What Can We Learn About Karst Aquifer Heterogeneity From Pumping Tests ?
Résumé
Due to the complexity and duality of flows, well-test interpretation into karst systems constitutes a challenging task for hydrogeologists. This is especially true when the pumping well intersects karst heterogeneities such as the conduit network. The method of diagnostic plots, widely used in oil industry, can be applied to karst hydrogeology. In this paper, the classical response of a well-test into a karst conduit is described on a log-log drawdown derivative curve. It allows identifying successive flow regimes corresponding to the contribution of various karst aquifer subsystems (fractured matrix, karst conduit, main karst drainage network) to the pumped well. In heterogeneous karst systems, the log-log diagnostic plot of drawdown and its derivative in the pumping well can help identifying departures in flow-geometry from the classical homogeneous radial case. Classically, the diagnostic plot can be divided into several portions with: (a) early data used for identifying the karst conduit storage; (b) intermediate data for identifying the type of aquifer model that should be used (e.g. double porosity, anisotropy...); and (c) late data for identifying the possible boundaries. This is illustrated on three examples from Mediterranean karsts in southern France. A one-month duratio pumping test on a well intersecting the main karst drainage network of the Cent-Fonts karst system shows (i) a preliminary contribution of the karst conduit storage capacity followed by (ii) linear flows into the fractured matrix. A pumping test on a well intersecting a small karst conduit of the Corbières karst system shows the existence of (i) bi-linear flow within both the karst conduit and the fractured matrix at early times, followed by (ii) radial flows within the fractured matrix and (iii) finally the contribution of a major karst cavity. A two-months duration pumping test on a deep confined karst aquifer under low permeability rocks into the Gardanne basin shows the existence of no-flow boundary conditions due to the basin extension. The use of diagnostic plots allows identifying the various flow regimes during pumping tests, corresponding to the response of the individual karst aquifer subsystems. This is helpful for improving the understanding of the structure of the karst aquifer and flow exchanges between subsystems.