Capture zone delineation in hard-rock aquifers: Theoretical insights
Résumé
Hard-rock aquifers present a high level of complexity that makes them difficult to manage. Most of the flows are focused within a limited number of preferential paths where water can travel over large distances. It is generally admitted that the productivity of these preferential paths relies on the presence of a conductive layer, usually a more weathered zone near the ground surface, that can provide them with water. The consequences of this particular organization of flows are important for the definition of a protection perimeter around a pumping well. Because of the high complexity of this kind of medium associated to a lack of reliable characterization, building a deterministic model is neither possible nor relevant. As a consequence, a better fundamental understanding of the possible flow configurations is crucial from a management perspective. Indeed, there is a need for evaluating the possible degree of uncertainty associated with these systems, and for determining the parameters on which to focus on. We handle this problem from a theoretical point of view, in order to assess the role of permeability structures typical of hard-rock aquifers on the shape and the extension of the capture zone of a well. We explore several possible configurations in semi-regional models integrating the full dynamic of the water circulation from the recharge in the weathered zone to the deeper aquifer. We show that the recharge of the deep aquifer is especially sensitive to the distant structure of the weathered zone and its relation to the zone of intense vertical circulations. Strong connections between surface and subsurface water bodies reinforce the 3D nature of the circulations and the interactions between deep pumping and surface recharge. From this perspective, the topography also exerts a strong control on the shape of the capture zone.