Influence of ponds on hazardous metal distribution in sediments at a catchment scale (agricultural critical zone, S-W France)

International audienceEight potentially toxic trace elements were investigated together with the physicochemical characteristics of sediments collected upstream, downstream, and from various ponds from three adjacent carbonated agricultural catchments in southwestern France. Geochemical indices with local references and multivariate data analysis revealed a significant enrichment of Cd, followed by Ni and Cu in sediments, originating from agricultural practices, especially in the upper catchments sensitive to erosion processes. Five other PTEs (As, Pb, Co, Cr, and Zn) mostly originated from the weathering process of the molasse. Clay minerals and/or iron oxides were the main controlling factors for most PTEs. In ponds, Cd was regulated by Ca co-precipitation favoured by the long hydraulic retention time and increased pH, whereas it was mostly controlled by silicates in stream sediments. Copper, Pb, and Cd were the highly extractable metals, suggesting the risk of a hazardous environment. Multivariate analysis revealed that several environmental factors affected the spatial variation of PTE concentrations in sediments as well as the trapping role of ponds (texture, slope, soil erosion, pond size and position, discharge, hydraulic retention time) in this area of intensive agricultural practices. Our conclusions are useful for pond management in a context of increasing water demand and climate warming