A methodology for the mathematical modelling of hard-rock aquifers at catchment scale, based on the geological structure and the hydrogeological functioning of the aquifer

Hard-rock aquifers can be considered to comprise several compartments, each having distinct hydrogeological properties; from bottom to top: (1) the fresh basement substratum, essentially transmissive where fractured, (2) the "weathered-fissured zone" characterized by almost horizontal fractures. This compartment has good hydrogeological properties: sufficient transmissivity to enable pumping of up to several cubic metres per hour, effective porosity of around 1%, (3) alterites, that cover the weathered-fissured zone to a variable thickness. Because of its clayey-sandy composition, this compartment is characterized by a relatively low permeability, but a significant water retention capacity. Based on this conceptual model of the genesis of the regolith cover, an economically feasible method of mapping the thickness and spatial distribution of the alterites and the weathered-fissured zone has been developed. The consequent knowledge, at the scale of the catchment, of the geometry of these different compartments that constitute the aquifer, opens up large development prospects in terms of well siting, but also for water resource evaluation, by enabling to regionalize hard-rock aquifer properties. It allows also to use multilayer mathematical modelling tools (finite differences of finite elements) to simulate the functioning of this type of aquifers, taking in account the geometry and the hydrodynamic properties of these different compartments. First applications of this methodology are ongoing, in France and India.