A comparison of subsurface biodegradation rates of methanol and tertiary butanol in contaminated and uncontaminated sites
The use of alcohols as inexpensive octane enhancers in gasoline has contributed to an increased concern about the potential contamination of groundwater. Being highly soluble in water, alcohols may easily separate from other, more insoluble gasoline components, and rapidly enter the groundwater flow system. The alcohols are relatively tasteless and odorless, and thus, may go undetected until potentially harmful concentrations are reached. This study was designed to determine the potential for alcohol biodegradation in a groundwater system that had been previously contaminated with gasoline containing tertiary butyl alcohol (TBA). Laboratory microcosms, utilizing actual aquifer material and groundwater, were constructed to determine the rate of alcohol biodegradation in a system closely resembling the subsurface environment. The only microorganisms used were those naturally present in subsurface soil obtained aseptically. Bacterial counts and degradation kinetics were evaluated at each of three subsurface depths (10, 26, and 45 feet) and results were compared to similar studies utilizing uncontaminated aquifer material.
Significant bacterial populations were found to exist at all depths studied in the contaminated subsurface system. Bacterial plate counts ranged from 10 6 to 10 7 colony forming units per gram of soil (dry weight). Methanol was found to be a readily degradable substrate. Complete degradation of up to 1000 mg/L was degraded in a matter of months. The biodegradation of methanol in the contaminated system was similar to that observed at pristine sites, indicating that a similar degradation mechanism is involved. TBA biodegradation in the contaminated system occurred and was accompanied by microbial growth. Complete TBA degradation of up to 100 mg/L occurred in less than one year. In contrast, TBA biodegradation in the uncontaminated systems occurred at a very slow rate, which appeared to be constant over time, and thus zero order. However, the zero order rate was found to vary directly with initial substrate concentration. Several mechanisms may explain TBA biodegradation, including the presence of a non-specific exocellular enzyme system. Such a system would describe observed results and suggest that a widespread potential exists for the degradation of a large number of organic compounds.