SEQUENTIAL EXTRACTION OF LEAD FROM SOILS ACROSS PHILADELPHIA: THE INFLUENCE OF HISTORICAL LAND USE AND TOPOGRAPHY

We analyzed concentrations of lead and other trace metals in soil samples from Fairmount Park in Philadelphia, PA, using a Thermo Niton XL3t portable X-ray fluorescence (pXRF) analyzer. The selected sites fall under three categories: undisturbed forested park land, recently disturbed meadows, and urban parks. The undisturbed park land and the meadows all have topographic relief, whereas the urban park lands are relatively flat landscapes. Total Pb concentrations range from 1.8% at the site of a former orchard to <100 ppm in undisturbed park land. Lead concentrations do vary with changes in topography. However, if the site is in proximity to point and non point sources of anthropogenic lead, then those sources override topography as the dominant controlling factor. Selected samples were digested and analyzed using ICP-MS for independent verification of pXRF results. Selected samples were also digested sequentially following the method of Tessier et al. (1979) to determine lead fractionation. In most samples the lead distribution was: Fe/Mn oxides > residuals > organic > carbonate > exchangeable fraction. Thus, most lead was in less biologically available fractions, either by ingestion or uptake by plants. Total lead concentrations may not reflect biological availability, and lead fractionation between phases did not change consistently with topographic position.