Impact of gently dipping discontinuities on basement aquifer recharge: An example from Ploemeur (Brittany, France)

Classically, geological surveys of water resources in hard-rock aquifers are made from aerial photographs or geophysical techniques that basically permit to detect vertical features. On the other hand, aquifers only linked to vertical structures are very limited in space and offer in most cases very poor water resources. In the present case, we do show that an outstanding aquifer in Brittany is linked to a geological context associated with low-angle faults possibly directly connected with vertical feeders. In particular, we show through a high-resolution gravimetric survey that the highly fractured contact between granite and micaschists, which constitutes the main path for groundwater flow, is a gently dipping structure. Combined gravimetric, magnetic and geological data allowed us to establish the overall aquifer geometry by running a three-dimensional forward model. In addition, details about the shape of the contact have been obtained using an iterative scheme based on the method of Oldenburg (1974). The gravimetric model confirms the presence of sub-vertical faults that may constitute important drains for the aquifer recharge. Thus, the geological context associated with exceptional water resources for this crystalline aquifer is characterized by a sub-horizontal to gently dipping contact between granite and micaschists. Only such a geological context can allow sufficient recharge to provide the main water supply for a town of 18,000 inhabitants at an average rate of about a billion of cubic meter per year since 1991. Thus, instead of looking for possible vertical structures like in classical hard-rock hydrogeology, it appears much more efficient to detect sub-horizontal permeable fractures and faults for providing consistent water resources.