The impact of common impurities present in gypsum deposits on in situ dissolution kinetics

International audienceThis study aims to bring elements of understanding of water-gypsum interactions in situ to better assess the development of dissolution cavities in areas where gypsum is present at depth. Different types of natural gypsum facies are tested, impure alabaster, sacharoidal and clay/carbonate matrices gypsum. An experimental protocol is first developed to study the effect of erosion and particle transport on gypsum dissolution. An original leaching experiment under constant flow is then elaborated to study the effect of erosion and particle transport on the dissolution of gypsum. The flow of released particles is globally low and mostly composed of solid impurities (insolubles). The distribution of insolubles at the water-gypsum interface is found to have a significant impact on the dissolution. Geochemical models are then used to investigate the influence of the most common minerals present in natural gypsum on the dissolution process and the chemical composition of groundwater at their contact. These findings, applied to in situ conditions, allow us to evaluate the relevance of the common use of a simple criterion such as dissolved sulfate content or electrical conductivity to identify a system saturated in gypsum. Lastly, an effective recession rate, derived from the dissolution rate and evaluated directly according to the groundwater saturation index of gypsum, is used to determine the intensity of natural gypsum dissolution in the study area