Pb Sources and Chemistry in the Urban Environment.

Sources of Pb include: 1) native bedrock (15-60 ppm), 2) agricultural amendments (sewage sludge), 3) atmospheric deposition (auto-Pb, fly ash, street dusts), 4) urban wastes (building materials, Pb-paint), 5) industrial wastes (iron smelting, coal-related, etc.), and 6) mine-related wastes (coal processing refuse). Geochemical forms observed in urban soils include operationally-defined phases targeted by sequential extraction analysis: 1) labile (water soluble, exchangeable), 2) “carbonate-occluded” (includes calcite, cerrusite and anglesite), 3) fixed organic (non-exchangeable organic forms), 4) “oxide-occluded” (crystalline and amorphous Fe and Mn oxides and apatite), and 5) residual (lattice-bound Pb in silicate minerals and aluminosilicate glass). Labile Pb is usually < 5% total, but may increase as total Pb increases. Anomalously high levels of labile Pb result from water-soluble organometallic compounds. The residual fraction often comprises 20-75% of total, and increases with increasing contamination. Spiking experiments suggest that Pb associated with Mn and Fe oxides, and in the fixed organic fraction, is not bioavailable. Calcareous soils may have 12-63% carbonate-occluded Pb, about 50% of which is bioavailable. Thus, the most labile Pb is: 1) complexed with water-soluble humic substances, 2) sorbed on ion exchange sites, and 3) sorbed or precipitated on surface of calcite. The remaining geochemical forms of Pb are generally not bioavailable. Typically, 60-95% of measured total is in a highly insoluble form, but labile Pb generally increases in contaminated urban soils with increasing organic matter content. Microartifacts also contribute to total Pb levels in urban soils, but waste artifacts comprised of immobilizing agents can have an ameliorating effect on Pb-contaminated soils.