LEAD STABILIZATION AND ARSENIC MOBILIZATION BY PHOSPHATE AND ALTERNATIVE AMENDMENTS: IMPLICATIONS ON URBAN SOIL REMEDIATION AND URBAN AGRICULTURE

Phosphate amendment is being promoted as a cost effective means of reducing Pb hazards in soil via the formation of stable lead-phosphate minerals. Limited studies have demonstrated the Pb stabilizing potential of phosphate amendments, but the magnitude of the effect remains incompletely characterized for urban soils - especially relating to the ability of phosphates to reduce Pb uptake by plants and bioaccessibility to human. Urban soils are often featured by their high carbonate contents where large percentages of Pb reside. Recent (May, 2012) CDC recommendations aimed at lowering blood Pb levels in children will likely increase the importance of amending agents as a tool in reducing soil Pb hazards. However, the application of phosphate in soil can potentially mobilize As, a known human carcinogen. There is uncertainty regarding the extent to which As can be mobilized by phosphates in different types of soils, and how it will affect the As uptake by plants. Better quantitative understanding is needed as to the potential benefits and risks associated with phosphate amendments. There is also a need to assess the Pb stabilizing and As mobilizing capacity of non-phosphate, alternative amending agents. This RARE study uses a combination of lab and field experiments to examine the Pb binding and As mobilization potential for Bone meal, triple super phosphate (TSP), and two types of composts, with or without iron filing. Six types of produce commonly found in urban gardens (lettuce, spinach, carrots, radish, tomato and egg plant) are grown in test plots at two contaminated sites and treated with afore mentioned amendments. A modified-Morgan method leach and an in vitro method (RBA) are used to assess plant availability and bioaccessibility, respectively, for soils before and after treatment. Plant tissues are analyzed for Pb and As contents to evaluate the uptake potential and the health risk upon consumption.