary with time, as well as the principal directions of the transmissivity, but both values approach their geometric means of the aquifer at large times. Analysis of the estimated transmissivity (T) and storage coefficient (S ) using well hydrographs from a single observation well shows that at early times, both the estimated T and S values vary with time. At late times, both estimates approach local averages near the observation well. The T value approaches but does not equal Teff , representing an average value over a broad area in the vicinity of the observation well while the S value converges to the value dominated by the storage coefficient near the observation wells (i.e., its average area is much smaller than that of the t value).