Evolution of Redox Processes in Groundwater - ACS Symposium Series

Chapter 26

Evolution of Redox Processes in Groundwater

Downloaded by SUNY ALBANY on April 10, 2013 | http://pubs.acs.org Publication Date (Web): September 2, 2011 | doi: 10.1021/bk-2011-1071.ch026

Peter B. McMahon,*,1 Francis H. Chapelle,2 and Paul M. Bradley2 1U.S.

Geological Survey, Denver Federal Center, Mail Stop 415, Lakewood, CO 80225 2U.S. Geological Survey, 720 Gracern Rd., Suite 129, Columbia, SC 29210 *[email protected]

Reduction/oxidation (redox) processes affect the chemical quality of groundwater in all aquifer systems. The evolution of redox processes in groundwater is dependent on many factors such as the source and distribution of electron donors and acceptors in the aquifer, relative rates of redox reaction and groundwater flow, aquifer confinement, position in the flow system, and groundwater mixing. Redox gradients are largely vertical in the recharge areas of unconfined aquifers dominated by natural sources of electron donors, whereas substantial longitudinal redox gradients can develop in unconfined aquifers when anthropogenic sources of electron donors are dominant. Longitudinal redox gradients predominate in confined aquifers. Electron-donor limitations can result in the preservation of oxic groundwater over flow distances of many kilometers and groundwater residence times of several thousand years in some aquifers. Where electron donors are abundant, redox conditions can evolve from oxygen reducing to methanogenic over substantially shorter flow distances and residence times.

Introduction The purpose of this chapter is to describe how reduction/oxidation (redox) processes evolve spatially and temporally in different groundwater systems. Redox processes, chemical reactions that transfer elections from donor compounds to acceptor compounds, are often catalyzed by microbial processes. Identifying the kinds of redox processes that occur in aquifers, documenting their spatial and temporal distribution, and understanding how they affect concentrations Not subject to U.S. Copyright. Published 2011 by American Chemical Society. In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


of natural or anthropogenic contaminants is central to assessing the chemical quality of groundwater (1–8). The chemical composition of mineral electron acceptors such as dissolved oxygen (O2), nitrate (NO3-), manganese (Mn4+), ferric iron (Fe3+), sulfate (SO42-), and carbon dioxide (CO2) are relatively simple and analytical methods for quantifying them are well developed. Because microbial populations utilize these electron acceptors based on the free energy released by the electron donor-acceptor reaction, the sequence of electron acceptor utilization is often predictable and in the order O2> NO3->Mn4+>Fe3+>SO42->CO2. For these reasons, frameworks for describing redox processes in groundwater have traditionally been based on the sequential uptake of electron acceptors (2, 6, 8, 9). This general framework forms the basis for the description of redox processes used in this chapter. Redox zonation in heterogeneous sediments, however, can be spatially complex due to variations in factors such as electron acceptor and donor reactivities, concentration of redox intermediate products, and chemical transport rates (10–14). Thus, redox zones may actually overlap in some settings or simply be indistinguisable at the scale sampled by well screens.

Sources of Electron Donors in Groundwater Reduced chemical compounds, that is, compounds capable of donating electrons in redox reactions, have the potential for serving as electron donors in groundwater systems. By far the most common electron donor supporting microbial populations in groundwater systems is organic carbon. Aquifer materials deposited in sedimentary environments commonly contain particulate organic carbon that can support microbial metabolism. Particulate organic carbon can be present in aquifer recharge areas, discharge areas, or points in between, depending on the geologic framework of the aquifer. Dissolved organic carbon (DOC) can be delivered to aquifers by various processes such as water percolating through the unsaturated zone (15–20), or by diffusion into aquifers from adjacent confining layers (21–23). DOC in groundwater recharge can originate from natural or anthropogenic sources. While natural DOC mobilized from surface sources is ultimately derived from decaying plant material (17, 24), studies using both biochemical methods (25) and spectrofluorescence methods (26) indicate that DOC in groundwater systems is predominantly microbial in origin. This, in turn, reflects the cycling of dissolved and particulate organic carbon to microbial biomass, which is then recycled back to DOC. As a result of this continuous cycling of carbon, the natural DOC delivered to and/or produced in aquifers tends to have relatively low bioavailability (25). Confining layers adjacent to aquifers can sometimes be a source of DOC to aquifers through the process of diffusion. For example, confining layers of the South Carolina coastal-plain aquifer system contained groundwater with concentrations of organic acids greater than concentrations in the adjacent aquifers (21) (Figure 1). These organic acids, which are more bioavailable than most DOC in groundwater, appear to have been produced by acetogenic bacteria in the confining layers (22) and consumed by respiring bacteria in the aquifers. The concentration gradient that developed in response to 582 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


these two processes drove a net diffusive flux of organic acids from the confining layers to the aquifers (21).

