Formatted Title
Low Temperature Remediation of a Carbon Tetrachloride DNAPL Site Using Thermally Enhanced Hydrolysis
Background/Objectives
A low-temperature thermal remediation (LTTR) project was completed at a site in the eastern US that focused on thermally enhancing the degradation of carbon tetrachloride (CT) through hydrolysis, including the removal of CT DNAPL. The site geology consisted of several feet of fill overlying ~30 feet of low permeability till, which was underlain by a deep permeable sand deposit. Thermal conduction heating (TCH) was used to treat the target treatment zone (TTZ) from 10 to 33 feet below the ground surface (ft bgs). The TTZ footprint was approximately 10,039 square feet (ft2) and comprised a treatment volume of approximately 8,254 cubic yards (yd3). The TTZ was defined as a source area where concentrations of CT were present in excess of 200 milligrams per kilogram (mg/kg) and the maximum observed concentration was 144,000 mg/kg. The project's remedial objectives were to remove CT DNAPL and reduce the CT present in the TTZ to a site-wide average concentration of less than 100 mg/kg, which was established to be protective of a nearby waterbody. The planned design was to heat the TTZ to 100°C to boil and volatilize the DNAPL and dissolved and sorbed CT sufficiently to achieve the remedial goals. Steam, air, and CT vapors would then be removed via extraction wells and treated in an off-gas treatment system. However, site conditions and concerns over vapor emissions required that heating be limited to sub-boiling conditions (<100°C) to minimize steam production, and system operation was modified to focus on low-temperature thermally enhanced hydrolysis of the CT. This was achieved by heating the TT to a minimum of 66.8°C (the co-boiling point of CT) and a maximum of 90°C. Heating to the co-boiling point ensured the transfer of the CT DNAPL mass to the dissolved phase for effective hydrolysis and limiting heating to <90°C prevented the production of excessive amounts of steam and associated vapor emission issues.
Approach/Activities
A total of 68 vertical TCH wells and 34 angled TCH wells extending off-site beneath an active rail line and petroleum loading area were used to heat the TTZ to hydrolyze the CT. For the first 60 days of operations, the heater output ranged between 250 to 300 W/ft, which is typical for a 100°C treatment approach. After 60 days of heating, power output was reduced to 65 to 75 W/ft for low-temperature heating. A network of 56 vertical extraction wells and 38 angled vapor extraction wells (VEWs) co-located to heaters were also installed to remove steam and CT vapors, however, under low-temperature operations, these only removed low amounts of CT and steam. Ten temperature monitoring points (TMPs) were used to monitor subsurface conditions throughout operations. Soil samples were collected to evaluate performance.
Results/Lessons Learned
After nearly 150 days of operations, the average temperature of the TTZ was 68°C with temperatures ranging between 47 and 85°C depending on depth. Starting soil concentrations of CT ranged between 102 and 144,000 mg/kg with an average concentration of 4,393 mg/kg. Following LTTR, the average soil concentration of CT was 7.52 mg/kg, well below the site remedial goal of 100 mg/kg and the LTTR project was determined to have met the remedial objectives for the site. Increasing subsurface temperatures in the TTZ from ambient (~10°C) to between 60 and 80°C increased the hydrolysis reaction rates of CT by two to three orders of magnitude, thereby reducing the half-life of the CT from 60,000 days at ambient temperatures to ~60 days at the targeted temperatures. Importantly, the LTTR approach utilized 34% less power than the 100°C treatment approach. In addition, by limiting temperatures to less than 100°C, boiling and subsequent vapor production was minimized and the need for vapor extraction and treatment was limited, which, when combined with the lower power usage, improves the sustainability of thermal remediation.