SPATIAL AND TEMPORAL VARIABILITY IN THE WATER REDOX CHEMISTRY OF THE M27 EXPERIMENTAL SITE IN THE DRAC RIVER CALCAREOUS ALLUVIAL AQUIFER (GRENOBLE, FRANCE)

The hydrogeochemistry of a small portion of a calcareous alluvial aquifer recharged by the Drac River (Grenoble, France) is characterized both in time and space. Significant changes in water chemistry are observed as the river water infiltrates into the aquifer. Processes occurring in the river bank (mostly biologically mediated) and in the aquifer (additional solid-liquid interactions) buffer some of the groundwater composition regardless of the variations in the river input. However, small zones (of enhanced biological activity due to low permeability and/or high organic carbon content) located away from the river bank can induce significant localized changes in groundwater quality (such as locally high dissolved Mn due to the dissolutive reaction of MnO2(s) and/or MnOOH(S)).