Figure 1. Concentrations of dissolved formate and acetate in groundwater of confining layers and aquifers in the coastal plain of South Carolina. (modified from reference (21) Organic carbon may be the most common electron donor for microbial metabolism in groundwater systems, but mineral electron donors such as sulfide and ferrous iron minerals can be locally important. For example, it has been shown that oxidation of pyrite coupled to the reduction of nitrate is an important process limiting the migration of nitrate in a variety of hydrologic settings including a glacial-fluvial aquifer in southwestern Canada (27), an agriculturally-impacted glacial aquifer in Minnesota (28), and a coastal-plain aquifer in Maryland (29). These inorganic electron donors could be present in geologic materials at the time of deposition or formation, or they could be produced in aquifers as the result of previous reduction with organic electron donors.

Sources of Electron Acceptors in Groundwater Of the commonly available electron acceptors, four of them (O2, NO3-, and CO2) are present in the atmosphere and in the unsaturated zone of many environments, soluble in near-neutral pH conditions of most groundwaters, and often enter the groundwater system through recharge processes. There also SO42-,

583 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


can be important anthropogenic sources for some electron acceptors at the land surface, such as nitrogen-fertilizer sources of nitrate in agricultural recharge (30) (Figure 2). Manganese and Fe3+, the other commonly available electron acceptors, primarily exist in the solid phase of rocks and minerals, and are less likely to be present in groundwater recharge. Once recharge water reaches the water table it becomes isolated from the atmosphere, which could result in the depletion of electron acceptors along groundwater flow paths unless there are subsurface sources to replenish them.

Figure 2. Nitrate concentrations in agricultural recharge and nitrogen fertilizer use in the United States, both in relation to estimated date of groundwater recharge. (data from references (28–35))

Manganese, Fe3+, SO42-, and CO2 have important subsurface sources in many aquifers. Iron oxyhydroxide minerals such as goethite and amorphous Mn4+ and Fe3+ solid phases can occur as coatings on other minerals. Iron within the structure of minerals such as smectite also can serve as an electron acceptor (36, 37). The minerals gypsum and anhydrite can serve as subsurface sources of SO42-, particularly in carbonate-rock aquifers (4, 38). Sulfate that has diffused from confining layers can also act as an electron acceptor for redox processes in marine sedimentary aquifers (39). Subsurface CO2 can be produced by dissolution of carbonate minerals and by microbial respiration in many types of aquifers.


Evolution of Redox Processes along Groundwater Flow Paths Flow Systems Dominated by Natural Sources of Electron


Unconfined Aquifers Unconfined aquifers commonly receive spatially distributed recharge. This recharge water moves vertically downward across the water table and causes flow paths from upgradient areas to move deeper in the aquifer. Such a pattern of groundwater flow in the recharge area of unconfined aquifers results in vertical gradients in both groundwater age and redox processes. Flow systems characterized by an abundant supply of natural electron donors could evolve from O2-reducing to methanogenic conditions in a relatively short distance below the water table. Flow systems characterized by a limited supply of electron donors may not evolve beyond O2-reducing or mildly anoxic conditions. Figure 3 presents an example of the evolution from O2-reducing to methanogenic conditions which occurred within a few meters below the water table in a glacial outwash aquifer in Minnesota (40). In contrast, figure 4 presents a fluvial aquifer in the High Plains of Kansas (41) where the system remained O2 to NO3- reducing, even at depths of 200 m below the water table.

