The role of aquifer and microbial heterogeneity on the transport and activity of bacteria in the Columbia Aquifer, Oyster, Virginia

Groundwater at many Department of Energy facilities is contaminated with radionuclides, metals, anions, chlorinated solvents, and fuel hydrocarbons. The goal of this thesis was to examine how heterogeneity in aquifer properties and heterogeneity in bacterial properties could affect bioremediation at these sites. Field-scale and complementary laboratory experiments were conducted to better understand bacterial transport and activity in physically and chemically heterogeneous, porous media. Experiments were conducted in the Columbia Aquifer at the South Oyster Bacterial Transport Field Site on the Southern Delmarva Peninsula near the town of Oyster, Virginia. New groundwater injection and sampling systems were developed in order to conduct bacterial transport and push-pull experiments. Field-scale bacterial transport experiments were conducted in the aerobic portion of the aquifer utilizing a CFDA/SE-stained, monoculture of bacteria. Laboratory and field data indicated that the monoculture of bacteria possessed a distribution of surface properties. This surface property distribution controlled bacterial attachment to the sediment with the physical and chemical heterogeneity of the aquifer playing a secondary role. Flow cytometry coupled with the CFDA/SE staining was utilized to determine in situ bacterial growth rates. During the field-scale bacterial transport experiment the in situ bacterial doubling time was estimated to be 14 days. Laboratory growth and transport experiments with the same bacterium indicated that daughter cells were not preferentially transported after cell division. Push-pull experiments were conducted over a vertical section of the microaerophilic aquifer. After injection of an electron donor, reduction of dissolved O₂, nitrate, Fe(III) and Mn(IV) began within one day with sulfate reduction beginning five days later. The rate of denitrification was slower than observed at sites contaminated with petroleum hydrocarbons. During denitrification some of the nitrate adsorbed to the sediments and the resulting N₂ formed a separate gas phase. The Fe(II) produced from Fe(III) reduction adsorbed to the sediments and precipitated. Aqueous and adsorbed sulfate were both reduced. The amount or rates of reduction were poorly correlated with aquifer properties. The presence of electron acceptors at all depths in the aquifer and bacteria capable of utilizing them minimized the importance of aquifer heterogeneity in controlling microbial activity in the aquifer. During both bacterial transport and stimulation experiments, aquifer heterogeneity played a secondary role in controlling each processes.