Hydraulic conductivities of low permeability sediments inferred from a triple pumping test and observed vertical gradients

Knowing vertical conductivities of thick aquitards is important for simulating regional or local ground water flow. These hydraulic parameters are best determined by calibration of these models. Collection of reliable data from heavily pumped ground water reservoirs can be problematic. Another way to infer these hydraulic parameters is the simultaneous interpretation of different parts of a multiple pumping test using an inverse numerical model. Here, the drawdowns observed in the indirectly pumped aquifers allow us to deduce the vertical conductivities of the aquitards with an equivalent accuracy as the horizontal conductivities of the aquifers. When the hydraulic resistances of the aquitards are too large, it becomes impossible to induce observable drawdowns in the indirectly pumped aquifers, and the inverse model loses valuable input data. Observing vertical gradients in and over the aquitards in case of important vertical flow can make up for this loss. These data allow us to estimate the specific discharges through the successive parts of the aquitard and hence their hydraulic resistances. This useful information allows us to consider all these hydraulic resistances in one group of hydraulic parameters during the simultaneous interpretation of the multiple pumping test using an inverse numerical model. As this procedure reduces the number of identified parameter groups, it enhances their reliability. In this paper it is demonstrated how this reliability can be deduced from a number of statistical parameters that can also be used in the collinear diagnosis. The proposed interpretation procedure is demonstrated on a triple pumping test performed in and around the most important aquitard in Flanders, Belgium. The obtained results should first be integrated in a local ground water flow model. By the performance of a number of such integrated hydrogeological studies, the lateral change in the hydraulic properties of this aquitard can be evaluated.