Legacies of Lead Paint Contamination in the Mineral Soils Adjacent to Historic Buildings

Abstract

For hundreds of years, compounds containing the element lead (Pb) have been added to paint in order to improve the texture and weather resistance. Across the United States, millions of homes and buildings were painted with lead-based paints up through the 1970s, when lead paint was phased out following medical research confirming the link between lead and a variety of neurological and developmental disorders. Over 170 million Americans are estimated to have been exposed to lead levels above safe concentrations, many of whom were likely exposed to soil lead while playing in yards as children. Although the phase-out of lead in paint began over 50 years ago, the legacies of contamination from lead paint are still present in the soil in a meaningful way. This is largely due to the immobile nature of lead in the soil, tightly binding to clays and other particles, preventing it from being leached out of the soil.

Previous soil lead studies have attempted to characterize spatial patterns of contamination at various scales, but this study is unique in the highly fine-scale sampling design around each structure. Up to 190 samples were taken around each structure, at a point density of 1 sample every 4-12 square meters, prioritizing areas closer to the base of the structure. Nine study sites were chosen in the vicinity of Durham, North Carolina, including 2 buildings that continue to be well-maintained, 5 dilapidated structures, and 2 footprints of buildings burned to the ground. For our study, we surveyed the top inch of the mineral soil using an Olympus Vanta portable X-ray Fluorescence Machine to get lead concentration in parts per million.

At all nine of our study sites, we found evidence of elevated lead levels in the mineral soil that can reasonably be attributed to legacies of paint contamination. Sites varied in the intensity of lead contamination, ranging from maximum values below 300ppm to multiple samples above 5000ppm, and each had its own unique footprint of soil lead. Interpolations of lead concentration were created in ArcGIS from the point data. Despite the variation, we were able to draw a variety of conclusions about the state of soil lead around historic structures: • Concentrations are typically highest adjacent to the base of the structure, and often decline rapidly with distance. • Concentrations often decline back down to the geologic background within 4-12 meters of the structure, related to the maximum concentration. • The corners of buildings are often hotspots for lead contamination, likely due to weathering patterns of paint. • Topography and erosion of soil can affect concentrations and directionality of elevated lead plumes. • We did not find any direct pattern between lead levels and a building’s height, age, location, or level of maintenance. • Destroyed/burned structures can show very high hotspots even within the footprint of the former structure. • Mulch and other ground covers may either protect the soil from contamination or insulate the soil lead from loss.

Statistical and geospatial analysis was used to help characterize the spatial patterns of the contamination at each site. For each structure, data of lead concentration versus distance from the building was used to generate a logarithmic regression that can be used to predict concentration at any given distance. For two structures, interpolations were georeferenced to estimate the area of spatial contamination at different thresholds. Other analyses were done on a site-specific basis, such as comparing concentrations at the different cardinal directions from one structure.

All of our results were communicated and interpreted to the landowners to help inform their knowledge of their properties. This is especially important considering most of our sites are open to the public, and multiple have outdoor programs for children. For each site, we estimated a total health risk, based on the levels of lead contamination and the potential for human exposure. Considering the levels still present in soils even after 50+ years, more research is needed into soil remediation methods, as the high values we found in soils demonstrate that lead still poses a considerable risk to humans.