A microcosm study was performed to investigate the biodegradation
potential of BTX compounds in unsaturated soils under aerobic and
anaerobic conditions. Toluene was used as a model compound at
concentrations of 100 to 200 mg/L. An uncontaminated, groseclose soil
ranging in depth from 0 to 18 feet was used in order to observe
differences in microbial degradation abilities in shallow subsurface
environments. Several metabolic groups were investigated including
aerobes, denitrifyers and sulfate reducers. Bacterial densities of
these groups were determined at each soil depth. Physical and chemical
parameters of the subsurface environments were also investigated to
observe their impact on microbial biodegradation potentials. These
included changes in soil particle size, moisture content, and pH with
increasing depth.
Substantial toluene biodegradatic:n tCXJk place in some, but not all
soils under both aerobic and denitrifying conditions. Biodegradation
rates varied considerably among aerobes and denitrifyers in similar
environments. In acidic, poorly drained clay soils of 3, 6 and 9 feet,
denitrifyers readily degraded toluene while aerobic microorganisms were
unable to mineralize the compcund. Evidence of toluene biodegradation
by anaerobic bacteria including sulfate reducers was also observed in
moist, clay soils although the rates were much slower. Currently, in
situ bioremediation techniques for benzene, toluene and xylene compounds
almost always rely on indigenous, aerobic organisms to degrade the
contaminants. However, results of this study indicate that aerobic
organisms capable of biodegradation may not exist in some subsurface
environments, even in soils very close to the surface. The
physical/chemical properties of unsaturated environments have
significant impacts on microbial capabilities as well as the
biodegradation potential of contaminants.
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