Inversion of a dual-continuum approach to flow in a karstified limestone: Insight into aquifer heterogeneity revealed by well-test interferences

Two large sets of interference testing data from a karstic aquifer (Hydrogeological Experimental Site, HES – Poitiers – France) are inverted using a dual-continuum approach to groundwater flow. The parameterization technique used to infer spatially distributed hydrodynamic parameters is free of any prior estimation regarding the spatial structure of the parameter fields; it merely seeks the minimal number of local values to obtain the best fit between flow simulations and data. A previous work inverting without any prior geostatistical assumption the same set of data based on a single flowing continuum produced nicely structured (spatially correlated) parameter fields. The dual-continuum approach renders fields as patchworks of subareas of quite uniform values. The statistical distribution of the hydrodynamic parameters is generally skewed toward high values, especially when interference data bear traces of rapid channeled flow in karstic conduits. It is now well established that continuous approaches to flow are well suited to simulate diffusive flow in fractured – karstified rocks, although single continuums may result in artificially high contrasts of hydraulic diffusion values between flowing and non-flowing areas. In the case of a dual continuum, the model has in its physics the capability to clearly separate conduction properties attached to fractures and channels from capacitive properties of the host matrix. We show that high contrasts of hydraulic diffusion disappear, resulting in a mildly heterogeneous medium in which high values of hydraulic diffusion are only located in areas where channeled flow occurs.