Fiber Optic and Hydrogeological Sensors for the Monitoring of Mass and Heat Transfers Through the Vadose Zone of a Cenozoic Limestone Aquifer (beauce, ORLÉANS, France)

International audienceThe Vadose Zone (VZ) is commonly defined as the partially saturated area which extends from the surface down to the water table and plays a major role within the Critical Zone. As a result of increasing public concern about the need to protect groundwater reserves for drinking and agricultural purposes, a growing body of work has been focused on studying VZ processes, which are still not well understood. In order to integrate observations over a wide range of spatial (nm- to km-) and temporal (minutes to decades) scales, an Observatory of Transfers in the Vadose Zone (O-ZNS) is being carried out near Orléans (France) in an agricultural field. The O-ZNS project aims to understand and quantify mass and heat transfers thanks to a large access well (depth-20m & diameter-4m) surrounded by several secondary boreholes and equipped with environmental monitoring techniques. In order to meet the scientific objectives of the project, several complementary sensors and environmental monitoring techniques are under consideration as part of the instrumentation the O-ZNS experimental site. In this context, three distributed fiber optic sensors were installed in July 2020, in a continuous loop through three boreholes (21 to 23 m deep) surrounding the central well. These sensors will allow continuous measurements of temperature (DTS), deformations (DSS) and acoustic detection processes (DAS) throughout the entire depth of the VZ. Fiber optic technology provides functions similar to conventional sensors, while offering many advantages like small footprint, low attenuation and immunity to electromagnetic fields. Within the framework of the O-ZNS project, many other hydrogeological techniques are also being explored for the monitoring of the water flow and solute transport along the VZ. This includes innovative solutions such as new probes for the monitoring of the water content in hard and deep materials of the VZ, new solutions for the sampling of water and gas at several levels in the VZ, or even the latest generation of lysimeters which represent an excellent tool for the study of transfers at an intermediate scale between the laboratory and the field