1. Field of the Invention
The present invention relates to a method for in situ bioremediation of contaminated soil and groundwater. In particular, the invention relates to remediation of contaminated soil and groundwater by the injection of nutrients to stimulate growth of pollutant-degrading microorganisms.
2. Discussion of Background
Manufacture, transportation, and use of chlorinated hydrocarbons often results in accidental releases of these pollutants that contaminate subsurface soils and groundwater. Such contamination damages the local ecosystem, posing serious potential health problems if local groundwater is used as a source of drinking water or irrigation water, or if the soil is used for growing crops. Adjacent ecosystems may be affected if the contaminants migrate from the site with the flow of groundwater. Contamination of groundwater by organic compounds is recognized as one of the most important pollution problems of the industrialized nations. For example, it is estimated that over 15% of community drinking water supplies in the United States are currently contaminated with chlorinated hydrocarbons. As reliance on groundwater for drinking water, irrigation, and industrial uses increases, the problem will assume increasing urgency.
Soil can be treated by excavating the contaminated subsurface materials such as soils, sediments, fractured rock, and the like. The excavated materials are treated to remove the contaminants or simply stored elsewhere. Groundwater may be removed along with excavated soil or be pumped to the surface of the earth for treatment. These direct methods of dealing with contaminated groundwater are relatively expensive.
Other methods are available for treating contaminated sites in situ. A horizontal well system for treatment of contaminated groundwater is described in commonly assigned U.S. Pat. No. 4,832,122 issued to Corey, et al. This system comprises two sets of wells, an injection well for injecting a fluid into a saturated zone on one side of or within a plume of contaminants and an extraction well for collecting the fluid together with volatilized contaminants from the plume on the other side of, above or within the plume.
A number of techniques for treating contaminated groundwater involve pumping the water above ground for filtration and treatment. Nutrients may be added to stimulate the growth rate of indigenous microorganisms that are capable of oxidizing the contaminants, or the microbial population may be supplemented with naturally-occurring or genetically altered exogenous microorganisms. As the microbial population increases, more and more microbes are available to degrade the contaminants. When the contaminant concentration decreases to acceptable levels, the nutrient supply is terminated and the microbial population returns to background, pretreatment levels. Lawes, et al. (U.S. Pat. No. 4,749,491) extract contaminated groundwater, mix it with an aqueous hydrogen peroxide solution and nutrients, and inject the mixture back into the contaminated zone through an infiltration well. Caplan, et al. (U.S. Pat. No. 4,992,174) and Jhaveri, et al. (U.S. Pat. No. 4,401,569) add microorganisms and nutrients to groundwater, then return the mixture to the ground.
Instead of pumping contaminated groundwater to an above-ground treatment center, nutrients and microorganisms may be supplied in situ via injection wells. Contaminants and oxidized byproducts may be removed, if desired, via nearby extraction wells. Systems are available for supplying water and nutrients (Ely, et al., U.S. Pat. No. 4,765,902), hydrogen peroxide and oxygen (Raymond, et al., U.S. Pat. No. 4,588,506), nutrients and oxygen (Raymond, U.S. Pat. No. 3,846,290) and denitrification organisms Hallberg, et al. (U.S. Pat. No. 4,683,064) to contaminated sites.
The principal existing technology for remediation of groundwater contaminated with trichloroethylene (TCE) involves pumping followed by air stripping. Contamination of unsaturated sediment can only be remediated by vapor extraction. Neither of these techniques results in destruction of the contaminants: the TCE is either discharged to the atmosphere or, where emissions restrictions are in effect, captured on activated carbon for subsequent disposal.
TCE can be mineralized in soil when exposed to air and natural gas (Nelson et al., "Biodegradation of TCE and Involvement of an Aromatic Biodegradative Pathway", 53, 5 (May 1987) pp. 949-954). Some studies indicate that chlorinated solvents slowly degrade in the presence of methane or propane (Wilson, "Aquifer Microbiology and Aerobic Transformations of Chlorinated Solvents", June 1986, Stanford University Seminar on Biological Approaches to Aquifer Restoration-Recent Advances and New Opportunities). However, these techniques are not applicable to large-scale in situ treatment of TCE-contaminated groundwater.
Laboratory studies have shown that chlorinated hydrocarbons, including TCE, are aerobically biodegradable in liquid cultures with methane-oxidizing bacteria known as methanotrophs (Fogel et al., "Biodegradation of Chlorinated Ethanes by a Methane-Utilizing Mixed Culture", App. Environ. Microbiol., 51, 4 (Apr. 1986) pp. 720-724). Methanotrophs are naturally occurring and widespread microbes. The primary enzyme in the oxidation chain is methane mono-oxygenase, an extremely powerful oxidizer with the capability of oxidizing a wide variety of normally recalcitrant compounds, including chlorinated hydrocarbons and especially TCE. The soluble form of methane mono-oxygenase (Type I) induces formation of TCE-epoxide from TCE. TCE-epoxide is extremely unstable and spontaneously breaks down to simpler daughter compounds such as formate. All the daughter compounds of TCE are either unstable, or small and easily metabolizable compounds. The final (and almost immediate) end products of methanotroph-induced TCE-epoxide formation are carbon dioxide (CO.sub.2) and various chloride salts.
Aerobic degradation of TCE results in nontoxic end products, avoiding the formation of toxic by-products of anaerobic degradation such as vinyl chloride. Unfortunately, methanotrophs occur naturally in very small concentrations, as low as ten cells per milliliter of groundwater. Therefore, under normal circumstances, they cannot be counted on to degrade pollutants to any significant degree.
There is a need for a cost-effective method for in situ remediation of subsurface soil or groundwater contaminated by chlorinated hydrocarbons such as TCE. The treatment should degrade the contaminants in situ, and result in nontoxic end products that need not be removed from the site. The treatment should be capable of reducing the subsurface concentration of contaminants to an acceptable level, preferably below detectable limits.