A mixed culture was enriched from surface soil obtained from an eastern United States site highly contaminated with chromate. Growth of the culture was inhibited by a chromium concentration of 12 mg/L. Another mixed culture was enriched from subsurface soil obtained from the Hanford reservation, at the fringe of a chromate plume. The enrichment medium was minimal salts solution augmented with acetate as the carbon source, nitrate as the terminal electron acceptor, and various levels of chromate. This mixed culture exhibited chromate tolerance, but not chromate reduction capability, when growing anaerobically on this medium. However, this culture did exhibit chromate reduction capability when growing anaerobically on TSB. Growth of this culture was not inhibited by a chromium concentration of 12 mg/L. Mixed cultures exhibited decreasing diversity with increasing levels of chromate in the enrichment medium. An in situ bioremediation strategy is suggested for chromate contaminated soil and groundwater.
Hexavalent chromium reduction kinetic parameters were estimated for several mixed cultures using molasses as the carbon source and nitrate as the terminal electron acceptor. Mixed cultures were enriched from diverse environmental sources, and kinetic parameter comparisons are made between the consortia and a pure culture grown under the same conditions. A statistical analysis of the results indicated that some kinetic parameters exhibited significant differences, whereas others did not. It should be noted that those parameters with statistically significant differences were nonetheless numerically similar. Conservative values, therefore, could be assumed, or site‐specific data obtained, for those parameters to design in situ bioremediation of chromium‐contaminated sites.
No abstract
The destruction of the Unit 4 reactor at the Chernobyl Nuclear Power Plant resulted in the generation of radioactive contamination and radioactive waste at the site and in the surrounding area (referred to as the Exclusion Zone). In the course of remediation activities, large volumes of radioactive waste were generated and placed in temporary near-surface waste storage and disposal facilities. Trench and landfill type facilities were created from 1986-1987 in the Chernobyl Exclusion Zone at distances 0.5-15 km from the nuclear power plant site. This large number of facilities was established without proper design documentation, engineered barriers, or hydrogeological investigations and they do not meet contemporary waste-safety requirements. Immediately following the accident, a Shelter was constructed over the destroyed reactor; in addition to uncertainties in stability at the time of its construction, structural elements of the Shelter have degraded as a result of corrosion. The main potential hazard of the Shelter is a possible collapse of its top structures and release of radioactive dust into the environment. A New Safe Confinement (NSC) with a 100 y service life is planned to be built as a cover over the existing Shelter as a longer-term solution. The construction of the NSC will enable the dismantlement of the current Shelter, removal of highly radioactive, fuel-containing materials from Unit 4, and eventual decommissioning of the damaged reactor. More radioactive waste will be generated during NSC construction, possible Shelter dismantling, removal of fuel-containing materials, and decommissioning of Unit 4. The future development of the Exclusion Zone depends on the future strategy for converting Unit 4 into an ecologically safe system, i.e., the development of the NSC, the dismantlement of the current Shelter, removal of fuel-containing material, and eventual decommissioning of the accident site. To date, a broadly accepted strategy for radioactive waste management at the reactor site and in the Exclusion Zone, and especially for high level and long-lived waste, has not been developed.
The Hanford Area is a U.S. Department of Energy (DOE) reservation in Southeastern Washington, where the primary mission for nearly fifty years was production of nuclear weapons materials. It is now the nation's largest superfund site and its sole mission is environmental remediation of the mixed wastes generated during plutonium production. A large fraction of these wastes are stored in 177 underground tanks and are the subject of the DOE's Tank Waste Remediation System (TWRS) Program. Since its inception the TWRS Program has been managed by a Maintenance and Operations (M&O) contractor .The DOE is now considering the privatization of a portion of this program and has recently issued a Request for Proposals (RFP) seeking new, qualified, private vendors. Successful bidders will be expected to build waste processing facilities with their own financial resources and to recover their costs by charging fixed prices for the various products delivered to the DOE. Because the TWRS Program is such a large, complex, and expensive undertaking, the privatization initiative will be conducted in two phases: a small proof-of-concept phase, followed by full-scale production. A primary objective of the proof-of-concept phase is to test this new contracting approach by determining the interest of private companies and demonstrating their technical capabilities.The key to a successful demonstration is establishing the right set of requirements to be satisfied by the private vendors. These requirements must be consistent with the existing requirements set developed over the past three years by the M&O contractor. This paper presents the results of a systems engineering effort that was conducted in support of the RFP preparation and had to be coordinated with an ongoing program. Much of the effort was focused on the specification of new proof-of-concept requirements that are directly traceable to corresponding requirements in the M&O's RDD-100® database. A new functions and requirements database was created for this first privatization phase using CORE®, a systems engineering support tool, produced by Vitech Corporation.
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