Effects of Soil Morphogenesis On the Bioaccessibility of Pb in An Urban Soil Chronosequence, Detroit, Michigan.

An urban soil chronosequence in downtown Detroit, MI spanning 95 years of weathering was studied to determine the effects of time on soil genesis and Pb geochemistry. The results show that soil morphogenesis impacts Pb bioaccessibility over time because the geochemical forms of Pb change in response to progressive melanization and weathering of anthropogenic materials. The primary artifactual weathering processes appear to be: 1) dissolution of mortar and Pb-based paint (cerrusite), followed by sorption of Pb by soot and soil organic matter, in ^A-horizons, and 2) leaching of organically chelated Pb, and coprecipitation with pedogenic calcite, in ^C-horizons. The older soils tend to be more contaminated, and contain more bioaccessible-Pb, than younger soils, but overall there is a poor correlation between bioaccessible-Pb levels and soil age. There is a strong correlation between total-Pb and bioaccessible-Pb, but total-Pb levels are controlled simply by random effects of the demolition process. Soils at all of the demolition sites studied are contaminated, but total-Pb concentrations are below the EPA action level of 400 mg kg-1. Carbonaceous and ferruginous anthropogenic microparticles are potential sinks for Pb. Preliminary data suggest that earthworms (Lumbricus terrestris) are reducing the amount of bioavailable-Pb in the rhizosphere through bio-uptake and casting activity, and thus could be used as a natural remediation tool for reducing the hazard associated with resuspension of contaminated urban soil.