Review articleCharacterization of redox conditions in groundwater contaminant plumes
Introduction
Redox conditions of a groundwater contaminant plume from a point source usually differ from the redox condition of the pristine aquifer. When sufficient organic matter and other reduced components leak from a point source into an aquifer, strongly reduced redox conditions will develop close to the source and the plume will develop a redox gradient along as well as transversal to the main groundwater flow direction. In the outskirts of the plume, the redox conditions will approach the redox conditions of the pristine aquifer.
The redox conditions of a contaminant plume constitute an important part of the chemical framework controlling the behaviour of the contaminants in the plume. Knowledge of the actual redox conditions, therefore, is important for interpretation of field observations, evaluation of plume development and risks to downgradient groundwater resources, assessment of natural attenuation as a remediation option, and in engineering of remedial measures. Examples of the importance of understanding redox conditions of plumes are: (1) Elevated concentrations of manganese in groundwater sampled downgradient from the source may not originate from the source, but be a result of redox processes in the plume; (2) the presence of a large reduced plume in an aerobic aquifer suggests that the plume has been there for some time; (3) the presence of strongly reduced redox conditions allowing for methane formation suggests a significant potential for natural reductive dehalogenation of, e.g., tetrachloroethene; (4) the feasibility of remediating a benzene plume by injecting oxygen strongly depends on the current redox condition of the plume as well as the redox buffering capacity of the aquifer sediment in the plume; and (5) laboratory studies of microbial activity and contaminant degradation is of limited value unless the samples are incubated or cultured at a redox state representing the in situ conditions.
The changes in redox conditions in contaminant plumes have been recognised for at least 30 years. Golwer et al. (1969) described a landfill leachate plume in terms of an anaerobic, a transition zone and an aerobic zone and observed elevated dissolved iron concentrations in part of the pollution plume. The redox concept of groundwater contaminant plumes was first introduced by Baedecker and Back, 1979a, Baedecker and Back, 1979b) and Champ et al. (1979), but apparently not used rigorously to describe actual plumes in any detail until the 1990s. Actually, two papers Hostettler, 1984, Lindberg and Runnells, 1984 addressing redox potentials of groundwater, in general, concluded that improved approaches were needed to better understand the redox conditions of groundwater.
Determination of redox conditions in contaminant plumes is still no simple task and no universally accepted procedures exist. However, the need to understand contaminant behaviour in plumes has fostered several approaches and attempts to characterize redox conditions of contaminant plumes. Although several of these approaches are rather pragmatic or restricted in their application, major progress has been made in recent years. Most of the development so far has taken place within the framework of research projects, but there is little doubt that determination of redox conditions will be an important issue in the future also in the context of actual contaminant plumes subject to mapping, risk evaluation and remediation on a routine level.
This paper reviews the different approaches described in the literature for determination of redox conditions in contaminant plumes. After an introductory presentation of the concepts of electron activity, redox potentials, redox conditions and redox buffering, the various approaches are described: electrochemical redox potentials, groundwater sample composition with respect to redox-sensitive parameters, hydrogen concentrations in groundwater, volatile fatty acid concentrations in groundwater, sediment characteristics (iron species, sulphur species, oxidation capacity (OXC), reduction capacity (RDC), extractants), microbial characteristics in terms of biomass composition, biomarkers (PLFA), and redox bioassays.
Section snippets
Electron activity (pε) and redox potential (EH)
Aqueous systems contain no free electrons, but the relative electron activity, as an intensity parameter, can still be defined (Stumm and Morgan, 1996).pε gives the hypothetical electron activity, {e−}, and measures the tendency of a system to accept or transfer electrons. In a highly reducing system, the tendency to donate electrons, that is the hypothetical “electron pressure”, or electron activity, is relatively large and pε is low. In contrast, high pε values indicate a
Background
The definition on electron activity presented in Section 2 and the link to the redox potential, via the Nernst equation, may suggest that immersing two electrical leads in a groundwater sample and connecting them to the input of a voltmeter may well result in a measurable electrical potential that is associated with the redox condition of the sample. Electrochemical redox measurements are easily performed and have been successfully applied in analytical chemistry to aquatic solutions of pure
Background
The redox half reactions presented in Fig. 1 and the overall redox reactions exemplified in Table 1, Table 2 involve reactants and products that are present in groundwater as dissolved ions or as dissolved gases. Outside a narrow interval containing the standard redox potential of a given reaction, either the reactants or the products dominate and their presence, therefore, reflects the current redox conditions. The primary redox-sensitive species in groundwater are the dissolved ions SO42−, HS−
Background
The use of H2, as a general indicator of the predominant redox process, was introduced by Lovley and Goodwin (1988). The paper indicated that each anaerobic redox process was characterized by a well-defined range of H2 concentrations. The proposed theory suggested that in a steady-state system, limited by the availability of organic matter, the H2 concentration is constant and controlled by parameters related to the physiology of the mediating bacteria, which again is related to the redox
Background
VFAs are produced as fermentation products through the degradation of organic matter (Fig. 2) and specific levels of volatile fatty acids, such as acetate, have been reported for different redox processes in different types of sediments, e.g., by Lovley and Phillips (1987a) and McMahon and Chapelle (1991). Of the VFAs, acetate and formate are observed in the highest concentrations, whereas propionate and butyrate are measured in lower concentrations. So far, only little data have been published
Background
Aquifer sediments contain large pools of redox-sensitive species in terms of minerals, precipitates and ions associated with exchange sites on particle surfaces. Important oxidized solid species are iron oxides, manganese oxides and sulphate present on exchange sites. Sulfate minerals may, in some cases, also constitute a significant pool of potential electron acceptors. Important reduced solid species are organic matter, sulphides, and iron and manganese carbonates. Reduced species, such as Fe
Background
Most redox processes in contaminant plumes are microbially mediated; and 30 years back, Farkasdi et al. (1969) distinguished between reduction, transition and oxidation zones in leachate plumes at German landfills. This investigation related the observed zones to microbial activity by enumerating sulphate- and nitrate-reducing (NO3− to NH4+) as well as denitrifying (NO3− to N2) bacteria in the plumes. On this background, it seems obvious that characterization of redox environments in
Discussion and conclusion
Redox conditions in a contaminant plume must be addressed to understand the biogeochemistry of the plume, and much progress, as reviewed in the previous sections, have been made in this regard in the last decade. However, it should be emphasised that our current documentation and experience on measuring redox conditions in pollution plumes is still rather limited.
⋅ The number of actual plumes, where assessment of redox has been addressed in any elaborated way (e.g., in more than 20 sampling
Acknowledgements
The preparation of this review would not have been possible without the valuable and much appreciated support from Ms. Birthe Brejl (illustrations) and Ms. Grete Hansen (bibliography and permissions to print published material), Technical University of Denmark, and Dr. Dale Carlson and Mr. Joseph Greer, the Valle Scandinavian Exchange Program, University of Washington, Seattle (logistics and telecommunication). Thorough reviews by Dr. J. Griffioen (TNO, the Netherlands), Dr. J. Lendvay
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