Microbial Processes and Down-Hole Mesocosms in Two Anaerobic Fractured-Rock Aquifers

By Don A. Vroblesky, Paul M. Bradley, and John F. Robertson

ABSTRACT

Aqueous chemistry, hydrogen-gas measurements, and downhole mesocosms were used to characterize the microbial terminal electron-accepting processes in anaerobic fractured-rock aquifers at Montville, Connecticut, and at the Mirror Lake research site, New Hampshire, and to examine the potential for the microbial community to degrade a target contaminant, toluene. At the most contaminated horizon in the petroleum-hydrocarbon-contaminated aquifer at Montville, the terminal electron-accepting process was methanogenesis. At shallower horizons, sulfate- or iron-reduction prevailed. The presence of organic carbon allowed similar depletion of oxygen in ground water at wells CO-11 and RR-1 (Mirror Lake site). At well CO-11, the advective transport of sulfate apparently was rapid enough or sufficient amounts of oxidized iron were available to prevent methanogenic conditions from developing. At well RR-1, however, the lack of more efficient electron acceptors and the increased availability of organic carbon resulted in methanogenic conditions. Downhole mesocosms placed next to fractures in the aquifer were colonized by native bacteria. The mesocosms provided a sampling approach to recover native bacteria and to use the recovered bacteria to examine the potential for contaminant biodegradation.