Investigating the role of deep weathering in critical zone evolution by reactive transport modeling of the geochemical composition of deep fracture water

Relatively little is known about the deep water circulation and role of weathering processes on Critical Zone (CZ) evolution. In this study, major fractures from four boreholes collected these (20–60 m depth) were studied to improve our understanding CZ in a granitic headwater watershed (Strengbach watershed, France). Geochemical analysis indicates that chemical composition m) clearly different than shallow subsurface waters (0–15 m), with higher mean concentrations stronger spatial variability across watershed. Reactive-transport modeling performed 1D KIRMAT code (KInetic Reaction MAss Transport) highlights minor mineral dissolution (i.e. dolomite) longer transit time (100–597 yr) along for explaining differences. The obtained results also shed light difference hydrogeochemical functioning between system (water sampled springs piezometers) (deep borehole fractures). characterized by spatially relatively homogeneous flow, high pore velocities, water–rock interactions porous regolith, while behaves much more as independent systems fractures, low variable mineralogy hydrodynamic conditions. A state equilibrium can be reached some primary minerals (biotite K-feldspar), feature never observed so far simulations Strengbach Even if exhibits solute concentrations, significantly lower velocities inferred fractured zones imply responsible very small part total hydrologic fluxes limited compared suggests an up bottom control evolution at millennial timescale. Our indicate majority fresh available supply human timescale originates waters, too being critical relevant societal needs type geological context.