Abstract

In situ Thermal Desorption (ISTD) was used to remediate a gasholder (18.9 m diameter, 5.5 m deep) containing residual coal tar at a North Adams, Massachusetts former MGP site. ISTD applies heat and vacuum simultaneously to remove organic contaminants from the subsurface. Coal tar had been observed but at ambient temperatures had been recovered only sparingly. After dewatering, TerraTherm applied ISTD to eliminate the dense non-aqueous phase liquid (DNAPL) and remediate the gasholder using a tailored heating approach, which achieved soil cleanup standards protective of groundwater, without excavation.

We first employed gentle heating (~80°C) for thermally enhanced DNAPL recovery, producing >60 000 L of coal tar/emulsion. Then we raised temperatures to attain the project goals. Within the mid-section (1.8–4.6 m depth), we achieved temperatures of 325°C to volatilize, boil, pyrolyze and oxidize the semi-volatile organic compounds (SVOCs). Within the lower portion (4.6–5.5 m depth), we attained more moderate temperatures of 100°C and vaporized the volatile organic compounds (VOCs). Overall the extracted vapor contained 75 000 kg expressed as naphthalene, treated in an aboveground thermal oxidizer.

Within the mid-section, elevated heating produced the following reductions in soil concentrations (mg/kg): benzene – 2068 to 0.35; naphthalene – 679 to 5.7; and benzo(a)pyrene – 20 to 0.33. Within the lower portion, more moderate heating effected the removal of all DNAPL. All constituents were below the remedial goals. National Grid judges the turnkey cost ($850,000 for ISTD) to be less than the excavation alternative. TerraTherm conducted the project under a guaranteed performance contract.

Key words:in situ thermal desorption, manufactured gas plant, gasworks, polycyclic aromatic hydrocarbons, thermally-enhanced free product recovery, thermal remediation