Changes in subsurface catabolic gene frequencies during natural attenuation of petroleum hydrocarbons

Subsurface hydrocarbon contamination associated with petroleum spills is a widespread occurrence that presents a significant threat to groundwater res ources. The natural attenuation test site (NATS) at Columbus Air Force Base , MS, provided a unique opportunity to monitor changes in the molecular mic robial ecology as well as stimulation of natural biodegradative processes u nder transient field study conditions. A large, synthetic jet fuel mixture containing BTEX compounds and naphthalene in a decane carrier was introduce d into the subsurface. Over 462 days, a plume of hydrocarbon contamination developed and stabilized at a distance of less than 15 m downgradient from the source area. Exposure of indigenous microorganisms to the contaminant h ydrocarbons was evaluated using an array of gene probes targeting common ge notypes associated with the aerobic biodegradation of BTEX and naphthalene. Each of the targeted genotypes alkB, nahA, nahH, todC1C2, and xylA showed significant responses to hydrocarbon exposure. Aerobic mineralization poten tials of selected contaminants were greater in sediments collected from wit hin the plume relative to uncontaminated areas, suggesting that an aerobic contaminant-degrading community successfully developed within the plume. An increase in aerobic degradation activity coincided with the arrival of the hydrocarbon front within the well field. The Natural Attenuation Study at Columbus AFB successfully linked adaptations of indigenous microorganisms t o hydrocarbon exposure during a transient field study.