HYDROLOGICAL AND HYDROCHEMICAL CHARACTERIZATION OF FAULT ZONES IN CRYSTALLINE MEDIA: IMPLICATIONS FOR GROUNDWATER FLUXES

Abstract:

Fault zones are heterogeneities that may greatly influence groundwater flow in crystalline regions. The quantification of fluxes, the origin of water and geochemical processes associated to groundwater flow in such context remain not well understood. This study mainly concerns a large-scale multidisciplinary field experiments performed on a specific site in Brittany (Saint-Brice en Coglès, France) where a permeable fault zone was identified at depth (200 m). The main objectives here are to constrain both fluxes dynamic and water sources involved during different seasonal regimes.

We demonstrate that the fault zone allows the discharge of regional groundwater at the watershed outlet. Using specific hydro-geophysical measurements (Heat Pulse Flow Meter), we estimate a natural discharge rate between 150 and 200 m³/d. The fault zone presents different geochemical signatures related to changes in hydrologic regime. They are linked to transient fluxes enhancement from different reservoirs. During the low hydrologic regimes, water with high resident time flows along the fault zone, with a contribution of inter-glacial origin (recharge temperature of 7°C deduced from noble gases interpretation). Water trapped in a low-permeability domain is mobilized to the fault zone and/or large-scale circulation loops are involved. During the high hydrologic regimes, modern water predominantly ensures the recharge of the system at a local scale. Results are compared to regional observations in the Armoricain Massif in order to establish mechanisms responsible for recharge and migration of groundwater at the basement scale.