Management of contaminated fractured rock sites remains one of the top environmental challenges for the U.S. Department of Defense (DoD). Use of chloroethene solvents such as tetrachloroethene (PCE) and trichloroethene (TCE) has led to extensive contamination of both soil and groundwater. In particular, chloroethene source zones containing dense non-aqueous phase liquids (DNAPLs) in fractured rock have proven very difficult to remediate. Project ER-201210 was designed to test the ability of bioaugmentation to treat a PCE DNAPL source zone in fractured rock at Edwards Air Force Base (AFB), California.
Bioaugmentation is a relatively low-cost technology to remove and degrade DNAPLs, proven to be highly effective in unconsolidated media but not yet demonstrated in fractured rock. The specific demonstration objectives included: (1) removal of >9% of the DNAPL mass per month, (2) reducing the total chloroethene flux by >90% after treatment, and (3) achieving complete dechlorination of PCE to innocuous products (ethene and ethane). In addition, monitoring was performed ten months after treatment to assess the potential for rebound from untreated contaminants in the treatment zones. The latter two objectives were attained throughout, but removal in the deep zone was slower than desired (about 5% per month). Further, aqueous concentrations rebounded in the deep zone ten months after treatment ceased.
Bioaugmentation involves injecting a mixed culture of bacteria that includesDehalococcoides sp.(DHC) strains capable of dechlorinating all the chloroethenes, along with a carbon source (lactate) and nutrients. The anaerobic biological activities both increase the solubility of the DNAPL constituents and biodegrade contaminants in place. PCE is degraded through TCE tocis- 1,2-dichloroethene (DCE), vinyl chloride (VC), and eventually ethene and ethane.
In this case, water containing amendments was recirculated through two target treatment zones, with extraction, treatment, and reinjection into separate depth intervals with discrete fractures. The field scale injections followed a detailed characterization of the target source zone that identified the two fracture intervals and quantified the DNAPL mass within each area through partitioning tracer testing. The amendments were recirculated through each depth interval to degrade PCE and its daughter products over a nine-month period, followed by a ten-month rebound period.
A small portion of a (presumably) much larger DNAPL source area was targeted, and the DNAPL mass and distribution were quantified in two separate depth intervals with discrete fractures. Conventional hydraulic and geophysical tools, along with partitioning tracer testing, were used to quantify the DNAPL distributions in the shallow and deep fracture intervals. The geophysical testing showed that DNAPL was present, that the well capacities within the source area were sufficient to distribute the amendments in conductive fractures, and that there was hydraulic connectivity in both zones in the two wells used for the field test. Initial lab testing was done to verify that bioaugmentation using CB&I’s SDC-9 culture could be effective, to assess the need for additional amendments (e.g., nutrients or pH buffer), and to evaluate the potential inhibitory or toxic effects of the partitioning tracers on SDC-9.
Pre-treatment characterization monitoring showed that very low levels of DNAPL (<1% of the fracture volume) persisted in several of the fracture zones, and that DNAPL was present in both the lower and higher transmissivity zones. During biological treatment, enhanced dissolution of the DNAPL sources was observed in both the shallow and deep fractures intervals. In the shallow fracture zone, the measured DNAPL mass removal was approximately 100%. However, the estimated DNAPL removal was only 45% over the same period in the deep zone. The difference in DNAPL mass removal between the two zones was attributed to the DNAPL architecture, as the flow field in the deep zone was more complex, and a greater extent of the DNAPL was present in mass transfer controlled zones.
Rebound testing indicated that there was no increase in the sum of chlorinated ethenes and ethene in the shallow zone ten months after active treatment. In contrast, the sum of chloroethenes and ethene concentrations did rebound significantly in the deep zone, probably because residual DNAPL mass was still present. These results highlight the relationship between DNAPL architecture and remedial performance.
The costs were evaluated using a consistent base case for the three most common fractured rock source zone treatment technologies—bioaugmentation, thermal conductive heating (TCH), and active pump-and-treat. The estimated costs for bioaugmentation were considerably less than for the other two technologies, with estimated net present value costs of roughly $1.4M, $5.3M, and$3.7M, respectively.
The challenges associated with DNAPL in fractured rock are similar to those encountered in unconsolidated media. However, these challenges are exacerbated by the complexities associated with the dual porosity nature of fractured rock, as well as the lack of insight into the highly complex DNAPL architecture at the field scale. The primary difficulties identified with implementing bioaugmentation at fractured rock sites were: (1) the complexity of the fracture flow paths, (2) the need for multi-level borehole sampling, and (3) the potential for biofouling at the injection wells.
Schaefer, C. E., E. B. White, G. M. Lavorgna, and M. D. Annable. 2017. Bioaugmentation in a Well-Characterized Fractured Rock DNAPL Source Area. Groundwater Monitoring & Remediation, 37(2):35-42.
Schaefer, C. E., E. B. White, G. M. Lavorgna, and M. D. Annable. 2016. Dense Nonaqueous-Phase Liquid Architecture in Fractured Bedrock: Implications for Treatment and Plume Longevity. Environmental Science & Technology, 50(1):207-213.
Lavorgna, G. M., C. E. Schaefer, T. Ault, M. D. Annable, E. B. White. 2015. Assessment and Biological Treatment of DNAPL Sources in Fractured Bedrock. Battelle Conference Proceedings, Miami, FL, May 18-21.