Cometabolism involves the injection of a dilute solution of nutrients such as methane and oxygen into the contaminated groundwater or soil. The microbes that metabolize these nutrients produce enzymes that react with the organic contaminant and degrade it to harmless minerals. The addition of methane or methanol has been demonstrated to degrade chlorinated solvents, such as vinyl chloride (VC) and trichloroethylene (TCE). Toluene, propane and butane have also been used to support the cometabolism of TCE.
Limitations and Concerns
VC concentrations may increase, as it is a breakdown byproduct of chlorinated solvents.
Where the subsurface is irregular (i.e., heterogeneous), it is very difficult to deliver the methane solution to every portion of the contaminated zone. Consequently, some contaminants sorbed onto small particles may not be degraded and will remain in the subsurface. Over the long term, these contaminants may desorb due to other environmental factors and re-contaminate the groundwater.
To use cometabolism in areas where there is clay, the area must be flooded with the methane-oxygen solution to achieve greater contact with the contaminants. This may increase the groundwater flow rate, and the addition of downstream wells may be required to capture contaminated water. A surface treatment system, such as air stripping or carbon adsorption, may be required to treat extracted groundwater.
Higher permeability zones are cleaned up much faster because groundwater flow rates are greater.
Safety precautions (such as removing all ignition sources in the area) must be used when handling methane.
Certain fuels can support the cometabolism of chlorinated solvents, and it is speculated that at many sites where fuel and chlorinated volatile organic compounds (VOCs) are combined, cometabolism may be “naturally” occurring.
To use this technology, the subsurface must be well characterized.
Technology Development Status
This technology is commercial.
Other Resources and Demonstrations
See description of Enhanced Bioremediation.
A field demonstration project was conducted at Moffett Naval Air Station, California, and another was conducted at the Energy Department’s Savannah River site, South Carolina. A full-scale application is in place at Air Force Plant 44 in Tucson, Arizona.
See http://water.usgs.gov/pubs/wri/wri994285/text/chlorinated.html for a technical overview.