The volume, extent, broad distribution, and complexity of DOE's contaminated soils and groundwater pose a unique and formidable challenge: to develop scientifically sound characterization, remediation, performance assessment, and long-term monitoring technologies that are cost-effective and result in acceptable risk to human health and the environment. In situ approaches to restoration and containment, including bioremediation, are methods of choice over excavation and pump-and-treat because much of the contamination is widely dispersed in the environment, is present in relatively dilute concentration, or is otherwise inaccessible because of its depth or its location beneath structures.
Environmental restoration is complicated by the diversity of subsurface environments at contaminated sites across the DOE complex footnote . These sites are located in arid, non-arid, cold, and hot climates and include a diversity of geological settings and depositional environments with unique conditions that must be understood and managed for bioremediation or other restoration methods to be effective. In addition, DOE's history of developing, manufacturing, and managing nuclear materials and weapons resulted in environmental contamination with complex and exotic mixtures of compounds. Management practices over the 50-year period resulted from direct disposal of wastes into cribs and trenches or by deep injection and indirect contamination following the loss of integrity of landfills and leaking underground storage tanks (Fig. 2). The costs associated with remediating many of DOE's largest plumes of contaminated groundwater and sediments have not been determined, but estimates to clean up DOE's contaminant soils, sediments, and groundwater range from tens to hundreds of billions of dollars (DOE, 1995).
The NABIR program will include leaders from the contaminant plume containment, remediation, and landfill focus areas in an advisory capacity. It will coordinate its efforts to ensure the two-way transfer of information and knowledge.
Classes of chemical contaminants present in sediments and groundwater at more than half of all DOE facilities include fuel hydrocarbons, chlorinated hydrocarbons, metals, and radionuclides. Compound classes most frequently detected at DOE sites are metals and chlorinated hydrocarbons, radionuclides, anions, fuel
hydrocarbons, and ketones. Organic acids, phthalates, explosives, alkyl phosphates, complexing agents (i.e., EDTA, DTPA, and NTA), and pesticides are also significant contaminants at individual sites. More than half of the DOE sites evaluated by Riley et al. (1992) were contaminated with mixtures of two or more compound classes. The most common mixtures reported were metals and radionuclides. Co-disposal mixtures of compounds has resulted in modified transport and toxicity properties that can increase ecological or health risk. For example, complexation with ligands enhances the mobility of metals and radionuclides, alters behavior at mineral surfaces, and affects availability to microorganisms.
Figure 2. The DOE legacy of contaminant plumes: 2500 billion liters of contaminated groundwater and 200 million cubic meters of contaminated soil.
Acquisition of this knowledge base must be accompanied by parallel advances in the development of effective performance assessments and advanced computational methods, identification of acceptable cleanup end points, and research in ethical, legal, and social impacts (ELSI). Researchers must identify and address stakeholders' issues and coordinate their efforts with other federal agencies and industry.
A full understanding of the natural processes involved in natural and accelerated bioremediation depends on the ability to build on laboratory experimentation by directly investigating phenomena in the field and conducting field experiments. Many of the scientific and technical results from a bioremediation R&D program on in situ treatment will also be applicable to waste management and minimization efforts such as the design and operation of bioreactors to treat a broad spectrum of dilute and concentrated waste streams from ongoing and past DOE operations.
