Assessment of inorganic lead species and total organo-alkyllead in some Egyptian agricultural soils

This study was carried out to assess the amounts of (i) total Pb in soil, (ii) inorganic Pb species: exchangeable (EXCH), carbonate (CARB), easily reducible (EASR), moderately reducible (MODR), organic matter and sulfides (ORGS), and residual (RESD) bound Pb, and (iii) total organo-lead as alkyllead, in alluvial and lacustrine soils of the Nile delta, Egypt. Wide ranges of soil Pb were found in the alluvial (18.2-1850 microg g(-1)) and the lacustrine (39-1985 microg g(-1)) soils. The topsoil was highly enriched with Pb relative to the subsurface soils, especially in highly contaminated soils. There was no significant relationship between soil type and Pb content. Amounts of soil Pb greater than the background level (14 microg g(-1)) are due to Pb deposited from various anthropogenic activities. The partitioning of soil Pb into different species varied according to the intensity of contamination. It followed the sequence: RESD > ORGS > CARB > MODR > EASR in the slightly contaminated alluvial as well as lacustrine soils. In the highly contaminated soils, it followed the sequence: ORGS > MODR > CARB > EASR > RESD in the alluvial soils, and the sequence: ORGS > CARB > MODR > EASR > RESD in the lacustrine soils. There is high binding capacity of organic matter and sulfides to Pb, especially in the highly contaminated soils. The concentrations of total alkyllead in soils varied markedly and were related to both intensity of contamination and depth in the soil. The subsurface soil (15-30 cm) was highly enriched by alkyllead (means 224 and 353 ng g(-1) in the alluvial and lacustrine soils, respectively) relative to the surface and deeper soils. The proportion of total alkyllead as a percentage of total Pb in the soil was generally very low. It did not exceed 1.6% in the slightly contaminated soils, and 0.6% in the highly contaminated ones.