[en] Limestone aquifers represent an important part of groundwater resources in the world. They are usually considered as fractured karstic rock formations. However, a different type, called 'weathered' or 'ghost-rock' karst aquifer has recently been highlighted. In this particular type of aquifer, the karst is not expressed as open conduits but consists in zones made of a residual alterite, resulting from the in-situ partial dissolution of limestone. These particular structures are crucial in some areas because they actually govern groundwater flow and transport in the aquifer. They have, however, never been studied from a hydrogeological perspective. In this study, we present a methodology to characterize and model such aquifers. Results show that the ghost-rock features are usually organized into networks, and that their hydraulic properties are variable within each feature, according to the limestone weathering intensity. Measurements in a specific aquifer in Belgium are presented. The modelling approach, derived from the common Equivalent Porous Medium approach, enables to spatially distribute the variable hydraulic properties within the features, from the hard fresh rock to the alterite, in finite difference grids. The method allows creating models including thousands of features and respecting correct connectivity relations. The approach is considered at different scales with varying grid cells sizes. Results show that the induced error increases significantly when the size of the grid cells exceeds the width of the main features controlling flow. The approach is applied on a case study consisting in dewatering operations in an exploited quarry located in a weathered karst aquifer. Presented results provide new insights regarding characterization and modelling of this kind of aquifers, and highlight their importance in hydrological studies.
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