Selenium mobility in a major Chalk aquifer (Lille metropolis, northern France): Contaminants cycles driven by geology, redox processes and pumping

Chalk groundwater in northern France presents selenium (Se) concentrations from <10 to 70 μg L−1, partly exceeding the latest European Framework Directive's drinking-water limit value of 20 μg L−1. Better to understand the heterogeneous dynamics of Se, we used a combination of geochemical, isotopic and geophysical tools on a study site belonging to the Metropolis of Lille, France. This approach provided a fine understanding of the Se fate in a dynamic redox system constrained by geology, related redox processes and pumping, illustrating how local versus global controls affect the Se cycling. A remarkable redox sequence controls element transfers in groundwater with a progressive creation of reductive conditions. Highlighted by geophysical tools, a wide fracture corridor in the Chalk formation disrupts the geological setting of this redox sequence. Under pumping, this corridor allows the mixing of oxygenated groundwater with groundwater under reducing conditions. The evolution of isotopic compositions of sulphate molecules confirms the global reduction trend of sulphates, while pyrite oxidation occurs very locally, together with high Se concentrations. Pyrite is expected to play a predominant role in Se mobility in the Chalk aquifer. Se and other redox sensitive elements (Fe, Mn, N and S) undergo multiple redox cycles, resulting in a Se-rich redox front that migrates downward over time within the water-level fluctuation zone of the porous Chalk. With the decreasing trend of water levels caused by global changes, a Se stock could be immobilized in the unsaturated zone, but nitrate content and redox conditions in the saturated zone will be major drivers for Se mobility.