Variability of the groundwater sulfate concentration in fractured rock slopes: a tool to identify active unstable areas

Abstract : Water chemical analysis of 100 springs from the Orco and the Tinée valleys (Western Italy and Southern France) and a 7 year groundwater chemistry monitoring of the 5 main springs were performed. All these springs drain from crystalline rock slopes. Some of these drain from currently active gravitational slope deformations. All groundwaters flowing through presently unstable slopes show anomalies in the sulfate concentrations compared to stable aquifers. Particularly, an increase of sulfate concentrations was observed repeatedly after each of five consecutive landslides on the La Clapière slope, thus attesting to the mechanical deformations are at the origin of this concentration change. Significant changes in the water chemistry are produced even from slow (mm/year) and low magnitude deformations of the geological settings. Pyrite nuclei in open fractures were found to be coated by iron oxides. This suggests that the increase of dissolved sulfate relates to oxidative dissolution of Pyrite. Speciation calculations of Pyrite versus Gypsum confirmed that observed changes in the sulfate concentrations is predominantly provided from Pyrite. Calculated amounts of dissolved minerals in the springs water was obtained through inverse modelling of the major ion water analysis data. It is shown that the concentration ratio of calculated dissolved Pyrite versus calculated dissolved gneiss rock allows us to unambiguously distinguish water from stable and unstable areas. This result opens an interesting perspective for the follow-up of sliding or friction dynamic in landslides or in (a) seismic faults.