Contributions and potential impacts of seven priority substances (As, Cd, Cu, Cr, Ni, Pb, and Zn) to a major European Estuary (Gironde Estuary, France) from urban wastewater

Urban wastewater metal inputs into coastal systems are of increasing interest to both scientists and managers facing restrictive environmental protection policies, population increase and changing metal applications. However, their impact and contribution to metal loads in estuarine and coastal environments is widely unknown due to the lack of (i) monitoring in both artificial and natural aquatic systems and (ii) an understanding of control parameters, such as spatial and temporal variations in hydrological conditions. We investigated the daily concentrations, fluxes and dynamics of seven EU priority contaminants (potentially toxic metals Cd, Cu, Cr, Ni, Pb, Zn and the metalloid As) transported by the Garonne River (La Réole site; watershed area ~ 57,000 km2) to those released into the freshwater reaches of the Gironde Estuary (Garonne Branch) by two main wastewater treatment plants (WWTPs) of Bordeaux under low river discharge and contrasting rainfall situations. During short intense summer rainstorms, wastewater flow into the WWTPs increased by up to 150% and 60%, respectively, resulting in an increase of 70% (As) to 200% (Pb) for fluxes entering the treatment plants. Overall resulting WWTP particulate and dissolved effluent concentrations were up to 2 (Cr), 3 (Pb, Cu and Ni) and 5 (Cd and Zn) times higher than measured upstream in the Garonne River, respectively. During low-discharge, maximum outlet fluxes at the WWTPs were similar to respective watershed-derived fluxes in the Garonne River. During rain events, Pb in all fractions, total and dissolved Cu and total and particulate Zn fluxes entering the fluvial estuary were close to the respective minimum flux values at the La Réole site. Furthermore, during rain episodes, particulate Cu and dissolved Zn fluxes from the WWTPs to the fluvial estuary were greater than those transported by the Garonne River at the La Réole site. Mixing of treated wastewater with highly turbid estuarine freshwater most likely results in a re-equilibration between the dissolved and particulate phases, although biogeochemical processes in downstream estuarine turbidity and salinity gradients may partly reverse contaminant adsorption on estuarine particles. Accordingly, urban wastewater contaminant release clearly impacts estuarine water quality before being expulsed to the Bay of Biscay.