Inputs of total and labile trace metals from wastewater treatment plants effluents to the Seine River

The Seine river basin has long been impacted by metal inputs from the Paris area, but the water quality has been gradually improving for the last 20 years. Among all metal pollution sources (surface runoff, industries), urban wastewater discharge has been shown to significantly contribute, during low-flow periods, to metal fluxes of the River Seine. This paper assesses the current wastewater contribution to metal inputs in the Seine river basin, based on sampling of nine wastewater treatment plants (WWTPs). Seven metals were targeted (Al, Cd, Cr, Cu, Fe, Ni and Pb) during dry weather periods. Since total and dissolved concentrations alone are not relevant enough for an ecological risk assessment, labile metals (free + weekly complexed) were also measured by means of DGT (diffusive gradient in thin film technique). Results show that WWTPs greatly reduce total metal concentrations but reduce labile metal concentrations only slightly. Estimations made for direct total metal inputs in the River Seine via treated effluent discharge confirm the decrease observed for the 1994-1995 period. Labile metals released by WWTP, were also considered by comparing fluxes in the effluent discharge of two different WWTPs to those flowing in the receiving river. Fluxes discharged by the largest plant were similar to those measured in the river during low-flow periods whereas they were negligible for the smaller one. Nevertheless, labile metal concentrations in both discharges were similar and the wastewater discharge's contribution to labile fluxes in receiving waters seems to depend mostly on the relative significance of the discharge flow compared to the receiving water flow. (C) 2008 Elsevier Ltd. All rights reserved.