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As a result of ending of cold war, U.S. Departments of Energy and Defense are faced with a new task of disposing tremendous amount of nuclear waste. The top left graph shows a storage site in Hanford, WA. These huge storage tanks are not permanent. When they are buried underground, they tend to leak with time. As a result, a plume of contaminants, including heavy metals and radionuclides along with organic compounds, migrates into our drinking water (top right). The inorganic contaminants tend to adsorb onto mineral surfaces, particularly oxides and hydroxides. Coincidently, negatively charged groundwater bacteria also stick to these mineral surfaces. The close contact between bacteria and the contaminants makes bacteria ideal candidate for bioremediation. Most living things, including humans, gain energy by transferring electrons from electron donor (organic carbon from food) to acceptor (oxygen, O2). In anaerobic (oxygen-free) subsurface environment (many aquifers), bacteria have abundant organic carbon from organic contaminants, but lack O2 for breathing. To survive, bacteria have to use oxidized forms of metals as electron acceptor (lower left, bacteria living on magnetite) in substitution for O2. In this electron transfer process and resultant change of oxidation state of a variety of metals, bacteria immobilize U(VI), Mn(IV), Cr(VI), and Tc(VII). When these elements are immobilized, they are removed permanently from groundwater. For example, U(VI) is soluble in groundwater. But when U(VI) is reduced to U(IV), U(IV) will form a mineral, urananite and is therefore removed from drinking water. Bacteria also mobilize Cd, Cr, Ni, Pb and Zn as a result of oxidation state change. If this takes place, groundwater can be pumped to surface, and cleaned up. Cleaned groundwater can be pumped back down to aquifer system for consumption.

Natural bacteria are unlikely to exist in large numbers. Therefore, meaningful environmental remediation using bacteria involves two ways: biostimulation and bioaugmentation. Biostimulation involves stimulation of natural bacterial activity by feeding them with essential nutrients. An alternative is to augment bacterial numbers by injecting laboratory-grown bacterial culture into a contaminant site. This process is called bioaugmentation. The vital requirement for the success of bioaugmentation strategy is delivery of the injected bacteria to a zone of contamination. Due to high cost of installing wells inaccessability of ofter highly radioactive contamination sites, it is desirable to inject bacteria from one well with some distance away from the zone of contamination, and have the inejcted bacteria cover all contaminant targets. If the injected bacteria are stuck near the point of injection, they will not be useful to clean-up contaminants. Injected bacteria tend to attach to mineral surfaces due to electrostatic interactions between negatively charged bacteria and positively charged oxides and hydroxides (where contaminant metals reside), and other processes, such as hydrophobic interactions. Therefore, it is essential to understand mechanisms of bacterial transport in natural conditions, and after this step, we can devise ways to have finite partitioning of injected bacteria between water and oxide and hydroxide surfaces. Laboratory (top graphs) and field-scale (bottom graphs) bacterial transport experiments are underway to investigate physical, chemical and biological factors responsible for the bacterial migration in natural porous media (sediments). This is a multi-displinary effort funded by the Department of Geology with principal investigators in microbiology, geology, environmental engineers, and chemists from the entire nation. The top left graph is a photograph showing laboratory setup with the core standing vertically, and injection takes place from bottom up. The top right shows kinds of data we gather, including bacterial breakthrough at the effluent and retention inside the core. The bottom left is a photo showing injection and sampling system in our field experimental sites in Oyster, Virginia, and the bottom right shows a plume of bacteria migrating through the experimental site (over 30 m distance in the longest dimension).