Environ. Sci. Technol. 1998, 32, 2142-2148
H2 Concentrations in a Landfill Leachate Plume (Grindsted, Denmark): In Situ Energetics of Terminal Electron Acceptor Processes R A S M U S J A K O B S E N , * ,† HANS-JØRGEN ALBRECHTSEN,‡ METTE RASMUSSEN,‡ HENRIK BAY,‡ POUL L. BJERG,‡ AND THOMAS H. CHRISTENSEN‡ Department of Geology and Geotechnical Engineering and Department of Environmental Science and Engineering, Groundwater Research Centre, Technical University of Denmark, DK-2800 Lyngby, Denmark
Empirical H2 concentration ranges are currently related to specific redox processes, assuming steady-state conditions at which only one microbiologically mediated redox process occurs due to competetive exclusion of others. Here the first H2 data from a landfill leachate plume are presented, and an alternative partial equilibrium approach is used. The approach implies that TEAPs (terminal electron-accepting processes) occur at negative ∆Gr values, close to thermodynamic equilibrium, and that the fermentative H2 production is overall rate limiting. It eliminates the steadystate prerequisite and may explain the occurrence of concomitant TEAPs. Concentrations of H2 and redox process reactants and products were measured in 52 sampling points, downgradient of the Grindsted Landfill (Denmark), and used to calculate in situ ∆Gr values of TEAPs, assuming partial equilibrium. H2 generally ranged from 0.004 to 0.88 nM, with most values around 0.2 nM. Fe reduction was, according to the empirically defined ranges, the most prominent TEAP, but concomitant methanogenesis and sulfate reduction occurred as well. This indicated a need for an alternative approach to explaining the H2 distribution, and the measured H2 concentrations are viewed as being controlled by a partial equilibrium. A derived theoretical relation between H2 concentrations and temperature indicates temperature effects to be more important than currently appreciated. Calculated in situ ∆Gr values can, combined with a threshold value, predict which TEAPs can occur via H2 oxidation. For our samples, ∆Gr for methanogenesis was always >-7 kJ/mol, and CO2 reduction should only occur in stagnant porewater at higher H2 concentrations or by direct interspecies transfer. In contrast, sulfate and Fe reduction occur close to or slightly below a threshold of -7 kJ/mol H2 and may occur concomitantly at partial equilibrium.
* Corresponding author. Telephone: +45 45 25 21 73; fax: +45 45 88 59 35; e-mail:
[email protected]. † Department of Geology and Geotechnical Engineering. ‡ Department of Environmental Science and Engineering. 2142
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 14, 1998
Introduction H2 measurements have been developed into a tool for deducing the redox state of natural anaerobic aquatic systems (1, 2). The concept is based on experimental data as well as microbiological and thermodynamic theory. According to this, a TEAP (terminal electron-accepting process) at steady state is characterized by a specific H2 level due to competetive exclusion of other microbiologically mediated TEAPs. The competitive edge is related to differences in the thermodynamic energy yield of the redox processes and physiological characteristics of the various bacteria (2). One of the advantages of using H2 is that the pool of H2 found in natural systems is so small that it is cycled within minutes, implying that the measured concentration relates to processes that actually occur in the sampled sediment. Recently, the possibility of using H2 measurements to map redox zones within petroleum-contaminated aquifers has been addressed (3-5). A precise knowlege of the ongoing redox processes in a plume is important for prediction of the fate of the pollutants in the plume. In this paper, we present the first H2 measurements from a mixed landfill leachate plume emanating from the Grindsted Landfill, Denmark. The redox conditions in the Grindsted Landfill leachate plume have previously been studied in terms of the distribution of redox-sensitive species in groundwater samples (6) and TEAP rates in unamended sediment bioassays (7). Both approaches have shown a redox state gradient from methanogenic conditions close to the landfill to aerobic conditions 250 m away. However, the redox processes in the plume do not occur in well-separated zones, but to some extent they occur simultaneously, indicating that competetive exclusion is not always fully efficient. This concomitance of redox processes has also been observed in a petroleum polluted aquifer (8) and in marine sediments (9). These observations call for an alternative approach to using H2 as an indicator of microbiologically mediated TEAPs. We propose that measured H2 and TEAP reactant concentrations may be combined, through simple thermodynamic calculations, into an actual potential in situ energy yield, representing the potential for a given hydrogen oxidizing TEAP. With this approach, we can explain the observation of concomitant TEAPs, evaluate where in the leachate plume the different TEAPs can take place, and derive relative reactivities of iron oxides. The existing approach to using H2 concentrations is only valid if the system studied is in a steady state, a prerequisite that is very difficult to be certain of. This is especially the case for systems with iron oxide reduction in which a range of iron oxides are present. In such a system, the most available iron oxide will steadily be removed, and the system might never reach a true steady state as long as there are iron oxides present. The approach proposed here is applicable also in systems that are not in a steady state.
