Structural controls of groundwater resources in fractured rock aquifers

Sarah Leray (1, 2) , Jean-Raynald de Dreuzy (2) , Olivier Bour (2) , Etienne Bresciani (1)

1 UC - Pontificia Universidad Católica de Chile
2 GR - Géosciences Rennes

As hard rock aquifers are gaining in importance for groundwater resources, significant advances have been made in the last two decades in characterizing their structure and understanding their hydrogeological functioning. Two conceptual models have been proposed so far to explain qualitatively the geological controls of water productivity. These models propose that groundwater resources in hard rock aquifers are typically associated with the weathered zone or with regional sub-vertical faults outcropping in the surface. Still, they fail to explain the hydraulic functioning of a few sites worldwide including the Ploemeur aquifer (Brittany, France). This system in particular hosts a slowly dipping highly transmissive fractured zone - a contact between overlying micaschists and a leucogranite - in addition to a set of sub-vertical normal faults. Numerical simulations on a simple conceptual hydrogeological model show that either the transmissivity or the deepening of a regional fractured zone control the resources in fractured media. The fractured zone indeed always allows some recharge to occur, even if in its vicinity only; and this, whatever the properties of the overlying rock. Conversely, when the fractured zone dips slowly, aquifer recharge extends spatially because of a thinner overlying unit and as such, even local fractured zones can bear a significant resource. In such conditions, the flow within the fractured zone may even become the limiting factor for groundwater occurrence. Local (kilometric) sub-horizontal structures can thus be as productive as sub-vertical regional faults. In the Ploemeur aquifer, the sub-horizontal geological contact then likely controls the recharge of the site and its high productivity. Comparison with other field cases in Brittany confirm the effect of the dip of the fractured zone on productivity. Simulation results not only reveal additional favourable hydrogeological conditions, they also show that the flow patterns in shallowly dipping fractured zone differ strongly from those located in regional subvertical fault zones. Such differences carry significant consequences in terms of management as well as identification methods.

Domains: Hydrology

Sarah Leray, Jean-Raynald de Dreuzy, Olivier Bour, Etienne Bresciani. Structural controls of groundwater resources in fractured rock aquifers. XV Congreso Geológico Chileno, Nov 2018, Conception, Chile. pp.643. ⟨insu-02048167⟩