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I ' 1. Theoretical Background 1.1 Literature Review 1 1 1 Biodegradation Processes Recent studies have indicated that many subsurface microorganisms are metabolically active and are able to degrade a wide vanety of contaminants Ghiorse and Balkwill(1983, 1985) observed population ' densities of approximately 106 cells per gram aquifer material at sites in Louisiana and Oklahoma using epifluorescent microscopy The microorganisms present appear to be predominantly bactena (Hirsh and Rades-Rohkohl, 1983), but a few higher fife forms have been detected(Wilson et at, 1983, Ghiorse and Balkwill, 1985,White et a[ , 1983) Some eucaryotic forms which may be fungal spores or yeast cells have been observed in the upper 10 m of a sod profile (Ghiorse and Balkwill, 1983, Federle et at, 1986) ' There are many environmental factors which limitthe biodegradation of subsurface organic pollutants, even in the presence of adapted microorganisms(Thomas et a1, 1987) These factors include the lack of an essential nutrient,substrate concentration,substrate inaccessibility,and the presence of toxicants (Alexander, 1975) The transport of contaminants in the subsurface also affects biodegradation ' Biodegradation of many organic pollutants in the subsurface may be limited by insufficient oxygen Lee and Ward (1985) found that the rate and extent of biotransformation of naphthalene,2-methyl naphthalene, dibenzofuran,fluorene, and phenanthrene were greater in oxygenated water than in oxygen-depleted water Recent research has also indicated that biodegradation can occur under anaerobic conditions Kuhn ' et al (1985) reported mineralization of xylenes in samples of river alluvium under denitrifying conditions In addition, benzene,toluene,xylenes,and other alkylbenzenes were metabolized in methanogenic river alluvium that had been contaminated with landfill leachate(Wilson and Rees, 1986) In addition to oxygen, other nutnents may limit the biodegradation of organic pollutants in the subsurface Inorganic nutnents,such as nitrogen and phosphorous, may be limiting when the ratios of carbon to nitrogen or phosphorous exceed that necessary for microbial processes (Thomas et al , 1987) Also,the presence of sulfate may inhibit methanogenic bactena that have been reported to dehalogenate and mineralize many chlonnated aromatic compounds(Suflita and Gibson, 1985, Suflita and Miller, 1985) 112 In-Situ Biorestoration Microbial processes may be used to degrade contaminants in-situ by stimulating the native microbial population (Thomas et of, 1987) Addition of electron acceptors, such as oxygen, and inorganic nutrients, typically nitrogen, phosphorous, and trace metals may provide the microorganisms with essential nutrients ' that are limiting in the presence of high concentrations of pollutants (Thomas et al, 1987) Inoculation of a specialized microbial population and the addition of surfactants to increase the availability of contaminants to the microorganisms can also be used In general, biological processes,when ' possible, may offer the advantage of partial or complete removal of the contaminants rather than simply transfernng the pollutants to another phase in the environment(Thomas et at, 1987) The application of in-situ biorestoration technology to site remediation is relatively new Raymond (1974) received a patent on a process designed to remove hydrocarbon contaminants from ground water by stimulating the indigenous microbial population with nutnents and oxygen The process was applied at a ' 1-1