Structural controls of groundwater resources in fractured rock aquifers
Résumé
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.