Relationship between hydraulic properties and material features in a heterogeneous vadose zone of a vulnerable limestone aquifer

Abstract

Abstract Limestone aquifers constitute important drinking water resources but are vulnerable to groundwater contaminations. They are also known to be often heterogeneous. However, the hydraulic properties of their entire vadose zone and the relationship of these hydraulic properties with their mineralogical and geochemical characteristics remain rarely studied. The hydraulic properties of soft (soil, powdery limestone, and calcareous sand) and hard (limestone rock) materials sampled throughout the entire vadose zone profile of the Beauce limestone aquifer (France) were determined with the multistep outflow method applied using a triaxial system. Physical, mineralogical, and geochemical analysis brought valuable information about factors contributing to the heterogeneity of the hydraulic properties and helped understanding water flow pathways within the vadose zone. The hydraulic properties of the soft materials were strongly related to their physical properties but also to the proportion and the nature of clay minerals. The massive rock presented a few thin microfissures where calcite was replaced by phyllosilicates, thus increasing its water retention capacity. The low hydraulic conductivity of the massive rock could explain the occurrence of perched water tables in the vadose zone. The weathered rock displayed fissures and vugs and was also characterized by the presence of phyllosilicates. The development of a secondary porosity and footprints of water–rock interactions and mass transfers highlighted the transformation of the limestone rock into a more permeable material. This study also pointed out the need to characterize the impact of natural fractures observed at the core scale on the preferential water flow at the field scale.