Household-Scale Spatial and Depth Distributions of Soil Lead in Minneapolis-St. Paul, Minnesota.

Far from being static repositories of the urban lead inventory, soils represent a dynamic pool of lead that can be re-mobilized through anthropogenic activities, particularly in light of increasing interest for utilizing urban soils for food production. Thus, management recommendations and land-use siting strategies for urban soils at the scale of individual households can affect ambient lead exposure and have a continuing impact on human health. In this work, we investigate the distribution of soil lead at the scale of individual households throughout the Minneapolis-St. Paul (Minnesota) Metropolitan Area through linked transect and core sampling. Through the in-situ use of portable X-Ray Fluoresence (pXRF), we reveal fine-scale patterns in both the spatial and depth distribution of soil lead. Our data show that over half of investigated cores now have their maximum lead concentrations at depths deeper than 10cm, to 20 or 40 cm in some cases, suggesting ongoing physical or chemical processes which are redistributing lead in urban soils. These processes have important implications: if anthropogenic lead inputs have been distributed deeper into the soil over time through biotic and abiotic agents, concentrations of lead at the surface may have been diluted, so that decades-old predictions of the surficial concentrations of soil lead may overestimate the current loading and risk at the surface. Conversely, understanding the depth distribution of lead is also important for making better recommendations for urban agricultural uses of soil, where contaminant screening in the top portion of the soil may not accurately reflect the total lead loadings that will be made available once the soil is mixed more deeply.