Scientists and environmental engineers have used bacteria to reduce by 98 percent the quantity of trichloroethylene, or TCE, in groundwater. According to a press release dated February 16, Lawrence Livermore National Laboratory scientists in partnership with Brown and Caldwell, an environmental engineering firm in Sacramento, California successfully demonstrated the use of bacteria to reduce levels of the solvent TCE from 400 parts per billion to less than 10 parts per billion.
The test was conducted in January this year at the Chico Municipal Airport in Northern California at a site Brown and Caldwell are cleaning up. Laboratory scientists injected about 13 pounds of the bacteria Methylosinus trichosporium OB3b 90 feet below the surface into a contaminant plume measuring approximately two kilometers long (one and one-quarter miles) by half a kilometer wide (one-third mile). The microbes clung to the soil particles, forming a spherical microbial mass roughly a meter in diameter around the end of the injection well. As water was drawn from the well, it passed through the microbial biofilter where enzymes in the bacteria acted on the TCE, breaking it down into carbon dioxide and water. Water brought to the surface was found to be 98 percent free of TCE.
TCE contamination is a problem at numerous U.S. Department of Energy, Department of Defense, and Environmental Protection Agency sites. TCE is a solvent used to clean tools and machine parts. During the days of nuclear production, TCE was used to purify uranium and plutonium products. Used TCE carrying small quantities of plutonium and uranium was disposed in ponds, where it was believed TCE would evaporate. Instead the TCE migrated through the soil where it contaminated groundwater. The microbes used at Chico were grown in a special bioreactor facility at Livermore and continue to degrade contaminants for a number of weeks before they either die or go into an inactive state. To continue the cleanup process, new microbes would have to be introduced.
The introduction of bacteria to bioremediate contaminated sites is an innovative concept. In previous bioremediation attempts, researchers injected nutrients into sites to increase the activity levels of resident microbes. Nutrient injection achieves large microbe populations, but often destruction of contaminants is not effective. Instead of feasting on contaminants, the choosy microbes fatten up on nutrients. Also, the nutrients may stimulate growth of other kinds of bacteria, causing proliferation of types not helpful in the cleanup. Another problem with nutrient injection is determining the correct nutrient mix to grow desired microbes without causing a blooming of bacteria around the bottom of the well hole. Richard Knapp, a geochemical engineer in the Laboratory's Environmental Programs Directorate says, "Ours is a revolutionary concept because we inject bacteria without nutrients. It is the first time anyone's accomplished this kind of bioremediation of TCE in groundwater."
Biofilter technology may prove to be a cheaper and faster alternative to the baseline technology of pump and treat. Future LLNL efforts will focus on development of custom-designed biofilters to fit particular contaminant and subsurface conditions.
