A history of lead pollution in nine takes in the Franche-Comté Region, eastern France, through the analysis of lakebed sediments

Résumé En

One of the main topics in any discussion of the state of the French Jura lakes is the latter's environmental quality. The latest studies carried out on Lakes Clairvaux, Grand Maclu and Saint Point have revealed significant quantities of trace metals. For many managers and researchers, the metals' origins remain unexplained and have frequently been a subject of dispute. Fishing and ecological-protection societies have blamed water treatment plants, agricultural muck-spreading and nearby factories for most of the contamination. The present study has made a geochemical analysis of the beds of nine lakes in the region based on sediment samples of an average thickness of 10 to 20 cm. The sampling and calculation of lead concentrations and isotopic ratios in lakes in Franche-Comté revealed widespread pollution dating from the mid-nineteenth century. The lakes are in a region highly exposed to prevailing north-westerlies blowing in from the ocean. Lead concentrations range from 10 ppm in the least polluted lakes to more than 150 ppm in the higher-altitude lakes. The dating of samples from the lake at Saint-Point village in Doubs department show that lead accumulation began in the 1850s, a period of intense industrial activity during Europe's, America's and France's Industrial Revolutions. The estimated sedimentation rate is around 0.43 mm p.a. Isotopic lead ratios, in particular of 206Pb/207 Pb, range from 1.17 to 1.19 at the bottom of the Lake Bonlieu core sample and from 1.13 to 1.16 at the top. This indicates two sources of contamination, first from the lead contained in coal up to the 1920s, followed by the lead in gasoline from the 1920s onwards. Contamination by the latter peaked in the 1970s, since when it has been declining as a result of the phasing out of leaded gasoline. However two of the lakes, Clairvaux and L'Abbaye, behave in slightly different ways from the others, as their sedimentation rates are significantly higher. These two lakes are accumulating sediment at around 0.6-0.8 mm p.a. The comparison of lead concentrations in the lakes analysed with those in Lakes Anterne (Haute-Savoie) and Bourget (Savoie), as well as those recorded in an ice core sample taken from Mont Blanc, confirm the anthropogenic origin and chronology of the inputs. Isotopic ratios are also comparable, in particular for the Anterne and Mont Blanc sites. The extent of contamination by lead and perfectly correlated volatile elements (Cd, As, Sb, Zn) is thus demonstrated.