The Geologic, Geomorphic and Geographic Controls On Lead and Other Heavy Metals In Philadelphia's Fairmount Park Soils.

Philadelphia is an extremely diverse city both currently and historically, including:  transportation, industrial, commercial, parks and residential areas. This urban tapestry of historical and current land use creates the potential for human exposure to heavy-metals in contaminated dust or bio-concentrated in food crops. In order to evaluate this potential public health hazard, we have begun to analyze open spaces in Philadelphia in order to define protocols and best practices for current parks and future sustainable garden sites.

We have obtained surficial soil samples from selected areas of the city and measured total concentrations of Pb and other trace elements in three size fractions with a Thermo Niton X-Ray Fluorescence XL3t Analyzer (XRF) as well as ICP-MS.  Most areas selected for this preliminary study are from largely undisturbed forested land in Philadelphia's Fairmount Park (the largest inner-city park in the U.S.), recently reclaimed meadows, and urban recreation centers. At each location a bulk sample was taken at various geomorphic positions or, if little to no topographic change was present at a specific site, then samples were taken systematically at specific intervals from structures.

Selected bulk samples were digested and analyzed using ICP-MS for independent verification of XRF results.   Selected samples were also digested sequentially following the method of Tessier et al. (1979) to determine distribution of lead and other heavy metals in various soil phases.  In initial results, Pb concentrations in largely undisturbed park land soils range from 17 to 1550 ppm. However, in soils from a former orchard, Pb concentrations are as high as 1.8%. Median measured values in largely undisturbed soils are ca. 230 ppm,  and concentrations are approximately log-normally distributed.  Pb concentrations in the largely undisturbed soils are generally less than those found in previous soil studies in Philadelphia.  In most samples the extractable Pb distribution was: Fe/Mn oxides > residuals > organic > carbonate > exchangeable fraction.  Thus, most Pb was in less biologically available soil fractions.