Section Eight
GROUNDWATER TREATMENT METHODS
23    Biodegradation of Organic Compounds in Anoxic Groundwater
Systems
Ward G. Wilson, Environmental Engineer
Black and Veatch Consulting Engineers
Philadelphia, Pennsylvania 19106
John T. Novak, Professor
Department of Civil Engineering
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061
INTRODUCTION
Contamination of groundwater supplies by organic chemicals has been recognized as a serious and
widespread environmental problem. To a large extent this contamination results from the leakage or
spillage of gasoline and other fuels. The underground storage tanks commonly used to hold gasoline
and other fuels have been identified as a major source of groundwater pollution. Corrosion, ground
movement, and poor sealing can cause leaks in underground storage tanks and piping which may go
undetected for years. A substantial portion of gasoline components may be sorbed to subsurface soil,
but the water soluble alcohol compounds used in place of lead as octane enhancers may travel
relatively quickly in an aquifer. Since the alcohols have a low odor threshold, they may reach
undesirable concentrations before they are detected by taste or odor.
Two alcohol compounds were of interest in this study, methanol and tertiary-butyl alcohol (TBA).
Methanol is used in pure form as a fuel on an experimental basis, but more commonly methanol and
methanol/TBA blends are added to unleaded gasoline. Both alcohols are also used in various chemical production processes.
Generally, organic compounds may persist for a much longer time in the subsurface compared to
their persistence in surface soil or water. Biodegradation can be a major removal process in the
subsurface for some compounds, depending on the solubility, structure, and other characteristics of
the molecule. The state of the groundwater environment and microbial population will determine the
rate and pathways of biodegradation. Degradation of both methanol and TBA have been demonstrated in subsurface microcosms,1 but the rate of TBA removal is usually very slow.
In this study microcosms were used to investigate the variations in alcohols degradation rates in
ground water systems. Also, alternative electron acceptors (nitrate and sulfate) were added to the
oxygen-poor environment to determine whether anaerobic respiration might occur. The microcosms
were dosed with base to determine pH effects on nitrate-dosed, sulfate-dosed, and non-amended
microcosms.
METHODS AND MATERIALS
Groundwater and subsurface soil were collected from a site in Dumfries, Virginia and used to make
up the microcosms. Soil and groundwater characteristics were as described by Novak, et a/.1 Soil
sampling was conducted with conventional drilling equipment at depths of up to 100 feet. All outer
edges of the soil samples were pared away aseptically to prevent contamination from surface microorganisms. The samples were collected in sterile, Teflon-capped, glass jars and kept on ice until they
reached our laboratories. Both soil and groundwater were stored at the subsurface ambient temperature.
In the past, groundwater and the deep subsurface have been considered sterile. Since the study
involved biodegradation, bacterial populations were enumerated by direct counting techniques (epi-
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