Chemical and isotopic fractionation of lead in the surface soils of Egypt

Chemical fractionation via sequential extraction (SEP) combined with isotopic analysis of Pb was used to investigate the origins and reactivity of Pb in 66 topsoil samples collected from 12 different locations in Egypt. The total soil Pb concentrations (TPb) covered a wide range (similar to 80-16,000 mg kg(-1)), but were only elevated in four industrial and urban locations within Cairo and Alexandria. In all the other locations values of TPb were generally low and were close to the average crustal Pb concentration of 14 mg kg(-1). The largest Pb fraction in all soils, with the exception of two industrial locations, was the 'residual' fraction (38-63% of TPb) followed by Pb bound to 'organic' and 'metal oxide' phases. The Pb isotopic signatures (Pb-206/Pb-207 vs (208) Pb/Pb-207) of all samples in all SEP fractions were highly variable, suggesting a heterogeneous mix of Pb contamination sources; however, they aligned closely to a binary mixing line between geogenic and petrol Pb sources. There were similar Pb isotopic patterns across all of the non-residual fractions with measureable data (F2 - F4) suggesting that the non-residual anthropogenic-Pb and geogenic-Pb have been assimilated into common pools within the soil. Binary and ternary source-apportionment models based on Pb isotopic ratios and abundances showed that the relative contribution of petrol-Pb and geogenic-Pb can be ascribed with reasonable certainty. However, the contribution of further sources can only be accounted for if the isotopic abundance of all end-members are known and are at the periphery of the soils dataset.