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Metal contaminated soil is a large remediation problem at military sites, with lead being the predominant heavy metal of concern. In situ solidification/stabilization methods are available that can reduce the environmental mobility of heavy metals by physically or chemically binding them in place; however, the metals remain in the soil in some form. The long-term fate of the remaining metals or chemical matrices they are bound within is unknown. Other issues include the potential unintended or indirect environmental effects resulting from the in situ application of a chemical binding/treatment solution. This project sought to validate the long-term immobilization of lead by phosphate amendment and assess the effects of erosion and precipitation on the in situ application of various forms of phosphate binders.
A treatability study was conducted to support a potential field demonstration of in situ immobilization of lead in soil. The treatability study attempted to demonstrate the immobilization of lead through the formation of stable lead complexes. In the study, treatment methods employed various phosphate-based binders and were coupled with appropriate leaching and vegetation monitoring methods to evaluate the stability of the treated soil and the potential for metals transport.
Variability in lead stability was observed in soil treated by all vendors of in situ phosphate stabilization methods. The main results of the treatability study were as follows:
Based on these results, a field demonstration of phosphate-based lead stabilization as an in situ treatment method was not recommended.
Field demonstration of the phosphate-based lead stabilization technology was not recommended for the following reasons: (1) human health risk reduction performance criteria were not met by any of the vendor amendments; (2) long-term stability of lead in the soil was still questionable based on the data collected; and (3) the plant-lead uptake study indicated a wide variability in lead availability. The variability was suspected to be the result of site-specific chemical and biological reactions, as well as plant species’ metal uptake characteristics, that may limit the use of the technology for in situ applications. Additional research is needed to investigate biogeochemical, microbial, and hydrological influences on the metals speciation and stabilization process. A better understanding of these factors is needed to predict the applicability and performance of phosphate amendments as a means of stabilizing metals on a site. Immobilization of metals will eliminate the risk of metals migrating to groundwater and surface water receptors and reduce the bioavailability of the lead remaining in the soil.