Estimating distributions of endogenous and exogenous Pb in soils by using Pb isotopic ratios

Metal contamination of soils by diffuse atmospheric deposition is a worldwi de phenomenon. The assessment of incorporation of exogenous metal contamina nts in soils is of major environmental importance, Once entering in the soi l's biogeochemical cycling, specific pedogenetic soil processes govern meta l distribution patterns with depth. In this paper, we attempt to estimate t he distribution of endogenous and exogenous Pb in two soils with contrastin g pedogenesis, both representative of undisturbed ecosystems. Pb isotope an alyses were performed using high-precision thermal ionization mass spectrom etry. Endogenous and exogenous Pb concentrations and exogenous Pb-206/Pb-20 7 ratios of the samples were calculated using bulk Pb and Sc concentrations and bulk Pb-206/Pb-207 ratios. Endogenous Pb distribution was in agreement with dominant soil characteristics, almost constant in the young Andosol a nd with a clear minimum and maximum in the eluvial and illuvial horizons of the Podzol. The distribution of exogenous Pb was closely related to that o f organic C in both soils, Exogenous Pb was evidenced in significant amount s at depth. Using moderate dispersive particle-size fractionation allowed u s to evidence the presence of exogenous Pb in functional soil compartments and to highlight preferential distributions of Pb, according to pedology.