Formatted Title
Thermally Enhanced Biodegradation of Dissolved TCE in LNAPL under Active Building Using Horizontal Wells
Background/Objectives
The subject building has a footprint of 172,000 square feet and overlies a 1MGal light non-aqueous-phase liquid (LNAPL) plume comprised of total petroleum hydrocarbons (TPH) with dissolved trichloroethene (TCE) and 1,1,1-trichloroethane (1,1,1-TCA). The TPH is composed of jet fuel and Stoddard solvent. The depth to the top of LNAPL is approximately 23 feet below ground surface. LNAPL temperatures were elevated proximal to former steam lines (as high as 140oF) as of 2017 (the lines were decommissioned June 2018). Volatilization of cVOCs from the LNAPL has created a significant cVOC vapor plume underneath the building, with initial cVOC levels >20,000,000 µg/m3. A soil vapor extraction (SVE) system with five horizontal wells has been in operation since May 2016, recovering >8,000 gallons of cVOCs. Three horizontal steam injection wells (screened below the LNAPL) were operated for 6 months in 2018 to enhance LNAPL volatilization and biodegradation, specifically the cVOCs. Steam injection was resumed in late 2021 using a steam boiler and is currently ongoing. There are two NAPL recovery wells screened in the LNAPL, i.e. above the steam injection wells. Optimization is planned to reduce long term costs.
Approach/Activities
Analyses of the LNAPL composition between 1998 and 2023 indicate that TCE concentrations have decreased by orders of magnitude and cis-1,2-DCE concentrations have increased over time. After steam injection was done in the first two quarters of 2018, vinyl chloride was detected in the LNAPL, and has been steadily increasing. Temperature data for the LNAPL collected in 2016 indicate temperatures over 110oF at a number of locations. Significant levels of cis-1,2-DCE were detected in soil gas, both in sub-slab and at depth. The composition of cVOCs and TPH in extracted vapor and soil gas are being evaluated as a secondary indicator of biodegradation. Product samples collected at the NAPL-water interface were analyzed for dehalococcoides (DHC), which have increased by up to three orders of magnitude. Reductase genes for TCE and vinyl chloride were also detected. Groundwater temperatures are (as of September 2023) as high as 95oF, and it is possible that populations may be compromised if temperatures continue to increase. An optimization strategy that involves injecting steam directly into NAPL Recovery Wells is being considered, which is expected to reduce the temperature increase at the NAPL-water interface. Measurement of heat shock proteins is planned to evaluate potential for presence extremophiles. In addition, Next Generation Sequencing (NGS) is planned.
Results/Lessons Learned
Biodegradation of cVOCs dissolved in TPH NAPL is possible. Elevated temperatures are likely promoting biodegradation of TCE at the LNAPL-water interface, with TPH serving as a continuous/limitless source of electron donor. Active injection of steam further below the NAPL increased temperatures within the LNAPL plume, resulting in biodegradation of cVOCs, and significant increases in concentrations and monthly removal rates. The appearance of vinyl chloride in the LNAPL after steam injection confirms that steam-enhanced biodegradation can address TCE (and other cVOCs) under adverse conditions. Concurrent with anaerobic degradation, the TPH is being aerobically biodegraded, which further increases the mass removal due to SVE. Injection of steam directly into the NAPL is expected to (a) increase the rate of TCE volatilization and (b) maintain or increase DHC population at the NAPL-groundwater interface. The NGS and heat shock protein data will be used to further optimize the removal action (including cessation of steam injection).