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DOD evaluates bioaugmentation for remediating low-pH aquifers

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The US Department of Defense (DOD) has completed pilot-scale work on the 'MAG-1' site at Fort Dix, NJ, to test suitability of anaerobic bioremediation for an aquifer with low pH. Project results will be used to design full-scale ground-water treatment at MAG-1. In addition, technical criteria gained from the demonstration will be integrated into DOD’s remediation strategy for its many facilities situated above low-pH Coastal Plain aquifers.

As part of recent DOD facility realignment, Fort Dix now operates as an Army Reserve and National Guard training reservation. The facility is located in central New Jersey's Pine Barrens region, within the recharge area of the Kirkwood-Cohansey Aquifer that serves as the primary source for the area's domestic wells. As a result of past discharge of chlorinated solvents during motor pool operations, the MAG-1 site contains a significant and cumulative trichloroethene (TCE) plume within a Class 1-A aquifer situated wholly beneath the property. The aquifer comprises primarily silty fine sand, with chlorinated volatile organic compound (CVOC) contamination primarily in the upper part of the formation in a saturated thickness of approximately 10 feet.

Baseline studies of the aquifer indicated a pH of 4.2 to 5.3 and a positive oxidation-reduction (redox) potential ranging from 40 to 145 mV. Sulfate concentrations were relatively low (27-61 mg/L), and dissolved iron concentrations ranged from 3,500 to 7,500 µg/L. Primary contaminants are TCE and cis-1,2-dichloroethene (cis-DCE), at pre-treatment concentrations of 30-1500 µg/L and 190-1300 µg/L, respectively; vinyl chloride (VC) concentrations were below detection limits. An initial qPCR (quantitative polymerase chain reaction) analysis of site samples for the dechlorinating organisms Dehalococcoides spp. (DHC) revealed that native DHC were present in relatively low numbers (below 105 DHC/L).

Although anaerobic bioremediation has been used widely for treating ground water contaminated with chlorinated ethenes (particularly tetrachloroethene [PCE], TCE, and cis-DCE), past applications have been ineffective in low-pH aquifers due to minimal natural biodegradation activity. In addition, PCE and TCE degradation that does occur tends to accumulate cis-DCE as a terminal end product, without proceeding to ethene. Formation of large contaminant plumes in low-pH environments over time reduces utility of common remedial alternatives such as in situ chemical oxidation and decreases practicality of adjusting pH for the purpose of increasing natural biodegradation.

Bioaugmentation suitability tests for MAG-1 began with laboratory microcosm tests. Results confirmed that dechlorination was extremely limited at the low natural pH; even pH adjustment resulted in limited dechlorination by indigenous microbial populations. Testing also suggested that native DHC were unable to efficiently degrade cis-DCE and a 'cis-DCE stall' was occurring in microcosms. In contrast, microcosms inoculated with a bioaugmentation culture (SDC-9™) and adjusted to pH levels greater than 6 demonstrated progression of TCE and cis-DCE dechlorination and production of stoichiometric amounts of ethene. Microcosm results were verified and an extended laboratory column study was conducted to gather additional data on dechlorination rates, bacterial transport, and aquifer buffering.

Bioaugmentation evaluation in the field involved construction of a recirculation system to capture ground water at an extraction well and re-inject it at an injection well approximately 40 feet upgradient. The design provided a re-circulating treatment loop approximately 15 feet wide and 10 feet below ground surface (bgs). In addition, two monitoring wells were installed between the extraction and injection wells. All wells were screened across the entire contaminated saturated zone.

Laboratory tests and ground-water flow models were used to estimate the amount of sodium bicarbonate required to raise ground water pH to above 6 and to determine the amount of electron donor (sodium lactate), nutrients (diammonium phosphate), and bacteria needed to facilitate in situ bioremediation. The ground-water circulation system initially operated at a flow rate of 0.5 L/min to create a 30-day hydraulic flow time between the injection and extraction wells.

Start-up operations in late 2007 included characterization of the flow system and efforts to reduce redox potential. The first bioaugmentation event involved injection of 10 liters of SDC-9 culture (containing a DHC titer of 1011 DHC/L) directly into the recirculation loop. Upon observation of a pH spike to above pH 9 and loss of bioaugmentation substrate, a second, identical injection was conducted approximately 100 days later. Each culture cost approximately $1,000.

DHC numbers in the aquifer subsequently increased to approximately 108DHC/L in a row of monitoring wells located 20 feet from the injection wells, which suggested significant in situ growth and distribution of organisms concomitant with CVOC dechlorination. SDC-9 bioaugmentation in the re-circulation loop reduced the CVOC concentration to approximately 20 ppb within one year (Figure 1), from a combined TCE and cis-DCE concentration of 1,500 ppb prior to treatment.

DOD evaluates bioaugmentation for remediating low-pH aquifers

It was difficult to achieve and maintain an acceptable pH in the treatment zone. During the first three months of operation, more than 1,700 lbs of sodium bicarbonate were injected into each recirculation loop. To reduce the excessive labor or automated loading costs associated with administering this volume, which was significantly higher than estimates derived from earlier testing and the column study, sodium carbonate was selected as an alternate buffering agent.

The sodium carbonate additions were found to cause the pH spike observed approximately three months after the first injection. Buffering in the form of carbonate also led to excess fouling of the injection wells, which necessitated periodic well redevelopment and a reduced volume of buffering agent. To maintain a suitable pH during the remainder of the demonstration, a total of 2,400 pounds of sodium carbonate was used.

Analysis of post-injection data indicated that DHC growth could not be stimulated in the aquifer without pH adjustment, despite existence of the organisms at measurable levels, and that pH adjustment alone could not facilitate biological degradation of cis-DCE and VC. Results demonstrated that complete dechlorination could be achieved by concurrently adjusting the pH to a level greater than 6 and inoculating the formation with bioaugmentation cultures.

Full-scale bioaugmentation at Fort Dix is expected to begin this summer, in accordance with final cleanup goals set by the New Jersey Department of Environmental Protection. The demonstration was conducted under DOD`s Environmental Security and Technology Certification Program, in conjunction with development of bioaugmentation guidance for DOD and the U.S. Department of Energy.

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