Rare Earth Element fluxes over 15 years into a major European Estuary (Garonne-Gironde, SW France): Hospital effluents as a source of increasing gadolinium anomalies.

New and rapidly developing technologies imply the emission of emerging potentially toxic contaminants such as Rare Earth Elements (REEs). Yet, the lithology-derived quantities and anthropogenic contributions, especially from urban areas, to annual REE fluxes into fluvial-estuarine systems remain widely unknown. The Garonne River drains water from ~20% of the French land surface hosting about 5,200,000 inhabitants and two large cities. Based on long-term monitoring (2003-2017) of water discharges and dissolved REEs concentrations at the outlet of the Garonne Watershed upstream from Bordeaux, this study aims at assessing REE anomalies and evaluating temporal evolution of annual dissolved REE fluxes into the Gironde Estuary. Additionally, potential urban sources (e.g. domestic, medical) in the urban area of Bordeaux (1,190,000 inhab.) were analyzed to evaluate respective signatures and contributions. Gadolinium (Gd) showed clear anomalies in all samples, with annual average anthropogenic concentrations ranging from 1.8 to 7.2 ng·L (0.011 to 0.046 nmol·L) in the Garonne River. If variations in annual Gd fluxes depend on hydrology, anthropogenic Gd fluxes have shown an overall increasing trend from 32 kg·year (204 mol·year) in 2003 to 75 kg·year (475 mol·year) in 2017. Sewer waters from the third largest hospital complex of France, the hospital group Pellegrin, contributed 25% to the incoming daily Gd flux into Bordeaux major Waste Water Treatment Plant (WWTP), owed to Gd use as contrast agent for Magnetic Resonance Imaging (MRI). Due to weak removal efficiency in the WWTP, the Bordeaux Metropole significantly contributes (>27 kg·year; 172 mol·year) to Gd fluxes in the Gironde Estuary. The temporal evolution of anthropogenic Gd fluxes in the Garonne River may be related with the growing regional population and the increasing number of MRI instruments, highlighting the importance of new high-tech applications in urban areas on contaminant fluxes and their potential harmful effects in fluvial-estuarine systems in the future.