The applicability of in situ groundwater remedies such as in situ bioremediation (ISB) or zero valent iron (ZVI) reduction in chlorinated solvent source zones (i.e., containing dense non-aqueous phase liquids [DNAPLs]) is often limited by the relatively long treatment timeframes required to meet remedial objectives at sites. Conceptually, the goal of this project was to evaluate moderate heating (i.e., 35-50°C) to accelerate the dissolution and desorption of residual trichloroethylene (TCE) contamination as well as the in situ degradation kinetics, and to minimize volatilization and therefore the need for soil gas extraction and treatment, as is typically required for high-temperature thermal applications. This field demonstration combined electrical resistance heating (ERH) with ZVI and ISB for TCE treatment in two separate test cells. The demonstration objectives included quantifying (1) the effect of low-energy heating on the extent and rate of contaminant degradation, (2) the impacts on the mass removal rate, (3) the relative contributions of biotic and abiotic contaminant degradation mechanisms at different temperatures, and (4) the costs and benefits of applying low-energy heating with in situ treatments.
ERH technology is used to treat soil and groundwater contamination, and can be especially effective in treating TCE and other volatile organic compound (VOC) contaminant sources in the DNAPL phase. ERH increases subsurface temperatures to the boiling point of water, at which point steam is created in situ and contaminants are directly volatilized, extracted, and treated in above-ground treatment systems. ISB is a demonstrated technology that relies on amendment injections to grow bacteria capable of dechlorinating chloroethenes. ZVI technologies rely on emplacement of ZVI in situ to facilitate abiotic reductive elimination reactions of chlorinated solvents, which does not generate hazardous degradation products. Combining subsurface heating (ERH) with in situ treatments (ISB or ZVI) has the potential to accelerate treatment rates of the in situ technologies because higher temperatures increase degradation reaction rates, and also increase the DNAPL dissolution and contaminant desorption rates, which are often the rate-limiting steps in DNAPL treatment with these technologies.
This project, located at the East Gate Disposal Yard at Joint Base Lewis-McChord near Tacoma, Washington, was conducted in three phases. Phase 1 consisted of initial characterization and verification of the suitability of ISB and ZVI test cells to meet project objectives. Phase 2 consisted of a field demonstration of ISB and ZVI without heating to establish the performance of the individual technologies at ambient temperatures. Phase 3 consisted of field demonstrations of low-energy ERH combined with ISB and ZVI at elevated temperatures.
In the ZVI test cell Phase 2 field demonstration, micron-scale ZVI particles were suspended within a shear-thinning fluid to increase their distribution within the subsurface. Approximately 190 kg of 2-micron-diameter ZVI particles were injected into the top 6 feet of an unconfined aquifer within the TCE DNAPL source zone, and were successfully distributed over 12 feet from the injection well. All monitoring wells showed indications of dechlorination including partial dechlorination products and high concentrations of ethene and ethane, by the end of the 2-month monitoring period at ambient temperature. Data indicated a mixture of abiotic reactions and biotic dechlorination reactions were occurring, as the daughter products included cis-dischloroethene (DCE) (biotic) but not vinyl chloride, and ethene and ethane (which probably resulted from abiotic processes).
For the ISB test cell, efficient degradation of TCE was established during Phase 2 via monthly injections of emulsified vegetable oil and powdered whey into the injection well for 9 months. A reactive treatment zone was established where geochemical conditions were generally reduced to support methane production, and reductive dechlorination of TCE to primarily cis-DCE with trace ethene was achieved at ambient temperature. However, relatively high groundwater velocities within the treatment zone resulted in relatively low retention of the amendments within the test cell, which was the reason that monthly injections were conducted.
Phase 3 was started at the same time in both the ISB and ZVI test cells by applying ERH to raise the temperature in the test zone to the target temperatures (30°C - 45°C and 40°C - 55°C respectively). The elevated temperatures increased the dissolution of contaminant into the groundwater and increased the rate and extent of dechlorination in both test cells. During this demonstration the total contaminant mass discharge increased by a factor of 4-16 within the ZVI test cell, and consisted primarily of the reductive daughter products (ethene and ethane), as the degradation kinetics were sufficiently high to keep the TCE concentrations low. For the ISB test cell, the total contaminant mass discharge increased by a factor of approximately 4-5 and the fraction of the total mass present as ethene increased dramatically during Phase 3 compared to Phase 2. In both test cells, the contaminant fluxes to the vadose zone increased by less than 1.5% at the elevated temperatures compared to ambient, indicating VOC losses to the vadose zone were minimal and vapor recovery and treatment likely would not be needed.
A detailed review of the costs for low-temperature ZVI and ISB suggests that low-temperature heating is less expensive than high-temperature ERH, but only incrementally so. Therefore, application of low-temperature heating combined with in situ treatment likely makes sense only for sites that contain relatively low to moderate VOC concentrations as residual DNAPL, so that the contaminant mass could be removed in less than 1-2 years of treatment. Sites with higher concentrations or significantly pooled DNAPL probably cannot be treated effectively using low-temperature heating. However, the benefit of heating to accelerate in situ reactions was clearly demonstrated, and therefore, combining in situ treatment with heating may be beneficial, especially for sites already considering high-temperature heating. In addition, in situ technologies could be implemented after thermal shutdown, to rapidly degrade any remaining contaminants in the treatment zone while the subsurface temperature remains elevated.