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.

Technology Description

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.

Demonstration Results

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:

  • Toxicity characteristic leaching procedure (TCLP) analyses indicated a substantial reduction of lead mobility, with most treated soil TCLP results below 5.0 mg/L. However, variations occurred in the samples as the treated soils aged during the 360-day monitoring period with lead TCLP results increasing and decreasing over time. In general, the data indicated a greater than 98.5% reduction in leachable lead in soil. The sequential extraction test (SET) results seemed to confirm this reduced mobility, with a shift in lead concentrations from the more soluble fractions in the control soil to the less soluble fractions in the treated soils. However, the variation in stability continues to call into question the long-term stability of the treated soils.
  • Bioavailability reduction was evaluated by comparing the untreated control and treated soil physiologically-based extraction test (PBET) results. The PBET lead concentrations in the treated samples indicated that a reduction in bioaccessible lead occurred as compared to the control data. However, the treated soils exhibited significant variations in PBET lead concentrations during the monitoring period. In addition, at no time during the monitoring period was the bioaccessible lead exposure risk reduced to a level considered safe for residential or industrial use.
  • Hyper-accumulating plant species were used to identify general trends in plant bioavailability. The plant data indicated that lead uptake can vary substantially according to the type of phosphate amendment. Lead uptake by plants is influenced by a combination of site-specific soil characteristics (i.e., mineral and organic constituents, biota, etc.), type of amendment(s) used, and the type and variety of local plants in the treated areas.
  • All of the vendor-treated soils failed to meet the 0.75 mg/L TCLP universal treatment standards performance criteria with the exception of the Forrester 0-, 14-, 28-, 60-, and 120-day samples and the RMT 360-day sample. With trends toward increasing TCLP lead concentrations observed, a determination supporting long-term stability could not be made.

Based on these results, a field demonstration of phosphate-based lead stabilization as an in situ treatment method was not recommended.

Implementation Issues

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.