Acquisition of water chemistry in a mobile fissured basement massif: its role in the hydrogeological knowledge of the La Clapière landslide (Mercantour massif, southern Alps, France)

Groundwater flowpaths of the La Clapière landslide are studied by chemical and isotopic water analysis and modeling of the landslide springs. In our case, it appears that sulfate can be taken as a chemical tracer of the different flowpaths. On the gneissic slope of La Clapière, geochemical modeling explains sulfate concentrations from 0 to 130 mg/l by dissolution of sulfurous minerals present in the gneisses. Such a variability of the concentrations measured in the springs is due to different infiltration paths: direct paths through the basement ([SO42−]=10–60 mg/l or indirect path through superficial deposits like screes or fluvio-glacial terrains which cover the basement ([SO42−]=60–130 mg/l). The high concentrations (600–800 mg/l) cannot be explained by thermodynamic geochemical simulations, and they reveal Triassic gypsum dissolution hidden under the studied slope. So, two main flowpaths are differentiated in the La Clapière landslide: (1) flow through low permeable Triassic deposits pinched under the foot of the landslide, (2) flow through a more permeable fissured basement with different flow paths; and, consequently, with different transit times due to the anisotropic nature of the top and the central part of the landslide.