Use of Dissolved H2 Concentrations To Determine Distribution of Microbially Catalyzed Redox Reactions in Anoxic Groundwater Derek R. Lovley'st Francis H. Chapelle,' and Joan C. Woodward7
Water Resources Division, U.S. Geological Survey. 430 National Center. Reston, Virginia 22092, and Water Resources Division. U.S. Geolcgial Survey. 720 Gracern Road, Suite 129, Columbia, South Carolina 29210 . . . .
The potential for using concentrations of dissolved Hz to determine the distribution of redox processes in anoxic groundwaterswas evaluated. In pristine aquifers in which standard geochemical measurements indicated that Fe(111) reduction, sulfate reduction, or methanogenesis was the terminal electron accepting process (TEAP), the HZ concentrationswere similar to the HZconcentrations that have previously been reported for aquatic sediments with the same TEAPs. In two aquifers contaminated with petroleum products, it was impossible with standard geochemical analyses to determine which TEAPs predominated in specific locations. However, the TEAPs predicted from measurements of dissolved HP were the same as those determined directly throughmeasurements of microbial processes in incubated aquifer material. These results suggest that HZconcentrations may be a useful tool for analyzing the redox chemistry of nonequilibrium aroundwaters.
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Introduction Anoxic reduction-oxidation (redox) reactions greatly influence the fate and mobility of organic and inorganic constituents in both pristine and contaminated aquifers. A major goal in hydrogeology is to deduce the redox reactions taking place in subsurface environments based on an analysis of the associated groundwater. In the anoxic groundwaters of deep pristine aquifers, the distribution of redox processes can often he readily discerned through simple inspection of groundwater chemistrybecause there is an orderly succession of anoxic redox reactions (Figure lA), each of which has readily indentifiahle chemical signatures ( 1 4 ) . However, when previously aerobic, shallow aquifers are heavily contaminated with organic compounds, such as petroleum products or landfd leachate, it is generally difficult to delineate the distribution of anoxic redox processes. This is because the redox processes are aligned along the groundwater flow path in the reverse order (Figure1B)of that observed in deep pristine aquifers (7-9). Thus, reduced products such as methane, Fe(II), and Mn(I1) that are actively produced near the source of organic contamination may persist in the groundwater as t Reaton, VA. 1 Columbia, SC.
Flsne 1.
Ideallzedd~mbuHonofterminaielectronacngprocesses pristine aquifers and (B. bonom)
(TEAPs) in (A, top) wnflned deep
shallow aquifers Contaminated with organic COmDOundS. Groundwater flow 1s from left to right.
it moves downgradient into areas where there is little or no ongoing production of these compounds. This complicates attempts to localize the redox reactions taking place in such aquifers. Historically,redox reactions in anoxic groundwater have been evaluated in terms of equilibrium thermodynamics and the master variahlepe (refs6,lO,and 11,and references cited therein). Although equilibriumthermodynamicsare useful for visualizing the potential relationships between the oxidized and reduced phases of various elements (12), in practice, the equilibrium thermodynamic approach cannot he used to discern the redox reactions in most natural waters. The principal reasons for this are that (1)
mh aIIIcb not s u b w m US. MpmM. Publlahed 1994 by ma A m n Chemical Society
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Sd.Technol., Vd. 28. No. 7. 1994 1205
ape does not actually exist (i.e., there are no free electrons); (2) the redox status of most natural waters cannot be accurately determined with electrodes because they respond to few of the geochemically significant redox couples; and (3) these low temperature environments almost never approach redox equilibrium so that it is generally impossible to specify a unique pe based on the distribution of reduced and oxidized species (6, 10-13). Therefore it is not surprising that there is no consistent relationship between the measured redox potential and the redox reactions taking place in sedimentary environments (3). A parameter that could identify the redox reactions taking place in anoxic groundwater, which was real (i.e., actually existed and could be measured in groundwater) and reflected the inherent nonequilibrium nature of most groundwaters, would be more useful than pe. At the circumneutral pH and low temperatures of most groundwaters, many of the most significant anoxic redox reactions such as nitrate reduction, Fe(II1) reduction, sulfate reduction, and methane production are catalyzed by microorganisms (14,151. In microorganisms, the flow of electrons from organic matter to inorganic electron acceptors requires electron transfer through various intermediary compounds and electron transport chain components before finally being passed to the electron acceptors from the external environment. Thus, the electron acceptors such as nitrate, Fe(III), sulfate, and carbon dioxide are referred to as terminal electron acceptors, and the reductions of these electron acceptors are known as terminal electron-accepting processes (TEAPs). A nonequilibrium analysis of redox processes that takes into account the biochemical constraints on the various TEAPs might provide a better description of redox chemistry in groundwater than the equilibrium thermodynamic approach. Hz concentrations have been proposed as a microbially based, nonequilibrium alternative to pe for elucidating which redox reactions are taking place in anoxic sedimentary environments (3). Hz is an important intermediate in the microbial oxidation of organic matter coupled to the reduction of many inorganic electron acceptors and, under steady-state conditions when TEAPs are generally segregated into distinct zones, there is a clear correspondence between HZconcentrations and the predominant TEAPs in aquatic sediments (3). For example, despite wide differences in such factors as rates of organic matter decomposition and pH, aquatic sediments in which methane production was the predominant TEAP typically had Hz concentrations of 7-10 nM; sediments with sulfate reduction as the TEAP had Hz concentrations of 1-1.5 nM; Fe(II1)-reducing sediments had Hz concentrations of 0.2 nM; and sediments in which nitrate or Mn(1V) reduction was the predominant TEAP had H2 concentrations less than 0.05 nM. A mathematical model based on well-knownprinciples of microbial metabolism can account for this association of specific Hz concentrations with specific TEAPs under steady-state conditions. It was suggested (3) that dissolved Hz might also be used to determine which redox reactions are taking place in groundwaters. In this paper, we show that the H2 concentrations in several pristine and contaminated aquifers are similar to the Hz concentrations in aquatic sediments with the same TEAPs and provide results that suggest that H2 concentrations can be used to discern the 1206 Environ. Sci. Technol., Voi. 28. No. 7, 1994
Wells 1A 18 1C 1E Water Level (m) 1.07 0.640.06 -1.22
7 Santee Aluvium
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SEA LEVEL Upper Pee Dee
Lower Pee Dee
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b 30m
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60m
90m
Figure 2. Hydrogeology of the Rimini well cluster.
