| Soil contamination by gasoline underground storage tanks is a critical environmental problem. The results herein show that in situ bioremediation using indigenous soil microorganisms is the method of choice. Five sites were selected for bioremediation based on the levels of benzene, toluene, ethylbenzene and xylene and the amount of total petroleum hydrocarbons in the soil. Bacteria capable of degrading these contaminants were selected from the contaminated sites and grown in 1,200 I mass cultures. These were added to the soil together with nutrients, water and air via PVC pipes. In the five trials described, the concentration of contaminants fell below the limits acceptable to the EPA within 60 days and the sites were accepted as decontaminated by the Texas Natural Resources and Conservation Commission. Twenty bacterial strains isolated from the contaminated soils were shown to selectively degrade benzene and the other toxic substances. The metabolic strategies of these bacteria were the same as those previously studied by other investigators. These microorganisms were also able to degrade hydrocarbons in the presence of other carbon sources such as glucose and those normally found in the soil. Sixty percent of the organisms studied degraded methyl-ter-butyl-ether probably by the process of co-metabolism. Previous reports claimed that this substance was extremely resistant to bacterial attack. It was also found that 28% of the bacteria contained plasmids. To test the role of plasmids on degradation, a gene probe for xylE from the plasmid pDK189 was constructed. It was shown that this gene, which encodes for catechol 2,3 oxygenase, was present in the chromosome of one organism but not in the DNA of the plasmids isolated. It is assumed that this work will serve as a basis for further research. |
