Demonstration of tailored levels of in situ heating for remediation of a former MGP site

Abstract

TerraTherm used its In Situ Thermal Desorption (ISTD) technology at full scale to remediate a gasholder containing residual coal tar at a former manufactured gas plant (MGP) site in North Adams, Massachusetts. Prior to the site being heated, coal tar DNAPL had resisted recovery. After dewatering, TerraTherm applied ISTD in a step- wise fashion, without excavation. To our knowledge, this is the first site where a multi- level in-situ heating approach has been applied. We utilized three levels of heating (Levels 1, 2 and 3), achieving low (80°C), moderate (100°C) and higher (325°C) soil temperatures, respectively. During Level 1, >16,000 gal (60,000 l) of coal tar/emulsion was recovered, while during Levels 2 and 3, >166,000 lb (75,000 kg) expressed as naphthalene were extracted and treated in the vapor phase. ISTD resulted in the following reductions in soil concentrations (mg/kg): Level 2, benzene from 3400 to 0.95, naphthalene from 14000 to 70, and benzo(a)pyrene from 650 to 100; Level 3, benzene from 2068 to 0.35, naphthalene from 679 to 5.7, and benzo(a)pyrene from 20 to 0.33. No DNAPL remained within the gasholder, and all constituents were below the remedial goals. National Grid judged the turn-key cost ($850,000 for ISTD) to be less than the excavation alternative. SITE BACKGROUND TerraTherm, Inc. employed its ISTD process, also known as In-Situ Thermal Destruction at full scale to remediate a gasholder containing residual coal tar and related constituents (i.e., benzene; polycyclic aromatic hydrocarbons (PAHs) such as naphthalene and benzo(a)pyrene; and petroleum hydrocarbons) at a former MGP site in North Adams, a city in northwestern Massachusetts. From about 1860 to 1952, coal carbonization and later, carbureted water gas manufacturing were conducted at the site. The facility included gasholders, storage tanks, switch houses, purifier boxes, retorts and other gas manufacturing equipment. Massachusetts Electric Co., which assumed ownership of the entire site in 1972, later became a subsidiary of National Grid. Although leaks are not known to have occurred from this gasholder, the potential for releases of coal tar to the adjacent Hoosic River was a significant concern. During decommissioning, the superstructure of the 62-ft (18.9-m) diameter cylindrical gasholder had been removed and its 18-ft (5.5-m) deep underground portion backfilled with a mixture of silt, sand, gravel, cobbles and debris (bricks, concrete fragments, wood, metal scrap, ash and clinker). Most of the pore spaces of the 2,013 cy (1,539 m 3 ) subsurface volume of the gasholder were initially filled with water. Residual coal tar was evident within the soil but coal tar dense non-aqueous phase liquid (DNAPL) had been recovered only to a limited extent during bailing of wells under ambient temperatures.