Lead in Egyptian soils: Origin, reactivity and bioavailability measured by stable isotope dilution

The current availability of Pb in Egyptian soils and associated plants were studied in 15 locations (n=159) that had been historically subjected to industrial and automobile Pb emissions. Isotopic dilution with enriched ²⁰⁴Pb was used to estimate the soil Pb labile pool (Pb_E); results showed that %Pb_E values were mostly <25% which is likely due to the alkaline nature of the soils. Nonetheless, lability of Pb was significantly higher in urban and industrial locations indicating greater reactivity of anthropogenic Pb in comparison to geogenic-Pb. A plot of ²⁰⁶Pb/²⁰⁷Pb vs ²⁰⁸Pb/²⁰⁷Pb showed that all soils were aligned close to a virtual binary line between two apparent end member signatures (petrol and geogenic-Pb) suggesting that they are the major sources of Pb in the Egyptian environment. Soils with greater Pb concentrations (urban and industrial locations) displayed a significantly greater ratio of labile petrol-Pb to labile geogenic-Pb in comparison to less-contaminated soils. However, this difference was marginal (±5%) suggesting that historically emitted petrol-Pb has substantially mixed with geogenic-Pb into a common pool as a result of prolonged contact with soil. The proportion of petrol-Pb in fruits and leaf vegetables was significantly (P<0.005) greater than that of the associated soils suggesting preferential uptake of the more labile petrol-Pb as opposed to the relatively immobile geogenic-Pb. However, it is also possible that the major source of Pb intake by Egyptian consumers is extraneous Pb dust enriched with petrol Pb rather than systematic Pb via roots uptake.