Using oral bioaccessibility measurements to refine risk assessment of potentially toxic elements in topsoils across an urban area

Abstract

Elevated concentrations of As, Cr, Cu, Ni, Pb, V and Zn in topsoils in Belfast, Northern Ireland have been found to exceed assessment criteria in the city and therefore may pose a risk to human health. Most generic assessment criteria (GAC) for potentially toxic elements (PTEs) in soils assume PTEs are 100% bioavailable to humans. Here we use in-vitro oral bioaccessibility testing using the Unified BARGE method (UBM) to measure what proportion of soil contamination dissolves in the digestive tract and therefore is available for absorption by the body. This study considers how PTE bioaccessibility in soils varies spatially across urban areas and refines human health risk assessment for these PTEs using site specific oral bioaccessibility results to present the first regional assessment of risk that incorporates bioaccessibility testing. A total of 103 urban soil samples were selected for UBM testing. Results showed low bioaccessible fraction (BAF) for the PTEs from geogenic sources: Cr (0.45-5.9%), Ni (1.1-46.3%) and V (2.2-23.9%). Higher BAF values were registered for PTEs from anthropogenic sources: As (8.0-86.9%), Cu (3.4-67.8%), Pb (9.1-106.2%) and Zn (2.4-77.5%). Graphs of bioaccessibility adjusted assessment criteria (BAAC) were derived for each urban land use type and PTE. These provide a visual representation of the significance of oral bioaccessibility when deriving BAAC and how this is affected by 1) dominant exposure pathways for each land use and 2) relative harm posed from exposure to PTEs via each pathway, allowing oral bioaccessibility research to be targeted to contaminants and pathways that most significantly impact risk assessment. Pb was the most widespread contaminant with 16.5% of sites exceeding the Pb GAC. Applying BAAC did not significantly change risk evaluation for these samples as many had Pb BAF>50%. In contrast, all samples that exceeded the As GAC were found to no longer exceed a minimal level of risk when oral bioaccessibility was considered. Oral bioaccessibility testing resulted in a 45% reduction in the number of sites identified as posing a potential risk to human health.

Keywords: Bioaccessibility; Human Health; Metals; Soil Contamination.