In-Situ Thermal Treatment of MGP Waste and Creosote

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 300C. 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 <100C. 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 m of predominantly silty soil to a depth of 32 m without costly excavation. Heating the heavily PAHand dioxin-contaminated soil to 325C resulted in the CA Dept. of Toxic Substances Control granting a No Further Action letter, releasing the site for unrestricted land use. INTRODUCTION MGP coal tar and creosote are characterized as being relatively viscous DNAPL with toxic and carcinogenic constituents such as BTEX, pentachlorophenol (PCP) and polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene (B(a)P). Spills and releases of these contaminants can degrade soil, sediment, groundwater and surface water bodies. NAPL can migrate long distances for decades before it finds its eventual position, evidenced by persistent seepage of NAPL to water bodies at many sites. In addition, dissolution into flowing groundwater can create massive problems for drinking water aquifers, such as under the city of Visalia in central California, where PCP and naphthalene from a 43-m (140-ft) deep creosote DNAPL source zone threatened to close down the municipal groundwater production wells, until it was successfully cleaned and delisted from the National Priority List (NPL) using ISTR (USEPA 2009). In general terms, there are three different levels of heating, and thus thermal treatment approaches applicable to MGP and creosote sites. One may be used alone, or two or more can be applied sequentially at the same site:  Level 1. Thermally Enhanced Free Product Recovery (TEFPR). The subsurface is heated to temperatures above ambient, typically between 70 and 90C, and the removal of DNAPL by pumping is enhanced. After cool-down, the residual DNAPL is relatively immobile. The subsurface can be heated using any one or a combination of the common ISTR technologies: thermal conduction heating (TCH), also known as In Situ Thermal Desorption (ISTD); electrical resistance heating (ERH) including the Electro-Thermal Dynamic Stripping Process (ETDSPTM); and/or steam injection/steam-enhanced extraction (SEE), selected based on hydrogeology and site considerations.  Level 2. Treatment at boiling point temperature (100C above the water table and steam temperature below). Steam stripping depletes the DNAPL of its more volatile and mobile constituents, rendering the DNAPL viscous and without significant leachability, which has been termed ISTS (Hayes 2002). Any one or a combination of the common ISTR technologies listed above (i.e., ISTD, ERH, SEE) can be used to achieve the target temperature, depending on the hydrogeology.  Level 3. Treatment at temperatures above the boiling point of water, with drying of the subsurface. Of the prevalent heating techniques, only ISTD can accomplish this, as no moisture addition is needed to deliver the energy. Target temperatures range from less than 200C for complete removal of naphthalene, to 335C for treatment of high molecular weight PAHs, including B(a)P, PCP and dioxins.