Modalités du transport des PCB dans la rivière Seine (France)

L'évolution de la pollution et les modalités du transport des P.C.B. en rivière ont été étudiées en Seine à Paris en relation avec les épisodes du cycle hydrologique durant un an.Les résultats analytiques montrent que la pollution occasionnée par les P.C.B. est supérieure à celle produite par les autres résidus de pesticides organochlorés, et ceci malgré la limitation réglementaire d'emploi des P.C.B. aux systèmes clos depuis 1975.Pour 88 % des échantillons analysés, les concentrations en P.C.B. dissous sont supérieures aux teneurs en P.C.B. adsorbés sur M.E.S. et atteignent jusqu'à 170 ng.l-1.Le transport se fait en solution pour 73 %, en moyenne, de la teneur totale, contrairement aux idées admises fondées sur la faible solubilité du polluant, mais qui ne prennent pas en compte le rapport en masse M.E.S./eau voisin de 25 x 10-6 en rivière.L'étude de la capacité d'adsorption des particules démontre qu'elle subit une évolution inverse de celle de la charge solide et du débit, ce qui explique la dominance des P.C.B. en phase aqueuse quelle que soit la teneur en M.E.S. Il en résulte que les valeurs élevées du coefficient de partage ne correspondent pas aux fortes teneurs en M.E.S. Les valeurs sont faibles pour la moitié d'entre elles et comprises entre 2 x 103 et 12 x 103 l.kg-1.The evolution of pollution by PCBs and their transport state in rivers was studied in the River Seine in Paris, in relation to the yearly hydrological cycle.Sampling took place just upstream of main waste waters (output) in the Paris area, and downstream from the junction of the Marne and the Seine, in order to establish the mean quality of water pumped upstream of these two rivers for drinking water production.Micropollutant concentrations were measured on 500 ml of raw water and 500 ml of water centrifuged at 6 000 G. Three extractions were carried out on these samples with a solvent mixture (85 % hexane, 15 % dichloromethane).In some cases extracts were purified on unactivated Florisil. After concentration down to 1 ml, extracts were measured through GC (falling needle injector; fused silica capillry column I.D. 0.25 mm, stationary phase SPB5; E.C.D. ni 63; vector gas He, make up Ar + 10 % CH4).Identification and quantification were carried out using an integrator and commercial Aroclor : 1242, 1254 and 1268 standards. In natural samples, the isomer composition is generally close to that of Aroclor 1254.Analytical data show that pollution linked to PCBs is higher than that due to other organochlorinated products, in spite of the official PCBs use limitation for closed systems since 1975.Except for a pollution peak of 990 ng.l-1, concentrations in raw water are irregular and vary from 25 to 250 ng.l-1.For 88 % of analyzed samples, dissolved PCB concentrations are higher than the concentrations of PCBs adsorbed onto suspended matter and reach 170 ng.1-1. The main transport (73 %) of the total flux is by solution, in contradiction with commonly accepted views based on the low solubility of PCBs.The study of adsorption capacity of solid particles demonstrates that its evolution is negatively correlated with the water flux and the solid load, which explains the prevalence of PCBs transport through solution, whatever the suspended matter concentration. Thereafter, it appears that high values of the partition coefficient are not linked to the concentration of suspended matter. For about half of them, values are low, between 2.102 and 12.101 l.kg-1