Origin of suspended matter and sediment inferred from the residual metal fraction: Application to the Marennes Oleron Bay, France
Détermination de l'origine des matières en suspension et des sédiments à partir de la concentration métallique résiduelle: application à la Baie de Marennes Oléron, France
Résumé
Total and HCl-available (extracted by HCl 1M) trace metal (V, Cu, As, Cd, Pb and Th) concentrations were measured in suspended particulate matter (SPM) and surface sediments from the Garonne and the Charente Rivers (Southwest France), from their respective estuaries and from the adjacent coastal zone including the Marennes Oleron Bay. The objectives were to explore the potential of trace element signatures in the residual (non-reactive) fraction to trace the origin of particles, i.e. major contaminant carriers, in complex coastal systems. The observation period covered 12 months and a wide range of hydrological conditions. Selective extractions (HCl 1M) showed that Pb, Cd and Cu in SPM and surface sediments from the entire system, including freshwater and marine environments, were highly reactive with potentially available fractions representing 64 ± 13 %, 60 ± 18 % and 43 ± 13 %, of the respective total metal concentrations. In contrast, V, As and Th showed low reactivity, with potentially available fractions lower than 19 ± 7%, 11 ± 3 %, and 1.6 ± 0.9 %, respectively. Two combinations of Th-normalised (i.e. grain size corrected) metal concentrations in the residual fraction (Vres + Asres)/Thres and (Cdres + Cures +Pbres)/Thres represented two distinct signatures corresponding to the Charente and Garonne River endmembers. The respective Mres signals showed that both SPM and surface sediments sampled in the Marennes Oleron Bay mainly (>54%) originated from the Gironde Estuary watershed. Furthermore, SPM from the tidal ranges of the Charente Estuary contained close to 100% of particles from the Gironde watershed. These results, together with SPM flux estimates for the Charente River, suggest that particle transport from the Gironde Estuary to the Marennes Oleron Bay accounts for 81 000 t a-1 to 435 000 t a-1, representing 5% to 29% of the total SPM expulsed by the Gironde Estuary. Application of this novel approach to determine SPM and sediment origin to other aquatic environments may require adapting the choice and association of residual metal concentrations.