The development of the O-ZNS on an agricultural plot of 2400 m2 is entering its final phase with the digging of the central well (4 m in diameter and 20 m deep). The scientific device integrates 8 boreholes of 20 to 25 m depth, 3 of which are already equipped with fiber optics to allow temperature measurements, acoustic signals and micromovements within the O-ZNS. A piezometer equipped with a multi-parameter probe makes it possible to monitor the evolution of the water level as well as the quality of the water, particularly with respect to several parameters (pH, Eh, conductivity, temperature, dissolved O2, nitrates). Water samples are also taken at regular intervals and screenings are conducted at the ISTO laboratory to identify the main pollutants found in the Beauce water table at the O-ZNS site (agricultural pollutants – pesticides, nitrates -, micropollutants, emerging pollutants, etc.). In order to control fluid dynamics and transfer times from the soil surface to the Beauce water table, water sampling campaigns also include the monitoring of CFC and SF6 tracers which have already made it possible to estimate, as a first approximation and by cross-checking with data obtained from analyses of samples carried out in the laboratory, a residence time of about 30 years. In addition, two probes measuring absolute and barometric pressure have been installed in the same piezometer in order to estimate the flow velocity and evaluate the permeability of the formations through the effects of the earth’s tides.
Image 1 – Excavation of the O-ZNS shaft using a hydraulic excavator (0-7 m depth)
The digging of the access well in the UZ started on 31/05/2021. This is done by a pass of about 1.30 m followed by the installation of a temporary wooden support. A scientific program accompanies the excavation work. The first part of this program consists of a precise sampling for each excavated pass at different depths with packaging specifically implemented for (i) conservation of a representative quantity of excavated materials, representing 2 to 3 m3 sampled in bags; (ii) water content measurements on a selection of samples representative of each pass; (iii) conservation under vacuum conditions of a selection of samples representative of each pass for chemical analyses to be carried out a posteriori in the laboratory; (iv) selection of samples judged remarkable presenting key/rare/specific geological phenomena. The second part of the scientific program accompanying the excavation of the shaft concerns the measurements carried out in the shaft after each run, during the period between the excavation of the shaft and the installation of the temporary wall (still bare wall). Thus, the shaft wall (with a digging diameter of 6 m) is accurately digitized by photogrammetry. LIDAR measurements with a thermal camera are also carried out. This digital geology will constitute the memory of the well for the construction of future hydrogeological models and digital twins necessary for the scientific exploitation of the structure and the orientation of strategic locations for the installation of sensors (realization of lateral and oblique drillings, installation of monitoring tools, etc.).
The central well of the O-ZNS offers a unique opportunity to observe the complex structuring of the vadose zone and its dynamics during many decades. Thus, thanks to the direct and highly resolved observations during the construction of the well we will seek to build a digital twin of the O-ZNS site. For this purpose, three levels of measurements are performed: (i) millimetric to centimetric observations are made in the laboratory on the cores from the drilling (digitization and 3D replication of geological objects); (ii) each digging pass is digitized in its entirety in 3D (LIDAR scan & photogrammetry); (iii) the first 50cm of the walls are scanned in depth to detect cavities or cracks in the massif (georadar, thermal camera). This spatialized data set will allow the construction of the digital architecture of the structure in order to preserve the geological and structural memory of the site necessary for future hydrogeological modeling and coupling leading to multiphase reactive transport models.
Image 2 – Sampling of excavated material from the O-ZNS pit with the hydraulic shovel (0-7 m depth)
A multi-method geophysical imaging program has been implemented at the O-ZNS site through several field campaigns over the past 3 years with a myriad of techniques sensitive to the specific geological structure of early karstification carbonate reservoirs and to fluid and heat transport processes. These techniques have been deployed on the surface and in boreholes. These methods should help to estimate the temporal variation and spatial distribution of water contents in order to highlight the movement of water in the pore and fracture network. Laboratory experiments on rock samples will complement this information to estimate water parameters. These methods include electrical resistivity tomography, georadar, spontaneous potential, gravimetry, magnetic resonance probing and seismic techniques. The campaigns will continue with the implementation of radar measurements in the well in order to characterize the fractures and karstic structures around the well from a depth of 7 m which corresponds to the roof of the “hard rock”. Magnetic resonance imaging is continuing on the surface, while the O-ZNS scientific team also plans to launch a few seismic campaigns during the digging work.
Image 3 – Photogrammetry and digitizing of the O-ZNS shaft walls (0-7 m depth)