Metal mobility in the groundwater of a paddy field in Camargue (South eastern France)

Waterlogged soils are submitted to temporal variations of aeration and aerobic–anaerobic conditions which influence redox conditions and thus element mobility. Cultivated paddy soils, that are an accurate redox-dynamic environment, can be found in Camargue (Rhône deltaic plain, SE France). In order to monitor fast physical and chemical variations in the waterlogged soil, a multiparametric probe was chosen to acquire continuous and in situ data. It can be showed that (i) Eh variations happen at quasi-constant pH during Mediterranean storms in non-irrigated seasons, and induce sharp variations of Fe(II) and Mn concentrations, and (ii) the soil is saturated with water under anaerobic conditions during long periods, where sharp pH fluctuations controlled by pCO2 evolution affect redox processes. During irrigation periods, Fe and Mn precipitate as oxides or co-precipitate with Al, Mg and Si at depth. These neoformed phases however can be dissolved when irrigation stops. The understanding of the reactions of the system, under varying conditions, is important in order to better assess its durability and anticipate the consequences of soil use or agricultural practices changes.