Patterns of intrinsic bioremediation at two U.S. Air Force bases

Intrinsic bioremediation of benzene, toluene, ethylbenzene, and xylenes (BTEX) occurs when indigenous microorganisms work to reduce the total mass of contamination in the subsurface without the addition of nutrients. A conservative tracer, such as trimethylbenzene, found commingled with the contaminant plume can be used to distinguish between attenuation caused by dispersion, dilution from recharge, volatilization, and sorption and attenuation caused by biodegradation. Patterns of intrinsic bioremediation can vary markedly from site to site depending on governing physical, biological, and chemical processes. Intrinsic bioremediation causes measurable changes in groundwater chemistry. Specifically, concentrations of contaminants, dissolved oxygen, nitrate, ferrous iron, sulfate, and methane in groundwater change both temporally and spatially as biodegradation proceeds Operations at Hill Air Force Base (AFB) and Patrick AFB resulted in fuel-hydrocarbon contamination of soil and groundwater. In both cases, trimethylbenzene data confirm that dissolved BTEX is biodegrading. Geochemical evidence from the Hill AFB site suggests that aerobic respiration, denitrification, iron reduction, sulfate reduction, and methanogenesis all are contributing to intrinsic bioremediation of dissolved BTEX. Sulfate reduction is the dominant biodegradation mechanism at this site. Geochemical evidence from Patrick AFB suggests that aerobic respiration, iron reduction, and methanogenesis are contributing to intrinsic bioremediation of dissolved BTEX. Methanogenesis is the dominant biodegradation mechanism at this site