Monitored natural attenuation (MNA) is a cost-effective remediation technology that is applicable to many sites. One of its limitations at sites contaminated with chlorinated solvents, however, is the commonly observed slow-down or stall of reductive dechlorination at cis-1,2-dichloroethene (cis-DCE). Electron shuttles, such as humic acid, fulvic acid and anthraquinone disulfonic acid (AQDS), possibly could be used to encourage complete dechlorination.

Electron shuttles are compounds that stimulate biodegradation of contaminants by facilitating electron transfer to and from bacteria. These compounds can accept electrons from bacteria that are oxidizing a contaminant such as vinyl chloride (VC) or cis-DCE and subsequently donate those electrons to a terminal electron acceptor like ferric iron (Fe³⁺). In the oxidative role, an electron shuttle serves as the initial electron acceptor accepting electrons from bacteria following oxidation of the contaminant. If another terminal electron acceptor such as Fe³⁺ is present, the reduced electron shuttle donates electrons to Fe³⁺ and, in doing so, is regenerated and ferrous iron (Fe²⁺) is produced. The regenerated electron shuttle is then capable of facilitating additional contaminant oxidation.

In a typical remediation scenario, the selected electron shuttle formulation would be added to groundwater through a series of injection wells or in a trench. Ideally, the electron shuttle formulation would be transported downgradient along the natural groundwater flow path, and as the formulation traveled downgradient, electron shuttles would accept electrons from bacteria that have oxidized VC and cis-DCE. The reduced electron shuttles would then donate the electrons to terminal electron acceptors and the regenerated electron shuttles would continue to shuttle electrons between the bacteria and the terminal electron acceptors until they were transported beyond the plume boundary or capture zone.

The objective of this project was to demonstrate and validate the ability of electron shuttles to enhance complete dechlorination of contaminants at chlorinated solvent-contaminated sites, focusing on the oxidative biodegradation VC and cis-DCE.

The electron shuttling capacities of 15 commercially available humic and fuvic acid products and six non-humic acid products were tested on soil and groundwater samples from the Space and Naval Warfare Systems Center Old Town Campus (SSC-OTC) site in San Diego, California. Of these products, three were then evaluated for their ability to promote anaerobic oxidation of radiolabeled VC. These products included a commercial humic acid product (Monterey Ag-Resources HA-12), a sulfonated humic acid (LignoTech Borregro HA-1), and indigo disulfonate. AQDS was also used for comparison because of its previously demonstrated ability to shuttle electrons and promote VC transformation.

The microcosm study, conducted by the U.S. Geological Survey, evaluated the fate of radiolabeled VC in the presence of the electron shuttling compounds. Analysis of select microcosms from each treatment was performed at 30, 95, and 190 days. After approximately 30 days, it was determined that a full time-point analysis would be performed following 95 days incubation. Subsequent to this evaluation, there was no evidence of ¹⁴C-VC loss or accumulation of ¹⁴C-products in any treatment system, and it was concluded that the biodegradation activity was low or insignificant. Another full time-point assessment was conducted after approximately 190 days of incubation, and based on the results, it was concluded that a potential for VC biodegradation was present in SSC-OTC site sediments but that the activity was low. Ultimately, the data indicated that electron shuttles with demonstrated electron shuttling capacity are not necessarily capable of promoting consistent anaerobic oxidation of VC to carbon dioxide in field samples.

Efforts were then redirected towards two related and parallel ventures that will facilitate application of in situ biogeochemical transformation processes and result in more cost-effective ways of mitigating the risks associated with inorganic and organic contamination of soil, groundwater, and sediment. The first effort was a workshop on in situ biogeochemical transformation, and the second involved sampling and analysis of Air Force biowall sites where biogeochemical transformations appeared to have been occurring. Workshop findings have been published, and a biowall analysis report is in preparation.

In this project, multiple compounds were evaluated for their potential to enhance dechlorination of VC and cis-DCE in SSC-OTC sediments. However, none of these products were capable of consistently promoting VC oxidation to carbon dioxide or reduction of ethane in groundwater and soil from the site. Whether these compounds are capable of promoting this activity at other sites is unknown. Nevertheless, use of electron shuttles for promoting oxidative transformation of VC does not appear to be warranted for the SSC-OTC groundwater.