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Access Type

WSU Access

Date of Award

January 2012

Degree Type

Thesis

Degree Name

M.S.

Department

Geology

First Advisor

Jeff Howard

Abstract

An urban soil chronosequence in downtown Detroit, MI spanning 95 years of weathering was studied to determine the effects of time on soil genesis, artifact weathering, and Pb geochemistry. The results show that ^A- and ^Ck-horizons can develop within a few decades, whereas B-horizon development occurs on a scale of centuries. Melanization of ^A-horizons was enhanced by airborne deposition of soot and flyash, and ^Ck horizons have formed by weathering of calcareous artifacts. Hence, urban soil genesis has been significantly affected by anthropogenic activities. Plaster and mortar artifacts may weather away completely within a few decades. Corrosion of iron-artifacts begins shortly after burial, and is enhanced by deicing salts and solution of gypsic artifacts. However, iron artifacts have survived 95 years of weathering. Weathering of iron artifacts has resulted in pedocementation and mobilization of Fe. This is attributed to fluctuating redox conditions (ferrolysis) produced by seasonal wet-dry and freeze-thaw cycles, and waterlogging caused by reduced permeability associated with densic horizons. All of the demolition site soils are contaminated with Pb, but total Pb concentrations are below the EPA trigger level of 400 mg kg-1. Soot and other carbonaceous microparticles appear to be controlling the bioavailability of Pb in ^A-horizons. Organically bound-Pb is being leached from ^A-horizons, and accumulating in ^Ck-horizons where it is sorbed by pedogenic calcite, corroded iron-artifacts and ferruginous microparticles. Pedogenic calcite and ferrihydrite, generated by weathering of calcareous and ferruginous building materials, are potential immobilizing agents for Pb and thus may have ameliorating effects on urban soils. Sand-sized microparticles related to coal combustion are locally abundant in the soils studied. They represent a previously unrecognized sink for heavy metals, but more study is needed because most of this material is likely found in finer size fractions which were not studied. More study is also needed of earthworms in urban soils because levels of bioaccessible Pb were found to be lower in the rhizosphere and earthworm casts than in the bulk ^A-horizons. Hence, earthworms may be significantly reducing the amount of bioavailable-Pb in the rhizosphere through bio-uptake and their extensive casting activity. If so, earthworms could be used as a natural remediation tool for reducing the hazard associated with resuspension of contaminated urban soil. ^A-horizon