Figure 3. Distributions of redox processes and groundwater age along flow paths in a glacial outwash aquifer in Minnesota. (modified from reference (40))



Figure 4. Distributions of redox processes and groundwater age along flow paths in a fluvial aquifer in Kansas. (data from reference (41))

The Minnesota aquifer was relatively thin and in an area of relatively high recharge rates so groundwater flow distances were less than a kilometer and residence times were on the order of decades or less. The Kansas aquifer was relatively thick and in an area of relatively low recharge rates so groundwater flow distances were tens of kilometers and residence times were on the order of millenia. Given these differences in hydrology and redox zonation, redox reaction rates in the Minnesota aquifer are assumed to have been much faster than those in the Kansas aquifer. Near discharge areas at streams or rivers in both systems, flow paths converged and turned upward with redox gradients becoming more horizontal (42). The ratio between groundwater residence time and chemical reaction time, sometimes expressed as the Damköhler number, has been used to quantitatively compare redox evolution in different flow systems (13, 14, 43). Whether an aquifer is transport or reaction dominated has important implications for any redox-sensitive chemical that has health and/or ecological concerns (13, 43).



Confined Aquifers Recharge in confined aquifers occurs in outcrop areas and flows laterally downgradient into confined parts of the aquifer. This pattern of groundwater flow generally results in longitudinal gradients in both groundwater age and redox processes in confined aquifers. Contributions of water and solutes from adjacent confining layers could alter these longitudinal patterns in the vicinity of aquifer/confining-layer interfaces (44). Thus, longitudinal gradients in groundwater age and redox processes represent a fundamental difference between confined and unconfined aquifers. A possible exception to this is contaminated unconfined aquifers, in which longitudinal redox gradients could develop, as noted below. As in unconfined aquifers, the timescale for evolution of redox processes in confined aquifers is controlled in part by the availability of electron donors and acceptors and groundwater flow rates. For the example shown in Figures 5A and 5B, an electron-donor rich glacial aquifer in Ontario, Canada (45), the flow system evolved from Fe3+ reducing to methanogenic over a flow distance of about 12 km and a groundwater residence time of less than 20,000 years. In contrast, the example in Figures 5C and 5D shows an electron-donor poor sandstone aquifer in the Kalahari Desert, Namibia (46) where the flow system remained NO3- reducing over a distance of about 90 km and a residence time of more than 20,000 years. Figures 5E and 5F illustrates the effect of abundant supplies of a single electron acceptor on redox patterns in the Floridan carbonate aquifer in Florida (38), a system with relatively abundant supplies of electron donor. Oxygen and NO3in recharge water were consumed relatively quickly in this flow system. Sulfate reduction became the next predominant redox process because of the limited supply of Fe3+ and abundant supply of SO42- from the dissolution of gypsum and anhydrite. Thus, the flow system remained SO42- reducing over a flow distance of about 100 km and a groundwater residence time of more than 10,000 years.

Redox Processes near Aquifer/Confining-Layer Interfaces Aquifer/confining-layer interfaces represent mixing zones that sometimes are capable of supporting greater redox activity than either hydrogeologic unit alone. Groundwater in aquifers can be a source of electron acceptors for redox processes occurring in confining layers. Similarly, groundwater in confining layers can be a source of electron donors for redox processes occurring in aquifers (44). In both cases, redox processes have the potential to evolve over very short distances. In the example shown in Figure 6, a NO3- contaminated alluvial aquifer in Colorado overlying an organic-rich marine shale (47), O2 and agricultural NO3in groundwater from the alluvial aquifer diffused into the highly reducing shale and were consumed over a distance of less than 2 m.



Figure 5. Distributions of redox reactants and products, and groundwater age, along groundwater flow paths in confined aquifers: (A and B) glacial aquifer, Ontario, Canada (data from reference 45), (C and D) sandstone aquifer, Kalahari Desert, Namibia (data from reference (46)), and (E and F) carbonate aquifer, Florida. (data from reference (38))

Flow Systems Dominated by Anthropogenic Sources of Electron Donors Many pristine aquifers are primarily electron-donor limited. However, when large amounts of metabolizable organic carbon, such as gasoline, are introduced to an aquifer the carbon limitation is relieved and microbial metabolism then becomes limited by the availability of nutrients and electron acceptors. Numerous studies have described how redox processes in groundwater systems are affected by petroleum hydrocarbon spills (1, 5, 48–51). An example of how these redox processes change in space and time was described by (52) at a leaking gasoline underground storage tank site in South Carolina. The spill occurred in a relatively permeable, fully aerobic (O2 ~ 6.0 mg/L), sandy, unconfined coastal-plain aquifer. 588 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 6. Distributions of redox reactants and products in groundwater near the interface of an alluvial aquifer and the Pierre Shale, northeastern Colorado. (modified from reference (47))

