Microscale controls on lead speciation in soils: a framework for sustainable remediation

The potential of a soil to immobilize heavy metal ions is dependent on the presence of adsorption sites, and the stability of metal species over the range of geochemical conditions present in the soil over time. Lead (Pb) is a cumulative toxin that is enriched in much of the urban pedosphere due to historical use of Pb-based paint and Pb-amended gasoline. Because in-situ remediation of Pb is possible if the bioavailable fraction can be rendered inert, understanding Pb-sorbent interactions is necessary to accurately and efficiently alter Pb speciation in soils. The objectives of this study are to 1) determine efficient ways to predict Pb behavior at the field scale, and 2) characterize microscale controls on Pb speciation. A combination of geospatial and analytical tools has been used across a variety of spatial scales to provide the first multiscale analysis of microenvironment impact on Pb speciation in soils. This research investigated Pb distribution at the field scale (in Burlington, VT), and mobility at the microscale. The field-scale study has shown that the relationship between total Pb and bioaccessible Pb is not linear, in stark contrast to the existing conceptual model of this relationship. It was determined that the disproportional influence of fine-fraction Pb in low total-Pb soils results in elevated bioavailability. Microscale investigations determined that there is a positive correlation between the density of reactive microenvironments and the release of Pb from contaminated soil, and that altered distributions of microenvironments significantly alters the rate of Pb release. This research identifies specific mechanisms controlling Pb behavior in soils at both the field and the microscale, which can be used to inform improvements to implementation of remediation