Removal of PCE DNAPL from Tight Clays Using In Situ Thermal Desorption

Abstract

AbstractThis paper presents a full‐scale thermal remediation of a brownfields site near San Francisco, California. In Situ Thermal Desorption (ISTD) was used for treatment of chlorinated solvents in a tight clay below the water table. The site had contaminants in concentrations indicating that a tetrachloroethene (PCE)‐rich DNAPL was present. A target volume of 5097 m3 of subsurface material to a depth of 6.2 m was treated for a period of 110 d of heating. Energy was delivered through 126 thermal conduction heater borings, and vapors were extracted from a combination of vertical and horizontal vacuum wells. Approximately 2540 kg of contaminants were recovered in the extracted vapors by the end of treatment. The PCE concentration in the clay was reduced from as high as 2700 mg/kg to an average concentration of 0.012 mg/kg within 110 d of heating (a reduction of >99.999%). Similar effectiveness was documented for TCE, cis‐1,2‐DCE, and vinyl chloride. A total of 2.2 million kWh of electric power was used to heat the site. Approximately 45% of this energy was used to heat the subsurface to the target temperature. Another 53% was necessary to boil approximately 41% of the groundwater within the treatment zone, creating approximately 600 pore volumes of steam by the end of the 110‐d heating and treatment period. Steam generation thus occurred within the clay. Partitioning of the contaminants into the steam and its removal comprised the dominant remedial mechanism. The steam migrated laterally toward the ISTD heaters, where it encountered a small dry region adjacent to each of the heaters, which served as a preferential pathway allowing the steam to migrate upward along the heaters to the more permeable vadose zone. There the steam was captured by a system of vertical and horizontal vacuum extraction wells. This vapor removal strategy facilitated effective thermal treatment of the tight clays located below the water table. Features of a robust design are extension of the heaters at least 1.2 m deeper than the treatment depth, and the installation of shallow horizontal vapor collection wells which allow for establishment of pneumatic control.