ELEVATED LEAD CONCENTRATIONS IN URBAN GARDEN SOIL: THE RISKS OF COMPOST AS A REMEDIATION TOOL

In the lead contaminated backyard gardens of Boston residents are provided with different sources of compost to enrich the soil and to dilute and sequester lead. In this study, four compost sources were tested for lead to assess the potentially negative additives introduced with compost. Compost samples were size fractionated and the total lead concentrations were assessed using pED-XRF. The average bulk concentration of lead in urban compost samples analyzed was 173 ± 22 µg g^-1. A trend of increasing lead concentrations with decreasing grain size was observed, with the < 37 µm size fraction containing lead concentrations at approximately two times the bulk. Since total soil lead concentrations do not provide an accurate assessment of exposure to labile soil/compost lead inventories, a sequential extraction procedure determined the metal speciation and indicated the bioavailability of the lead present in compost. The mobility of several heavy metals is dependent on the total organic matter (TOM) content of the soils therefore making compost a popular, inexpensive urban remediation scheme. Used as a soil amendment, compost can change the soil chemistry increasing lead sorption thereby decreasing lead bioavailablity. The pH and total iron, manganese, and phosphorus level of each compost source was measured. Manganese levels were found to exceed the unamended garden soil by an order of magnitude providing additional sites for the sorption of lead within the compost/garden soil mixture and potentially lowering the overall bioavailable lead fraction. Therefore, while the use of compost has the potential to introduce additional lead into gardens, the changes in bulk soil characteristics may both immobilize soil lead and improve soil fertility. This study hopes to advise gardeners as to which compost source is most effective at lowering labile lead concentrations and to suggest a distinct mineral/compost amendment to optimize lead immobilization.