Leaching Assessment Methods for Evaluating the Effectiveness of In Situ Stabilization of Soil Material at Manufactured Gas Plant Sites

In situ stabilization and solidification (ISS) is a remediation technique used at former manufactured gas plant (MGP) sites to reduce the rate and extent of contaminant release to groundwater. Assessment of the effectiveness of this treatment method with regard to containment of polycyclic aromatic hydrocarbons (PAHs) poses significant challenges due to the characteristics of the contaminants and the limitations of traditional assessment approaches for predicting leaching behavior. This technical update provides an overview and status of a project to develop an alternative leaching assessment methodology for estimating the release of PAHs from ISS-treated soils. Laboratory testing was carried out on cores of ISS-treated soils from three former MGP sites and a laboratory formulated control material. A new batch leaching test for monolithic samples was developed for this project to estimate long-term maximum release from groundwater-saturated ISS materials. A comparison of PAH releases for two ISS materials from a single MGP site showed that the test conditions imposed by regulatory protocols appear to be overly aggressive toward PAH releases. Scenario release calculations using broad-based assumptions not representative of any particular site determined by an assessment approach still under development were as much as one order of magnitude lower than those from regulatory tests. In addition, leaching potential under percolation conditions was also measured, and results indicate that current regulatory tests may overpredict release of PAHs in the vadose zone. For the column data, interactions between concentrations of PAHs in the leachate and total dissolved organic carbon do not seem to be a strong factor in PAH release. The role of discrete dissolved organic carbon (DOC) fractions (e.g., humic and fulvic acids) on the concentration of PAHs is still under investigation. Geochemical modeling of the interaction between DOC and PAH concentrations, and the development of scenario-based release assessments for PAH releases for each ISS material are discussed.