Remediation of coal tar at former Manufactured Gas Plant (MGP) sites, and creosote associated with wood preservative sites is challenging due to the viscous nature of the dense non-aqueous phase liquid (DNAPL) and the modest solubility and vapor pressure of the contaminants of concern (COCs). DNAPL seepage into surface water bodies, as well as groundwater plumes formed by the most mobile constituents like benzene, pentachlorophenol and naphthalene, are frequent environmental problems associated with coal tar and creosote. In Situ Thermal Remediation (ISTR) technologies have been shown to overcome these limitations through one or more of the following approaches: (1) decreasing DNAPL viscosity by one to two orders of magnitude, making the DNAPL pools recoverable; (2) removal of the most volatile and mobile COCs by steam stripping, making the residual mass immobile and non-leachable; and/or (3) complete removal of the COCs if the soil can be dried out and treated at temperatures above 300oC. Three case studies of thermal remediation of DNAPL sites are presented, covering the range of aggressiveness of heating and degree of treatment, as follows:
- Approach (1) was demonstrated at full-scale at a former MGP site owned by National Grid in North Adams, MA. There, over 60,000 l of formerly highly viscous coal tar was recovered over a four-month period from a buried gasholder by thermally-enhanced freeproduct recovery, conducted by heating the gasholder contents to <100C.
- Approach (2) was initially developed through extensive laboratory testing of MGPcontaminated soil by the Gas Technology Institute, and named In Situ Thermochemical Solidification (ISTS). ISTS has recently been the subject of a comprehensive evaluation conducted under sponsorship of the Electric Power Research Institute (EPRI) on soil contaminated with coal tar from a former MGP site in the Southeast US. The results indicate that heating the subsurface to 100ºC is sufficient to remove the benzene, toluene, ethylbenzene and xylenes (BTEX) and naphthalene fractions from the soil and coal tar, thereby rendering the soil inert with respect to the potential for leaching of these constituents to groundwater. Toxicity Characteristic Leaching Procedure (TCLP) and Synthetic Precipitation Leaching Procedure (SPLP) tests on post-heated samples indicated that BTEX and naphthalene were not leachable (i.e., the leachate was nondetect for these constituents). This modest level of heating nevertheless solidifies and stabilizes the remaining, higher boiling coal tar residuals as an asphaltic material, no longer a NAPL.
- Approach (3) was employed at full-scale at a former wood treater site owned by Southern California Edison in Alhambra, CA. In Situ Thermal Desorption (ISTD) was used to treat 12,400 m3 of predominantly silty soil to a depth of 32 m without costly excavation. Heating the heavily PAH- and dioxin-contaminated soil to 325oC resulted in the CA Dept. of Toxic Substances Control granting a No Further Action letter, releasing the site for unrestricted land use.