This demonstration/validation project, performed at Naval Air Station (NAS) Jacksonville to target a tetrachloroethene (PCE) source area in clay materials, was conducted to validate the performance of an electrokinetic (EK) technique to promote uniform and effective distribution of remediation amendments (e.g., electron donors, electron acceptors, chemical oxidants) in low-permeability (low-K) and heterogeneous subsurface materials.
Recent advances in the understanding of mass distribution in subsurface environments has highlighted that in many cases, a significant portion of the source mass is stored in low-K materials. The main limitation of current in situ remediation applications in low-K materials using conventional hydraulic recirculation or injection techniques is the inability to effectively deliver the required amendments to the target contaminant mass. Estimated costs to Department of Defense (DoD) for adopting hydraulic containment at more than 3,000 chlorinated hydrocarbon sites could surpass $100 million annually, with estimated life-cycle costs of more than $2 billion. EK-enhanced amendment delivery will: (1) broaden the applications of cost-effective in-situ remedial alternatives at many DoD sites where the presence of low-K materials previously precluded the consideration of in-situ technologies; and (2) provide an effective source remediation solution at sites where source mass in low-K materials would otherwise result in long-term hydraulic containment with significant remediation life-cycle costs.
The overall goal of this project was to demonstrate and validate EK-enhanced amendment delivery for in situ bioremediation (EK-BIO) via enhanced reductive dechlorination (ERD) of a PCE source area in clay. Based on the performance monitoring data, this project met the following performance objectives:
The EK-enhanced amendment delivery technology entailed the establishment of an electric field in the subsurface using individual electrodes installed in a network of electrode wells. An EK control system was used to power the electrodes with direct current (dc) and supply amendment solutions to treatment wells. The electrical current and voltage gradient established across the dc electric field in the subsurface provided the driving force to transport remediation amendments, including electron donors, chemical oxidants, and bacteria throughout the treatment area.
The TTA for this project had dimensions of approximately 40 feet by 40 feet, with a treatment depth interval (a clay layer) of 19 to 23 feet below ground surface (bgs). The constructed EK system included nine (9) electrode wells and eight (8) supply wells located within the TTA. The remediation amendments distributed by the EK remediation system included electron donor (lactate, provided as potassium lactate), pH control reagents (potassium carbonate), and a dechlorinating microbial consortium (KB-1®) containing Dehalococcoides (Dhc). Following the system startup, initial site conditioning, and bioaugmentation of the TTA, the project included two (2) separate active EK operational stages of five months each, with a six-month incubation period between the two active stages.
Groundwater monitoring data from EK application showed that within the TTA: (1) total organic carbon (TOC) or volatile fatty acids (VFAs, such as acetate, propionate, etc) increased by more than 5x from baseline; (2) PCE concentrations decreased by more than 80%, coupled with evident increases of dechlorination daughter products and ethene; and (3) biomarkers (Dhc and vinyl chloride reductase gene [vcrA]) increased by several orders of magnitude over the course of the project. Soil sampling data showed that within the TTA, PCE concentrations in the clay decreased by an average of 88%.
This project demonstrated a critical and distinct advantage of the EK-enhanced amendment delivery technique, namely the effective delivery of remediation amendments and ensuing PCE treatment in low-K materials. EK-enhanced delivery was shown to be a safe and controllable approach. This technology also represents a remedial alternative of excellent environmental performance with a very low energy demand. The total energy usage by the EK system during the 14 active months of the project was 1,585-kilowatt hour (kW-hr), which is equivalent to operating two 100-watt (W) lightbulbs over the same time interval.
EK-BIO is mainly a variation on standard enhanced in situ bioremediation (EISB), whereby EK is used to more effectively deliver the required amendments through low-K materials. Based on the information and experience obtained from this project, there are three main cost drivers and technical aspects to consider in future implementation, including: (1) footprint, depth interval, and volume of target treatment zone and contaminant mass; (2) presence and location of above-ground and subsurface utilities; and (3) site geochemistry, particularly pH, iron, and potential corrosion related to geochemical conditions.