Objective:

Monitored natural attenuation (MNA) is a cost-effective technology that is applicable to many, but not all sites. The U.S. Environmental Protection Agency (EPA) estimates that 20 percent of sites contaminated with chlorinated solvents are amenable to natural attenuation, and this number may actually be less. An example of the limitation of natural attenuation at chlorinated solvent-contaminated sites is the commonly observed slow-down or stall of reductive dechlorination at cis-dichloroethene (cis-DCE). The objective of this project is to demonstrate and validate the ability of electron shuttles to enhance complete dechlorination.

Technology Description:

Electron shuttles are compounds that stimulate biodegradation of contaminants by facilitating the transfer of electrons to and from bacteria. These compounds can accept electrons from bacteria that are oxidizing a contaminant such as vinyl chloride (VC) and subsequently donate those electrons to a terminal electron acceptor such as ferric iron. Electron shuttles can facilitate oxidative and potentially reductive modes of contaminant biodegradation. Examples include humic acids, quinones such as the humic acid analogue, anthraquinone disulfonic acid (AQDS), phenazines, and thiol-containing molecules such as cysteine.
The project team will utilize two different approaches. The first involves direct injection of electron shuttles into an injection gallery comprised of three to five wells installed within a cis-DCE/VC plume. Concentrated solutions of humic acids and a bromide tracer will be injected and oxidative degradation of cis-DCE and VC will be monitored on a monthly basis. The second approach introduces electron shuttles passively through a 10 to 20-foot long trench of bark mulch. The difference between the direct injection and the bark mulch trench approach is that the latter releases organic carbon in addition to humic acids and creates reducing conditions.

Expected Benefits:

EPA estimates that close to 3,000 Department of Defense (DoD) chlorinated solvent-contaminated sites will eventually require remediation. Even at sites currently undergoing remediation, partial reductive dechlorination of trichloroethene (TCE) often results in accumulation of cis-DCE and VC. Both cis-DCE and VC can be remediated using this technology. Since ferric iron is a prevalent electron acceptor, this technology is applicable to numerous sites at substantial savings when compared to the pump-and-treat technology. (Anticipated Project Completion - 2008)

Contact:

Ms. Carmen LeBrón
Naval Facilities Engineering Service Center
1100 23rd Ave., ESC411
Port Hueneme, CA 93043-4370
Telephone: (805) 982-1616
Fax: (805) 982-4304
E-mail: lebronca@nfesc.navy.mil