Résumé
Coastal aquifers often display seawater intrusions resulting in the formation of a salty water wedge progressing inland. The mixing zone water has a chemical composition in disequilibrium with the rock forming carbonate minerals inducing significant mass transfers that may modify porosity and permeability irreversibly. Yet, these changes in the aquifer properties and the controlling mechanisms are poorly described due to the large variability of the (bio)chemical and hydrodynamic environment and the difficulty of monitoring both the fluid and the rock changes over pertinent durations. In this study we first characterize a saltwater intrusion into a coastal carbonate aquifer of Mallorca Island (Spain) and monitor its evolution with time, from 2003 to 2011, using repeated electrical conductivity logs of the formation and the saturating fluid, as well as regular pore-water sampling and permanent downhole multi-parameters monitoring of the water. Secondly, we investigate the geochemical processes occurring within the mixing zone by a coupled analysis of the sampled water composition and the characteristics of the rock, which are investigates with borehole images, thin sections and X-Ray tomography. The results show that the mixing zone groundwater is undersaturated with respect to calcite. The maximum calcite undersaturation is observed at the saltwater side of the mixing zone, which is not consistent with the prediction of the theoretical conservative mixing of the freshwater and seawater end-members. The observation of calcite dissolution features suggests that the undersaturated water in the mixing zone might lead to noticeable porosity development at these depths. Also, the localization of the dissolution features slightly deeper to the maximum undersaturation suggests an upward motion of the mixing zone. The data also highlight a pH anomaly (acidification) localized at the center of the freshwater-saltwater, which cannot be explained by conservative mixing, but however correlates with the presence of microbiological activity.