TEAPs in anoxic groundwaters in which it was impossible to determine the TEAP based on more traditional geochemical analyses.
Materials and Methods Sampling Sites. Four pristine aquifers were sampled near the town of Rimini, SC. Wells were screened in four sandy horizons separated by clayey confining bed material (Figure 2). These sediments were deposited by a series of prograding deltas during late Cretaceous and Paleocene time (16). The well site is located on the flood plain of the Wateree River and is a classic example of a groundwater discharge area. The water levels of three of the four wells are above land surface so that they flow under natural artesian pressure when uncapped (Figure 2). One of the contaminated aquifers studied was a shallow water-table aquifer near Bemidji, MN, that has a lens of crude oil floating on top of the water table (17). Aromatic hydrocarbons leaching from the oil are degraded in a plume of anoxic groundwater which extends downgradient from the oil lens (9,18). Geochemical modeling has indicated that methane production and Fe(II1) reduction are the important TEAPs within the anoxic plume (9). For this study, the site previously referred to as site B (19) was investigated. The water table at this site was 9.2 m below land surface, and the well was screened at 11.5 m. The other contaminated aquifer studied was a shallow water-table aquifer at the Defense Fuel Supply Center in Hanahan, SC, which is contaminated with petroleum hydrocarbons. The hydrology and chemistry of this site have recently been described (20). The two sites sampled were MW 20and MW 12. The water at table at these sites was 2-3 m below land surface, and the wells were screened at 4.6 m. The groundwater at both of these sites was anoxic and contained relatively high concentrations of dissolved Fe(I1) and methane. Hz Sampling, At the Rimini and the Hanahan sites, H2 was measured with the previously described "bubble
strip method" (21). Briefly, groundwater was pumped through a standard gas-sampling bulb (Fisher Scientific, 250 mL) with a peristaltic pump that was downstream of the sampler. The bulb was continually flushed with water for several minutes to eliminate air bubbles. A bubble (20 mL) of H2-free N2 was introduced into the bulb through the septum with a syringe and needle. As water continued to flow through the bulb, the gases dissolved in the groundwater equilibrated with the gas phase. The gas phase was sampled and analyzed for H2 over time until the H2 concentration stabilized (less than 5 % change in 5 min). The concentration of dissolved H2 in the water was calculated as follows: Hz(dissolvad) = (Lp)/(RT), where H2(diasolved)is the concentration of dissolved Hz in moles per liter; L is the Ostwald coefficient for H2 solubility (22); R is the universal gas constant (0.0821 L X atm X K-' X mol-I), P is pressure (atm), and T i s the temperature (K). At Bemidji, Hz was measured with a sampler that was designed to retrieve discrete water samples at depth. A detailed description of this sampler and a comparison of this method with the bubble strip method and other techniques for estimating dissolved H2 in groundwater will be published separately (23). Briefly, the sampler consisted of a glass chamber (34-35 mL) with ports and nonmetallic valves at either end. The sampler was lowered into the well, the valves at each end were opened, and water was drawn through the bottom inlet and out the outlet at the top with a peristaltic pump at the surface. Pumping was continued until 10-12 sampler volumes of water had been passed through the sampler. Then the valves on either end of the sampler were closed simultaneously and the sampler was brought to the surface. Two syringes (2.5 mL) with needles were flushed with H2-free N2 and one (syringe A) was filled with 2 mL of the gas. The needles on the syringes were injected through a butyl rubber stopper in the side of the sampler. The N2 from syringe A was injected into the sampler while 2 mL of displaced water was collected in syringe B. The sampler was shaken for 1min to equilibrate the dissolved H2 with the gas phase. The 2 mL of displaced water in syringe B was pushed back into the sampler, and 2 mL of water was collected in syringe A. After another min of shaking, the gas bubble was then pushed into syringe B and analyzed for H2. Based on H2 solubilities, calculated as described above, this bubble contained 77% of the H2 that was initially dissolved H2. The concentration of H2 that was dissolved in the initial groundwater sample was calculated accordingly. Appropriate precautions against potential H2 contamination of samples (23) were observed. Water for H2 sampling was pumped through tygon tubing rather than siliconetubing to prevent the leaching of H2 into the water. Peristaltic pumps that were downstream from the samplers were used because submersible pumps can produce substantial quantities of H2. The wells at Rimini and Hanahan were constructed of PVC casing and screens so that no metal came into contact with the groundwater. At Bemidji, the well casings were PVC, but the well screens were stainless steel. However, laboratory studies demonstrated that stainless steel well screens that had been preexposed to water did not produce H2 at rates that would detectably affect measured Ha concentrations. At least three well volumes of groundwater were withdrawn before sampling for H2. This procedure resulted in stable Hz
Table 1. Water Chemistry in Pristine Aquifers at Rimini,
sc
well 1A
1B 1C 1E
depth
Hz
(m)
(nM)
02
67 46 27 9
0.2 0.1 1.5 10.0
9.4
