Evaluating trace elements in urban forest soils across three contrasting New England USA towns and cities by pXRF and mass spectrometry

Abstract

Urban forest soils perform important ecosystem services, such as the sequestration of potentially toxic elements, but some can serve as a source for human exposure if ingested or eroded by wind. Assessment of trace elements in urban soils through traditional methods is costly but portable X-ray fluorescence (pXRF) can allow for rapid screening for hazardous concentrations. Our objectives were (1) evaluate trace elements (As, Cd, Cr, Cu, Pb, and Zn) at 460 locations in and around urban forests across three town and cities in New England and across multiple land-uses, (2) evaluate accuracy and precision of pXRF compared with traditional digestion-inductively coupled plasma analyses, (3) determine if pXRF measurements are improved by processing the soil (oven drying, rock fragment removal, and organic matter (OM) removal). Our findings show that urban forest soils have extensive Pb accumulations (42 out of 460 with >200 mg/kg), while As and Zn exceeded regulatory limits in limited outlier samples. Urban forest soils adjacent to multi-family residences have higher Pb and Zn concentrations than recreational and open lots. Fortunately, Cd, Cr, and Cu were far below hazardous concentrations. Our comparisons suggest pXRF measurements of Cu, Pb, and Zn can be considered accurate while Cd and Cr may be screened by pXRF. However, As should not be considered qualitative nor quantitative due to 70% type 1 (false positive) error on samples. While the accuracy and precision were nominally improved through soil drying, removal of rock fragments, and removal of OM (<5% decrease in relative error), our results show processing soil samples is likely not needed.

Keywords: Toxic metals; Urban soils; X-ray fluorescence; pXRF methodology.