Lead-isotopes as tracers of pollutants in soils

In order to gain insight into Pb pollution of soils, a Pb isotope study was performed on soil profiles from selected sites in Switzerland. Samples were analysed from the Swiss National Park, where high Pb contents have been observed despite the absence of obvious pollution sources. Other samples were taken in populated areas N of Zurich where soil pollution was expected. Major goals of this study were (1) to distinguish anthropogenic from natural pollution by Pb isotopic means, (2) to determine the magnitude of pollution penetration, (3) to assess the extent of pollution, and (4) to identify the anthropogenic sources of pollution. For major pollutants, the Pb isotopic compositions have been determined on samples collected between 1993 and 1995. Fly ash from waste incinerators showed a constant isotopic composition (206Pb/204Pb=17.92–17.96), independent of the plant and sampling year. In contrast, samples representative for petrol Pb displayed a larger spread (17.13–17.52) but generally had the least radiogenic composition among the pollutants. Sewage sludge showed an intermediate composition (17.74–17.88). Aerosols as omnipresent pollutants had compositions (17.49–17.77) between car exhausts and fly ash from waste incineration reflecting their mixed nature. For air fall deposition, a mean Pb isotopic composition (206Pb/204Pb=17.99) has been calculated for the 20th century taking into account the time-dependent evolution of aerosols. Due to the Pb isotopic contrast between unpolluted soil (206Pb/204Pb>18.8) and the anthropogenic contaminants, pollution could be detected in all but one soil profile. Penetration of pollution reached to depths of between 20 and 30 cm. Calculated anthropogenic net Pb inputs amounted to approximately 2.2 g m−2 in weakly polluted soils (National Park) and reached values of 15.7–17.5 g m−2 in soils closely exposed to pollutants. In soils with low anthropogenic Pb inputs, pollution could be explained by air fall deposition. In stronger polluted soil profiles, the Pb isotopic composition of the pollutant did not allow us to clearly identify the pollution sources because of anthropogenic inputs from several sources.