Biogeochemical Processes: The Foundation for In Situ Bioremediation

Transformation and degradation processes differ; but the fate of many contaminants, from petroleum hydrocarbons to radionuclides, is influenced by microbial activity.

Over the past two decades, it has become widely accepted that microorganisms, and to a lesser extent plants, have the ability to transform and degrade many types of contaminants. Transformation and degradation processes differ depending on physical environment, microbial communities, and contaminants; but the fate of many contaminants, from petroleum hydrocarbons to radionuclides, is influenced by microbial activity. These processes form the foundation for bioremediation both natural bioremediation, which relies on intrinsic rates and processes, and accelerated bioremediation, which seeks to enhance desirable processes through the addition of amendments.

Microorganisms degrade or transform contaminants by a variety of mechanisms. Petroleum hydrocarbons, for example, are converted to carbon dioxide and water or are used in generating new cells by aerobic bacteria. In this case, microorganisms use the petroleum hydrocarbons as a primary food source. Chlorinated hydrocarbons can be degraded, but the degradation takes place as a secondary or co-metabolic process. Enzymes created during aerobic utilization of carbon sources such as methane fortuitously degrade the chlorinated solvents. Under anaerobic conditions, chlorinated solvents such as trichloroethylene (TCE) are degraded through a sequence of steps, where some of the intermediary by-products may be more hazardous than the parent compound (e.g., vinyl chloride). Inorganic contaminants such as nitrate can also be degraded by microbial activity and plants. Depending on environmental conditions, nitrate can be converted to nitrogen gas or used as a nutrient to support cell production.

While metals and radionuclides cannot be degraded by biological activity, they can be transformed from one chemical form to another or transported from soils to above- and below-ground plant tissues. Fungi, for example, can convert dissolved arsenic and selenium to gaseous forms through methylation. Bacteria similarly have been shown to reduce mercury to its volatile elemental form. Bacteria have been shown to change the oxidation state of some heavy metals (e.g., chromium, selenium, and mercury) and radionuclides (e.g., uranium) by using them as electron donors or acceptors. In some cases, the solubility of the altered species will increase and consequently the altered species will more easily be flushed from the geologic host material. In other cases, the opposite will occur, and the contaminant will be immobilized in situ. The immobilization (or sequestration) of chemical and radioactive contaminants through in situ biological processes can make an important contribution to site remediation.

Because of the impressive capabilities of microorganisms to degrade and transform contaminants, systematic attempts to understand and harness these capabilities will provide tremendous benefits.

In light of the impressive capabilities of microorganisms and plants to degrade and transform contaminants, systematic attempts to understand and harness these abilities should provide tremendous benefits. In fact, these remediation ideas are not new: they have provided the foundation for many ex situ waste treatment processes (including sewage treatment) and a host of in situ bioremediation methods that are in practice today (Hinchee et al., 1994). For example, bioremediation technologies such as bioventing and land farming have become accepted practice for remediating petroleum hydrocarbon-contaminated soils and sediments (CISB, 1993). In addition, several projects, including one at DOE's Savannah River Site, have demonstrated that TCE can be co-metabolically degraded in situ by providing methane as a carbon source. Explosives and polychlorinated biphenyls (PCBs) have been biodegraded in field-scale experiments (Harkness et al., 1993). Bacterial transformation and immobilization of trace metals such as selenium also have been implemented on a limited basis (Benson et al., 1993). Technologies to promote volatilization and dissipation of selenium have been demonstrated in surface water and soils (Frankenberger and Benson, 1994).

Over the last decade, progress has been made in expanding the number and type of contaminants to which bioremediation can be applied. Significant progress has also been made in the number of practical methods for implementing in situ bioremediation. For example, alternative strategies have been developed for delivering chemical additives, such as oxygen. Chemical additives for increasing the bioavailability of recalcitrant organics have been identified. Techniques such as hydrofracturing have been developed for improved delivery of nutrients or microorganisms in low-permeability geologic media. In addition, methods have been developed for creating passive treatment systems such as biofilters (Taylor et al., 1993). Novel concepts for using microbially produced biopolymers as in situ plugging agents have also been explored (Li et al., 1994). These advances are important steps towards establishing bioremediation as one of the viable solutions for in situ remediation of contaminant mixtures in a wide range of complex environments. And of great importance, data are now available that can demonstrate the cost-effectiveness of bioremediation in comparison to physical and chemical remediation methods (Saaty and Booth, 1994; Wijesinghe et al., 1992; Atlas, 1995).

NABIR will build on all recent activities and advances in bioremediation and will focus on filling the existing knowledge gaps.

NABIR will build on all recent activities and advances in bioremediation and will focus on filling existing knowledge gaps. As described in the next section, significant work must be done to build a more robust scientific foundation to support widespread application of bioremediation, especially of contaminant mixtures, metals, and radionuclides. In addition, NABIR will identify the ethical, legal, and social impacts that must be addressed before natural and accelerated bioremediation at DOE facilities is acceptable to both the public and the regulatory authorities.

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