Trajectories of legacy and emerging metal contaminants in French river sediments
Résumé
Trace metals and metalloids (TMMs) are substancesdisseminated in the critical zone (CZ) as a result of i. mechanicalerosion processes or chemical alteration of bedrocks anduncontaminated soils, and ii. releases from multiple sourcesrelated to anthropogenic activities during several millennia. Inrivers, legacy and emerging TMMs, including Rare EarthElements (REEs), are predominantly or partially adsorbed onsuspended particulate matter. Therefore, riverine transport ofnatural and anthropogenic TMMs depends on hydraulicconditions, and storage occurs in depositional zones (e.g.reservoirs, floodplains, ponds). As rivers are the receptacle formany substances released into the CZ, analysing temporal trendsof TMMs in dated sediment cores can reflect the trajectory ofcontaminant pressure in watersheds, as sediments are consideredas testimonies of past and current anthropogenic activities, andmay provide information about the potential future impacts ofemerging pollutants.This study aims at establishing regional geochemicalbackgrounds (RGB) and the trajectories of legacy and emergingTMMs in dated sediment cores at the outlets of the main Frenchriver watersheds (Rhône, Loire, Eure-Seine, Lot-Garonne).These investigations are supported by temporal TMM and REEtrends in watersheds characterized by contrasting geologicalfeatures. Preliminary results showed that mean REEconcentrations are approximatively twice higher in sedimentdeposits collected in the Loire River than in the Seine and RhoneRivers for which mean REE contents are relatively similar.Moreover, European Shale (EUS)-normalized concentrations ofREEs highlighted positive anomalies of light- and medium-REEsas Cerium (Ce), Samarium (Sm), and Europium (Eu). Theseanomalies determined in the Rhône and Loire Rivers, and in theEure-Seine continuum have not been systematically measured inthe most recent deposits as expected. As REE anomalies are notexclusively due to anthropogenic signals, but may also derivefrom lithology (calcareous versus crystalline rocks) andbiogeochemical processes, it appears essential to discriminate thecontributions of anthropogenic signals and RGB to totalconcentrations. Based on these results, we discuss the potentialof increasingly used Technology-Critical Elements (TCEs),recorded in sediment archives, as markers of the beginning of the4th industrial revolution.