Stable lead isotopes ratios in major french rivers and estuaries

Abstract

Lead isotopes ratios have been determined in suspended matter and water samples collected from major rivers and estuaries in France to assess their present level of Pb pollution. Most of the anthropogenic Pb found in the environment comes from Pb alkyls added to petrol. In France most petrol Pb comes from imported Pb ores which mainly have very characteristic Pb isotopes ratios (for example, a low 206Pb/ 207Pb ratio). Indeed, the 206Pb/ 207Pb ratio of aerosols sampled in France near a highway and a carpark was found to be very low (1.09-1.11). Despite not being so important with respect to atmospheric and water pollution, anthropogenic Pb produced by the iron, steel and copper production industries has also been considered and a slightly higher 206Pb/ 207Pb ratio (1.141) has been found from measurements made in the north of France near industrial plants. Conversely, higher 206Pb/ 207Pb ratios (1.197-1.206) were found in the pre-industrial sediments discharged by the rivers studied. These ratios are considered to be representative of local natural Pb during pre-industrial times. The measured Pb isotopes ratios in the suspended particulate matter compared with the two end-members show that the most polluted river is the Seine, followed by the Rhône and then the Garonne and the Loire. It is shown that anthropogenic Pb pollution is more easily detected when the solid discharge is low, such as in the Seine river. Conversely, a high particulate load can dilute the level of pollution by anthropogenic Pb and in this case Pb isotopes ratios are very similar to the natural level. As far as estuaries are concerned (the Seine estuary was not studied), variations of Pb isotopes ratios are generally observed and are attributed either to natural geochemical processes (Gironde estuary) or to man's influence (Loire and Rhône estuaries).