Superfund Research Program
Bioremediation of Groundwater Contaminated with Carbon Tetrachloride
Carbon tetrachloride is a suspected human carcinogen that has been found at about 20% of the waste sites on the Superfund program's National Priority List. This persistent environmental contaminant has migrated into the groundwater on some Superfund sites as a result of its widespread past use as a grain fumigant, a degreasing agent, and a solvent. Once an aquifer is contaminated with carbon tetrachloride, it may be unusable as a source of drinking water for decades because carbon tetrachloride tends to degrade very slowly in subsurface environments. This groundwater contaminant not only contributes to the loss of usable water resources, but it can also migrate to nearby wetlands, streams, and lakes, where it may impact aquatic ecosystems.
To protect humans from the potential health risks of carbon tetrachloride and prevent further migration of contamination, environmental scientists have been challenged to devise ways to cleanup affected groundwater. Bioremediation - the use of microorganisms to convert hazardous substances to less toxic or more easily degraded byproducts - is showing considerable promise for the cleanup of carbon tetrachloride in aquifers. Part of what makes this technology so desirable for groundwater remediation is that it can be adapted to work in situ, or where contamination resides in the subsurface.
A team of researchers at Michigan State University has developed a treatment system for the in situ bioremediation of carbon tetrachloride in groundwater. The system uses a microbe known as Pseudomonas stutzeri KC (strain KC) that can convert carbon tetrachloride into carbon dioxide and other harmless end products. Unlike other naturally occurring microbes found to degrade carbon tetrachloride, strain KC is unique in that it does not produce toxic chloroform. This microbe also thrives in anoxic environments, such as those found in aquifers, so it is well suited to grow in the subsurface for in situ applications.
Throughout 1998, the researchers have conducted a field demonstration at a carbon tetrachloride contaminated aquifer in southwest Michigan. The overall design of the demonstration includes a series of 15 delivery wells spaced one meter apart, an aboveground system for weekly addition of nutrients and microbes, and an extensive monitoring well system. The delivery wells were used to create a subsurface "biocurtain" 30 to 80 feet below the ground surface that intercepted about 50 feet of the contaminant plume. Strain KC, along with nutrients that stimulate the growth and metabolic activity of the microbe, was injected into the contaminated aquifer to create the microbial curtain. Contaminated groundwater was treated as it passed through the biocurtain, and monitoring wells downstream of the curtain gauged the effectiveness of treatment.
After only three months, the biocurtain reduced carbon tetrachloride concentrations averaging 30 ppb in the upgradient groundwater to below 1 ppb at most monitoring stations, demonstrating the on-site transformation of carbon tetrachloride. The microbe was also very successful in colonizing the aquifer, demonstrating the long-term survival of strain KC underground. These results indicate strain KC can be successfully used in a full-scale system.
While this approach to in situ bioremediation has been successful, the researchers are continuing to investigate ways to improve the technology. The scientists are evaluating other methods of delivering organisms and substrates to the subsurface to further reduce capital and operating costs. They are also carefully monitoring any by-products that may result from the bioremediation to ensure that toxic by-products are not being produced. Furthermore, the researchers have identified genes in strain KC associated with the degradation of carbon tetrachloride, and hope to use this knowledge to monitor and increase the activity of strain KC, as well as to increase the range of chemicals that can be detoxified by strain KC.
This research is significant for developing a successful in situ remediation strategy for carbon tetrachloride. By treating the contaminant in the subsurface, this technology eliminates the need to pump groundwater to the surface for treatment, which not only reduces the above ground disposal problems associated with conventional "pump and treat" methods, but also minimizes worker contact with the contaminant. Moreover, the full-scale treatment system, including the nutrient and microbe delivery methods, is functioning well and can be readily applied to other sites and other organisms. Altogether, this in situ bioremediation technology shows significant promise as an effective, low-cost alternative or supplement to the current methods of cleaning up carbon tetrachloride in groundwater.
For More Information Contact:
Michael J. Dybas
Michigan State University
Center for Microbial Ecology
East Lansing, Michigan 48824
To learn more about this research, please refer to the following sources:
- Dybas MJ, Barcelona MJ, Bezborodnikov S, Davies S, Forney L, Kawka O, Mayotte T, Sepulveda-Torres Ld, Smalla K, Sneathen M, Tiedje JM, Voice TC, Wiggert D, Witt ME, Criddle CS. 1998. Pilot-scale evaluation of bioaugmentation for in situ remediation of a carbon tetrachloride-contaminated aquifer. Environ Sci Technol 32(22):3598-3611.
- Mayotte T, Dybas MJ, Criddle CS. 1996. Bench-scale evaluation of bioaugmentation to remediate carbon tetrachloride-contaminated aquifer materials. Ground Water 34:358-367.
- Dybas MJ, Tatara G, Criddle CS. 1995. Localization and characterization of the carbon tetrachloride transformation activity of Pseudomonas sp. strain KC. Appl Environ Microbiol 61(2):758-762. PMID:16534941
- Tatara G, Dybas MJ, Criddle CS. 1993. Effects of medium and trace metals on the kinetics of carbon tetrachloride transformation by Pseudomonas sp. strain KC. Appl Environ Microbiol 59(7):2126-2131. PMID:8357248
- Criddle CS, DeWitt JT, Grbic-Galic D, McCarty PL. 1990. Transformation of carbon tetrachloride by Pseudomonas sp. strain KC under denitrification conditions. Appl Environ Microbiol 56(11):3240-3246. PMID:2268146
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