Hydrogeochemistry: an investigation tool to evaluate infiltration into large moving rock masses (case study of La Clapière and Séchilienne alpine landslides)

Isotopic and hydrogeochemical methods have been used to investigate groundwater movement inside the La Clapiere and Sechilienne alpine landslides in southern France. The delta exp 18 O data were used to determine the infiltration altitudes of the two areas. The infiltration results indicate that the landslides are recharged from beyond the landslides' perimeters. Hydrogeochemical data on major ions were collected from springs. Numerical simulations of water-rock interactions were then undertaken. The major petrographic contrast between the limited sedimentary rocks and the more common mica-gneiss/micaschist results in a marked change between the measured and calculated groundwater contents. This contrast of 800 mg/l of SO sub 4 in the Triassic rocks but only 100 mg/l for the waters from the metamorphic strata at La Clapiere is significant. Two different groundwaters have been identified in both landslides: (1) a perched shallow saturated zone near the slope summit; and (2) a deep saturated zone located at the foot of the slope. Chemicalmonitoring of spring waters in the two zones has allowed an assessment of the infiltration within the slope over time. There is a good correlation between the sulphate content of the perched waters and rate of slope movement, with a sulphate dilution peak corresponding to an acceleration in the movement of the landslip. However, there is no correlation between the chemistry of the deep aquifer and the speed of movement. It would appear therefore that the hydromechanical behaviour of the landslide depends on the vertical leakage from the perched aquifer down to the basal aquifer and the near-surface effects of the water movement.