Historical contamination of three french peri-alpines lakes sediments by PCBs : influence of local sources and watershed characteristics
Résumé
Polychlorinated biphenyls (PCBs) are considered as persistent organic pollutants, due to their bioaccumulation and toxicity. They were introduced in France during the 1930s and production has been stopped in the 1970s. They are released in the environment, transported in the atmosphere and deposited and stored in soils, sediments, or glacier's ice. These compounds exhibit a high tendency to adsorb the particles and they accumulate in the lake sediments. Dated sediment profiles have been used to infer deposition of PCBs to lakes. The goal of this study is to reconstruct the historical inputs of PCBs into three French peri-alpine lakes and to distinguish between those impacted mainly by local and regional sources from those affected mainly by atmospheric deposition. Dated sediment cores from lakes Bourget, Annecy and Geneva, covering the last 130 years were analyzed to get an overview of the historical trend of PCBs deposition. Analyses of sediment cores revealed that concentrations of PCBs started to increase in the 1940s and decrease after the 1980s, due to their interdiction. The concentration-time profile is in good agreement with the available information on global production data. PCB concentrations in lake Bourget are very high and reached a maximum of 1800 =g/kg dw in 1973. Today, the annual flux of PCBs in this lake is about 0.008 =g/cm²/year and has reached 0.451 =g/cm²/year during the 1973-76 periods. For lake Geneva, concentrations of PCB were lower and have reached 50 =g/kg dw during the 1970-76 periods while annual flux was 100 times lower. The identified PCB mainly consisted of highly chlorinated compounds for the lake Bourget, while they consisted of low chlorinated biphenyl for lake Geneva. PCB concentrations in lake Annecy are very low and comparable to the one found in high altitude mountain lakes contaminated by atmospheric deposition (LALLEE, 2009). These results indicate different origins of the lakes contamination. High contamination of lake Bourget can be attributed to the local industries of PCBs production in the lake watershed (JUNG, 2009). Despite regulations, the PCB concentrations are still high in lake Bourget and could be attributed to the phenomenon of volatilization from nearby polluted soils of bio-resistant highly chlorinated congeners. For lake Geneva, its bigger size and the fact that PCB have never been manufactured in the nearby Switzerland might explain its lower contamination as compared to lake Bourget, while the profiles of PCB congeners support an origin from soil leaching and wastewater treatment plants. In contrast, the very low contamination rate of lake Annecy attests of the absence of any direct local PCB source and can be explained by a much less industrialized watershed such as the limitation of direct sewage water inputs to the lake as soon as the early 1960s.