The spill is thought to have occurred sometime in 1992 or 1993. By 1994, the hydrocarbon plume immediately downgradient of the spill location had become anoxic and Fe3+ reduction had been initiated (Figure 7A). Four years later, however, the core of the plume was actively methanogenic, SO42- reduction predominated downgradient of the methanogenic zone, and Fe3+ reduction predominated near the discharge area at a small ditch (Figure 7B). The rapid change of predominant redox processes from Fe3+ reduction to methanogenesis near the spill, a much more rapid change than observed at most sites (1), reflects the extremely low amount of Fe3+ present in this system. With so little available Fe3+, the system rapidly shifted to SO42- reduction and methanogenesis over just a few years. This process was accompanied by an increase in the density of methanogenic microoganisms in the core of the contaminant plume as well. This example illustrates how excess electron donors caused by anthropogenic contamination can radically alter ambient redox processes in groundwater systems and microbial ecology (52). 589 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 7. Distribution of redox processes in a gasoline-contaminated unconfined aquifer in South Carolina. (modified from reference (52)) The observed rapid shift from O2-reducing to methanogenic conditions at this site is different from what commonly is observed in unconfined aquifers dominated by natural sources of electron donors (Figures 3 and 4), as is the spatial pattern of dominant redox processes. At the contaminated site in Figure 7, redox conditions were highly reducing at the upgradient end of flow paths and oxidizing at downgradient ends. Just the opposite pattern was observed in the uncontaminated flow system in Figure 3. Moveover, substantial longitudinal redox gradients developed in the contaminated system, whereas redox gradients in the uncontaminated flow system were largely vertical.

Effect of Groundwater Mixing on the Evolution of Redox Processes In the above examples, evolution of redox processes occurred along groundwater flow paths that were generally unaffected by processes such as pumping, leakage through long well screens, or leakage through natural fractures that could mix groundwater of different ages, origins, and compositions. Such processes have the potential to alter the predicted spatial pattern of redox processes in aquifers by redistributing electron donors and acceptors in the flow system more quickly than would otherwise occur under flow conditions unaffected by 590 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


mixing. Pumping and leakage through well screens, for example, introduced NO3- from shallow sources into deeper Fe3+ and SO42--reducing groundwater in Nebraska and Florida, thereby shifting redox conditions in those systems to NO3- reducing (43). Leakage through natural fractures introduced methane and possibly other electron donors from deep sources into a shallow, oxic aquifer in Colorado, thereby causing anoxic conditions to develop in the vicinity of fracture zones (53). Recognizing the potential impacts of groundwater mixing also is critical for shallow, unconfined aquifer systems with anaerobic contaminant plumes. For example, natural attenuation of chloroethene-contaminated groundwater is complicated by the variation in redox character between different chloroethene compounds. Polychlorinated parent compounds, like tetrachloroethene and trichloroethene, are highly oxidized, tend to serve as electron acceptors during biodegradation, and attenuate most efficiently under reducing conditions. However, regulatory emphasis at chloroethene-contaminated sites is often on the production and accumulation of reduced daughter products like vinyl chloride (VC), which can serve as electron donors during biodegradation and attenuate most efficiently in the presence of O2. Because VC is produced by microbial reductive dechlorination under anaerobic conditions, aerobic VC biodegradation is often deemed insignificant and the lack of accumulation of ethene, the product of VC reductive dechlorination, is interpreted as evidence of incomplete degradation, a so-called degradative “stall.” Trace concentrations of O2 have been documented widely in shallow, anaerobically-active chloroethene plumes and generally dismissed as an artifact of atmospheric contamination during sampling or as the metabolically insignificant O2 residual of aerobic consumption at the oxic/anoxic interface. However, a recent study of DOC and O2 supply in shallow groundwater indicates that O2 profiles can be explained by mixing of oxygenated recharge with shallow, nominally anoxic groundwater (54). This suggests that low O2 concentrations observed in shallow, anaerobically-active aquifers may, in fact, reflect extensive precipitation-driven advection of O2 into the “anoxic” plume. The potential impacts that a precipitation-driven flux of O2 into nominally anoxic chloroethene plumes may have on contaminant attenuation are particularly relevant, because mineralization of VC to CO2 can be substantial at O2 concentrations well below the field standard for nominally “anoxic” conditions (55–57). Failure to recognize the impacts of mixing of oxygenated recharge in anaerobically-active chloroethene plumes can lead to the incorrect diagnosis of a degradative “stall” and adoption of expensive and ineffective remedial actions.

