Carbon-13 and carbon-14 contents of groundwater located in sandy aquifer outcrops: driving influence of the unsaturated zone

Better assessment of carbon isotopes cycle in the Unsaturated Zone (UZ) is a key point for estimating the 14C residence time of relatively young groundwater. In order to approach the UZ carbon cycle and to quantify C-fluxes, the distribution of the isotopic composition (13C-14C) of each carbon phase (Dissolved Inorganic Carbon, CO2, carbonates) have been investigated on two experimental sites i.e. respectively the carbonate-free Fontainebleau sands (Paris Basin, France) and the carbonate Astian sands (South France). Similar matrix properties on both sites allowed identical in situ sampling protocols based on water-gas permanent samplers specifically developed for this work. In a carbonate-free UZ, the isotopic composition of water depends essentially on CO2 diffusion. While, in a carbonated UZ, the carbonates force the isotopic composition of dissolved inorganic carbon and CO2. These carbon exchanges lead to a depletion of matrix carbonates by precipitation of secondary calcite. (author)