Mass balance of organic contaminants at a large basin scale: case of the Seine River basin (France)

International audienceThe Seine River basin drains an area of 78 650 km² which accomodates 25% of the French population and 30% of its industry. Thus, it is highly impacted by anthropic activities. As such, the Seine River basin has been the place of numerous studies, mostly achieved within the PIREN-Seine research programme, aiming to quantify several groups of organic micropollutants in different environmental compartments and urban matrices. Among the monitored organic persistent pollutants, Polycyclic Aromatic Hydrocarbons (PAHs), Polybrominated Diphenyl Ethers (PBDEs), Polychlorinated Biphenyls (PCBs), Phthalates (PAEs), Alkylphenols (APs) and perfluoroalkylated substances (PFAs) have been considered. These compounds originate from very contrasting sources in the environment. PAHs are mainly emitted during combustion processes such as household heating and road traffic. PBDEs were mainly used as flamme retardants in plastic and textile industries. Although PCBs are forbidden in France since 1987, they have been extensively used as electric insulators and they can still be found in the environment. Phthalates are still used nowadays as plasticizer. Finally, PFAs can be found in many daily consumers goods as waterproof textiles or non-stick coatings. Along with their diversified sources, differences in their physico-checimal properties strongly impact their environmental fate.This study aims to establish mass balances at the Seine River basin scale for these 6 groups of organic compounds. Mass balances constitute efficient tools for understanding transfer pathways of pollutants in the environment. Databases collected by several research teams working on the Seine River basin were used for the calculations. Along with stocks in soils, most of major environmental fluxes were quantified including atmospheric fallout, erosion from soils, river exports and wastewater treatment plant discharges. A focus on urban mass balances was also carried out considering wastewater, runoff, and sewage discharges at the Paris City scale.First results show very contrasting dynamics depending on organic contaminant groups. PAHs were found to be the most abundant compounds in the soils of the Seine basin. The other contaminant stocks were smaller (PAHs > PAEs > Aps > PCBs > PBDEs > PFAs), likely due to differences in emission sources. In atmospheric fallout, Aps were found to be the most abundant compounds. AP, PAE and PAH atmospheric fallouts were homogeneous across the basin, but highest PBDE and PCB fluxes were measured on Paris City and on agricultural areas respectively. These results suggest that PBDEs present very specific sources and that agricultural soils may act as secondary sources for PCBs. The largest fluxes in the Seine River downstream Paris City were measured for Aps and PAEs and reached about 10 t.yr-1. The lowest fluxes were observed for PBDEs and PCBs (ca. 50 kg.yr-1). Soil erosion seems to be a predominant contributor to PAH and PCB fluxes in the Seine River. For PAEs, wastewater treatment plant discharges appears as a significant contributor. Concerning PBDEs and APs, erosion and urban discharges were approximatively proportionnal to fluxes in the river. However, so far, some sources remain unidentified or poorly quantified and futher treatment of results is required