Redox Processes in Regional Aquifers of the United States An analysis of water samples collected from 5,135 domestic wells was used to compare redox processes in regional aquifers of the United States (9, 58). A regional analysis can be useful because of the unique perspective it provides regarding the effects of climate, geology, and hydrology on redox processes. The Snake River Plain basaltic-rock aquifer in Idaho exhibited the largest percentage 591 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


(100%) of oxic samples and the Silurian-Devonian carbonate-rock aquifer in the Midwest exhibited the smallest percentage (13%) (Figure 8). The high percentage of oxic samples in the basaltic-rock aquifer reflects the relatively fast rates of groundwater movement from fracture flow and limited supply of electron donors in that aquifer. The low percentage of oxic samples in the carbonate-rock aquifer reflects, in part, a relatively large supply of electron donors in that aquifer, but also electron-donor abundance in overlying glacial deposits that in some areas served as confining layers for the carbonate-rock aquifer. On average among the eight lithologic groups examined, sand and gravel aquifers in the western United States such as the High Plains, Basin and Range basin fill, and Central Valley aquifers contained the largest percentage (85%) of oxic samples. These western sand and gravel aquifers are largely oxic, even though they often contain old groundwater (Figure 4), because they are electron-donor limited. The glacial sand and gravel aquifers contained the smallest percentage (43%) of oxic samples because they contained relatively abundant supplies of electron donors such as organic carbon and pyrite. Some aquifers, such as the Coastal Lowlands aquifer system in the Gulf Coastal Plain, the Pennsylvanian sandstone aquifer in parts of Ohio, Pennsylvania, and West Virginia, and the glacial sand and gravel aquifers in the central United States, contained large percentages (25 to 39%) of samples with mixed redox conditions (Figure 8). In part, this redox heterogeneity reflects the heterogeneous nature of the sediment in these aquifers compared to aquifers like the Central Valley and Basin and Range aquifers (Figure 8). Even at the scale of this assessment, considering redox processes explained many of the water-quality trends observed in these regional aquifers (9, 58).

Conclusions The variability in the source and distribution of electron donors and acceptors in aquifers has important implications for the evolution of redox processes in groundwater systems. Highly reducing redox conditions generally indicate an abundance of electron donors compare to electron acceptors. The distance along groundwater flow paths over which redox processes evolve is largely a function of the relative rates of redox reaction and groundwater flow. If redox reaction rates are greater than groundwater flow rates, reducing conditions are expected to develop over relatively short flow distances. Vertical redox gradients predominate in the recharge areas of unconfined aquifers, whereas longitudinal redox gradients predominate in confined aquifers. An important exception is unconfined aquifers dominated by anthropogenic sources of electron donors, such as gasoline, where substantial longitudinal redox gradients can develop and the gradients are the reverse (more reducing to less reducing) of what is observed in uncontaminated flow systems. Groundwater mixing also has the potential to alter the predicted spatial pattern of redox process in aquifers by redistributing electron donors and acceptors in the flow system more quickly than would otherwise occur under flow conditions unaffected by mixing. Understanding mixing effects on redox processes can be particularly important for predicting spatial patterns of redox processes and contaminant degradation potential in groundwater systems. An 592 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 8. Pie diagrams indicating the percentages of domestic well samples that were oxic, suboxic, anoxic, or diagnostic of mixed redox processes in selected regional aquifers of the United States. (modified from reference (58)) (see color insert)

analysis of redox processes in regional aquifers of the United States indicated that, on average, sand and gravel aquifers of the western United States such as the High Plains, Basin and Range basin fill, and Central Valley aquifers where the most oxic and glacial sand and gravel aquifers of the northern United States were the least oxic. Some aquifers, such as the Coastal Lowlands aquifer system in the Gulf Coastal Plain, the Pennsylvanian sandstone aquifer in parts of Ohio, Pennsylvania, and West Virginia, and the glacial sand and gravel aquifers in the central United States, contained large percentages of samples with mixed redox conditions. In part, this redox heterogeneity reflects the heterogeneous nature of the sediment in these aquifers compared to aquifers like the Central Valley and Basin and Range aquifers. 593 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

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