I. Introduction
A. THE PROBLEM: DOE LEGACY WASTES IN THE SUBSURFACE
For more than 50 years, the U.S. created a vast network of more than 113 facilities for research, development, and testing of nuclear materials. As a result of these activities, subsurface contamination has been identified at over 7,000 discrete sites across the U.S. Department of Energy (DOE) complex. With the end of the Cold War threat, the DOE has shifted its emphasis to remediation, decommissioning, and decontamination of the immense volumes of contaminated groundwater, sediments, and structures at its sites. DOE is currently responsible for remediating 1.7 trillion gallons of contaminated groundwater, an amount equal to approximately four times the daily U.S. water consumption, and 40 million cubic meters of contaminated soil, enough to fill approximately 17 professional sports stadiums.*
It is estimated that more than 60% of DOE facilities have groundwater contaminated with metals or radionuclides. The only contaminant that appears more often than metal and radionuclide contaminants in groundwater is chlorinated hydrocarbons. More than 50% of all soil and sediments at DOE facilities are contaminated with metals and radionuclides, the contaminants found with the highest frequency in soil at all DOE waste sites.† Indeed, while virtually all of the contaminants found at industrial sites nationwide can also be found at DOE sites, many of the metals and especially the radionuclides found on DOE sites are unique to those sites.
Current technology for treatment of groundwater contaminated with metals and/or radionuclides is “pump and treat,” followed by disposal or reinjection of treated water. This process can be costly and inefficient due to the difficulty of completely removing the contaminated groundwater and sorption of contaminants on mineral surfaces. DOE’s Office of Environmental Management (EM), which is responsible for the cleanup, has stated that advances in science and technology are critical for DOE to reduce costs and successfully address these long-term problems.* DOE’s Environmental Quality R&D Portfolio includes environmental restoration and long-term stewardship as its highest priorities. A recent analysis of the portfolio (September 2000) suggested that R&D in these two areas is inadequate. The NABIR program aims 1) to provide the fundamental knowledge to support the development of new bioremediation technologies and 2) to advance the understanding of key processes that control the effectiveness of containment as a means of long term stewardship.
NABIR has the distinction of being the only federal program that funds fundamental bioremediation research on metal and radionuclide contaminants in the environment. The program’s greatest strength is in focusing talents and expertise from many disciplines to address challenging research questions. The products from NABIR will influence the development of effective bioremediation technologies as well as contribute new knowledge about the function of subsurface ecological systems at the microbiological and geochemical levels. These advances can lead to more effective stewardship of natural resources as well as to remediation of DOE sites.
B. BIOREMEDIATION OF METALS AND RADIONUCLIDES
The catalytic potential of microorganisms in nature is enormous, and yet still relatively untapped for use in environmental cleanup. Bioremediation is the use of microorganisms to decrease, eliminate, or contain hazardous and/or radioactive wastes to environmentally safe levels. While bioremediation of organic contaminants involves their transformation to benign products such as carbon dioxide, bioremediation of metals and radionuclides involves their removal from the aqueous phase to reduce risk to humans and the environment. Microorganisms can directly transform metals and radionuclides by changing their oxidation state to a reduced form that leads to in situ immobilization. Or, microorganisms can indirectly immobilize metals and radionuclides through the reduction of inorganic ions which can, in turn, chemically reduce contaminants to less mobile forms. The long term stability of these reduced contaminants is as yet unknown. Other mechanisms whereby microorganisms can influence mobility include alteration of pH, Eh, oxidation and complexation.
Currently, the fundamental knowledge that would allow the cost-effective deployment of in situ subsurface bioremediation of metals and radionuclides is lacking. Research on in situ bioremediation of metals and radionuclides has received less attention than research on solvents, fuels, and other organic contaminants; however, successful in situ applications of bioremediation of petroleum products and chlorinated solvents provide experience from which scientists can draw. Scientists in DOE's NABIR program are exploring the potential of bioremediation to solve DOE problems of radionuclide and metal contaminants in subsurface environments.
The focus of the NABIR program is on radionuclides and metals that 1) are of great concern at DOE sites, and 2) are tractable by means of bioremediation. Thus, research is focused on the metals chromium and mercury, and the radionuclides uranium, technetium and plutonium. Radioactive contaminants such as tritium and cobalt are not a focus because of their relatively short half lives, and strontium and cesium are not addressed because they are not readily amenable to biotransformation. Research is focused on the subsurface below the zone of root influence and includes both the vadose (unsaturated) zone and the saturated zone (groundwater and sediments). NABIR research is oriented toward application in areas that have low levels of widespread contamination because it is too costly to clean up those situations with existing technologies. Chromium, uranium and technetium can be especially mobile in the subsurface under certain conditions; they are risk-driving contaminants at some DOE sites. The effects of co-contaminants such as nitrate, complexing agents (such as EDTA) and chlorinated solvents (such as trichloroethylene and carbon tetrachloride) on the behavior of metals and radionuclides in the subsurface is also of interest to the NABIR program.
