Scientific Investigations Report 2006-5056
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5056
Evaluation of redox conditions in ground-water contaminant plumes is a prerequisite for evaluating the effectiveness of natural attenuation processes because ground water redox conditions greatly control the occurrence and rate of nearly all biodegradation processes (Bradley, 2003). Most biodegradation processes involve redox reactions in which electrons are transferred from one compound (the electron donor) to another (the electron acceptor). Depending on the particular compound and redox condition, CVOCs can serve as either electron donors or electron acceptors. Possible electron donors in contaminated ground water include various organic-carbon compounds in landfill leachate and hydrocarbon fuels. The less chlorinated VOCs, such as VC and dichlorinated ethenes and ethanes, also can act as electron donors under aerobic or mildly reducing redox conditions. Possible electron acceptors include the inorganic constituents oxygen, nitrate, manganese (IV), iron (III), sulfate, and carbon dioxide, as well as highly chlorinated VOCs such as PCE, TCE, and TCA. VC and dichlorinated ethenes and ethanes also may act as electron acceptors under strongly reducing redox conditions.
Indigenous microorganisms that require carbon and energy to sustain their growth facilitate most ground-water redox reactions (Chapelle, 1992). A variety of microorganisms often competes indiscriminately for the supply of available electron donors in the ground water; however, these microorganisms generally are more specialized in their use of the available electron acceptors. Oxygen-reducing microorganisms (those that use oxygen as an electron acceptor) can out-compete all others when DO is present. However, if the supply of DO in ground water is depleted and electron-donating compounds are still available, oxygen reducers will become dormant and nitrate reducers will predominate until the supply of nitrate is depleted. This sequence continues through manganese reduction, iron reduction, sulfate reduction, and carbon-dioxide reduction (methanogenesis). The result of this competitive exclusion is the formation of somewhat discrete redox zones in an aquifer.
Redox conditions generally are considered either aerobic when DO concentrations are about 1 mg/L, or anaerobic when DO concentrations are less than 1 mg/L. Anaerobic redox conditions usually can be further specified (and are named) according to the inorganic compound acting as the predominant electron acceptor in a given part of an aquifer. Common anaerobic redox conditions in ground water are nitrate reducing, manganese reducing, iron reducing, sulfate reducing, and carbon-dioxide reducing (methanogenic). Nitrate reduction, manganese reduction, and iron reduction commonly are together referred to as mildly reducing conditions, whereas sulfate reduction and methanogenesis commonly are referred to as strongly reducing conditions. That distinction is made because different types of biodegradation processes are favored under mildly and strongly reducing conditions.
Determination of redox conditions in contaminated ground water is not a simple task and no universally accepted procedures exist. The rationale behind characterizing redox conditions in ground water at OU-1 is summarized here. Additional information about characterizing redox conditions is available in Wiedemeier and Chappelle (1998); Weidemeier and others (1998); Christensen and others (2000); and Cozzarelli and others (2000).
Redox conditions in contaminated ground water can sometimes be deduced by quantifying various oxidized and reduced inorganic compounds in ground-water samples (Wiedemeier and Chappelle, 1998; Christensen and others, 2000; and Cozzarelli and others, 2000). Identifying aerobic conditions is relatively simple; they are predominant if DO concentrations in ground water are greater than about 1 mg/L. Identifying various anaerobic redox conditions (in which DO concentrations are less than 1 mg/L) is more difficult. If nitrate concentrations exceed about 0.5 mg/L in anaerobic ground water, then nitrate reduction is likely. If anaerobic ground water lacks nitrate, and if reduced manganese (Mn(II)) or iron (Fe(II)) concentrations increase along a ground-water flow path, then manganese or iron reduction is indicated. If anaerobic ground water lacks nitrate, if sulfate (oxidized sulfur) concentrations decrease along a ground-water flow path, and if hydrogen sulfide (reduced sulfur) concentrations exceed about 0.05 mg/L, then sulfate reduction is indicated. Finally, if anaerobic ground water lacks nitrate, sulfate, and hydrogen sulfide, and if methane concentrations exceed about 0.2 mg/L, then carbon dioxide reduction (methanogenesis) is indicated.
Many conditions at the OU-1 landfill complicate the determination of redox conditions. Contaminated ground water beneath landfills often is not at a thermodynamic equilibrium, so several electron-accepting processes may occur simultaneously (Christensen and others, 2000; Cozzarelli and others, 2000). As non-saline ground water mixes with saline ground water in a near shore environment such as the OU-1 landfill, the natural supply of sulfate may increase substantially along a flow path and effectively mask any sulfate consumption by redox reactions. Redox-sensitive constituents such as sulfate, methane, or iron (II) may leach to ground water from many locations in a landfill, and could mask any concentration changes resulting from redox reactions in local ground water. Redox-sensitive constituents such as methane, iron (II), and manganese (II) often migrate away from their point of production, which blurs interpretation of where one redox zone ends and another begins. Precipitation of iron, manganese, and sulfur constituents also can affect identification of a redox condition at a given point. Redox conditions and concentrations of redox-sensitive constituents can change dramatically over very short distances in contaminant plumes, making it difficult to obtain a ground water sample representing a single discrete redox zone, even in monitoring wells with relatively short (5 ft) open intervals. These confounding conditions cannot be avoided, but an awareness of the conditions can put the interpretation and use of identified redox zones into the proper perspective.
An alternative method for identifying the predominant redox processes in anaerobic ground water is through direct measurement and interpretation of dissolved H2 concentrations in ground water (Lovely and others, 1994; Chapelle and others, 1995). Hydrogen is continuously produced and consumed by different microorganisms during anaerobic decomposition of organic matter. For natural ground waters, different microorganisms that facilitate nitrate-, manganese , iron-, sulfate-, and carbon dioxide-reduction reactions exhibit different efficiencies using H2 (Lovely and Goodwin, 1988). Nitrate-reducers are efficient at using H2 and keeping dissolved H2 concentrations in ground water at levels of less than 0.1 nM. Manganese- and iron-reducers use H2 less efficiently and keep H2 concentrations between 0.1–0.2 and 0.2–0.8 nM, respectively. Sulfate-reducers are less efficient still and keep H2 concentrations at between 1 and 4 nM, and carbon-dioxide reducers are relatively inefficient, resulting in H2 concentrations greater than 5 nM. The result of competition for H2 is that each anaerobic redox condition is characterized by a distinct H2 concentration in ground water (Lovely and others, 1994; Chapelle and others, 1995).
In practice, identifying redox conditions from specific steady-state H2 concentrations is not applicable to all contaminant plumes (Hoehler and others, 1998; Jakobsen and others, 1998; Christensen and others, 2000). Uncertainty in identifying a predominant redox condition from H2 concentrations alone is due to factors such as variation in the iron-oxide minerals that serve as electron acceptors, ground water temperature effects on equilibrium H2 concentrations, and overlapping and non-exclusive redox conditions. Despite those limitations, H2 concentrations do indicate redox conditions in a relative sense, in that higher H2 concentrations are consistently detected in more strongly reduced ground waters. In terms of contaminant biodegradation, identifying the presence of strongly reducing conditions or knowing where H2 concentrations exceed 1 nM may be more critical than knowing the specific inorganic compound that is the predominant electron acceptor. Quantifying oxidized and reduced inorganic compounds as well as steady-state H2 concentrations throughout a contaminant plume can be used with reasonable confidence to identify favorable and less favorable conditions for contaminant biodegradation.
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