CONTRIBUTIONS TO PXRF MEASUREMENT ERROR FOR TRACE METALS IN URBAN SOILS ACROSS SOIL PROPERTIES AND CONDITIONS

Urban environments are enriched in toxic metals, but analyses of soils can be costly and time consuming, and thus can be inaccessible to many communities that face the greatest risk of exposure. Portable XRF (pXRF) offers rapid measurements of toxic metals in urban soils, however there is an incomplete understanding of soil characteristics on error and uncertainty. We collected 150 to 175 surface soil samples (0 to 15 cm) from Lexington MA, Springfield MA, and Hartford CT to test the effect of moisture, rock fragments, and organic matter content on the ‘perceived’ concentrations of As, Cd, Cr, Cu, Pb, and Zn. Soil samples were first analyzed at field moisture content four times, then dried and re-analyzed four times. The samples were further processed to remove rock, root, and anthropogenic fragments and re-analyzed four times. Lastly, samples were combusted at 550° C to remove organic matter and re-analyzed four times. All samples were digested using aqua regia and analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to establish a ‘true’ concentration value for the six metals for all samples. Deviation of the ‘perceived’ concentrations from the ‘true’ concentrations were quantified. Additionally, precision of the four pXRF measurements across processing conditions was determined using nonparametric statistics. This information will be used to help quantify expected error for pXRF measurements under varying levels of field conditions and laboratory processing.