Materials and Methods Field Site. The site is described with regard to geology and sediment geochemistry in ref 10. The profile in Figure 1 summarizes the main geological features of the sandy aquifer along the center line of the plume. The groundwater flow is toward the right side of the figure (NW) with estimated flow velocities of 50 m/yr in the upper Quaternary sediments and 10 m/yr in the lower Miocene (micaceous sands) sediments (6). Annual infiltration is approximately 400 mm/ yr. The groundwater temperature is 8-10 °C. S0013-936X(97)00858-4 CCC: $15.00
1998 American Chemical Society Published on Web 05/28/1998
FIGURE 1. Geology along the flowline transect at the Grindsted Landfill site. Dots indicate sampling points. Note the boundary between the upper Quaternary sediments and the lower Tertiary micaceous sands. The landfill contains municipal as well as industrial waste (11) and has no measures to prevent leachate from migrating into the aquifer. The age of the plume is estimated to >20 yr. Sampling and Analysis. MLSs (multi-level samplers) were installed at eight distances from the landfill to a depth of 10 m below surface. A 3/4 in. stainless steel pipe was driven into the aquifer, a bundle of 4 mm Teflon tubing was inserted into this, and the steel pipe was retrieved (12). The lower 5 cm of the Teflon tubing was perforated to form a screen. Fifty-five sampling points were installed. The MLSs were left in the ground for more than 3 months in order to avoid elevated H2 concentrations in the samples (12). H2 samples were taken using the bubble-stripping method developed by Chapelle (13), adapted to smaller screens and lower flow rates (12). Water was continuously pumped with a peristaltic pump at 100 mL/min through a 4 mL bubble of N2 gas retained in a vertically held glass tubing. Equilibrium with the gas bubble was obtained in approximately 20 min, and a sample of the gas bubble was taken with the glass tubing held horizontally through a septum over a bulge in the tubing. Samples were taken after 20 and 25 min, and if the difference was less than 5%, it was assumed that equilibrium had been obtained. The H2 concentration was determined with a RGD2, reduced gas detector, from Trace Analytical (Menlo Park, CA). The separation of gases was done with a 3 ft long, 1/8 in. wide, 13× 40/60 column at ambient temperature. Calibration was carried out using dilutions of 20 or 50 µL/L
gas standards of H2 in N2 with a nominal precision of (10%. Gas concentrations in microliters per liter headspace were converted into concentrations in the water phase using Henry’s law (1 µL/L headspace ) 0.88 nM, at 8 °C). Other measured groundwater parameters in the samples were determined as described in ref 6. Free Energy Calculations. The energy available to the microorganisms from H2 oxidizing TEAPs has been calculated as Gibbs free energies of reaction (∆Gr), from Gibbs energies of formation (∆G°f), at a groundwater temperature of 8 °C. Reactions, equations, and thermodynamic values used in the calculations are shown in Table 1. Similar calculations of in situ energy yields in a field setting are found in ref 14 for sulfate reduction, methanogenesis, and methane oxidation in a marine sediment. Corrections for temperature were made using the enthalpy, and the Van’t Hoff equation in the form:
∆G°T2 )
∆H°(T1 - T2) + T2∆G°T1 T1
where ∆H° is the enthalpy, T1 is the absolute temperature at standard conditions (298.15 K), and T2 is the absolute groundwater temperature (281.15 K), see refs 15 and 16. From the equations of Table 1, it is clear that the energy yield for a given reaction is related to the chemistry of the local environment. For example, pH will be very important to the ∆Gr of iron oxide reductionswith decreasing pH (increasing H+ activity), the energy available to the bacteria will increase as the ∆Gr decreases. To make calculations as precise as possible, solute activities rather than concentrations were used. The derivation of solute activities was based on activities obtained from speciations made with PHREEQC (17) of earlier full analysis samples from the same plume transect (6). For the full analysis samples, analytical relations relating activities of the relevant species calculated by PHREEQC to concentrations of the major and most important complexing ions present in both data sets were derived. These relations were then used to estimate the activities for the samples in Table 2. Given that the natural log of the activity enters into the equations (Table 1), these corrections are not critical, and corrections using just activity coefficients lead to very similar values. Changing the HS- activity from 10-7 to 5 × 10-7 only increases the calculated ∆Gr for sulfate reduction by 1 kJ/mol H2. Also the error introduced by using interpolated alkalinities where data were missing should be small. Previous studies have shown that in steady-state systems a threshold ∆Gr of ∼-15 to -7 kJ/mol H2 for a given TEAP
TABLE 1. Equations and Thermodynamic Values Used for Calculating In Situ Gibbs Free Energies for Terminal Electron-Acceptor Processes (TEAPs) with H2a TEAP
eq used for calculating in situ ∆Gr
H2 + 2FeOOH + 4H+ h 2Fe2+ + 4H2O
∆Gr ) ∆GT + RT ln
4H2 + SO42- + H+ h HS- + 4H2O
∆Gr ) ∆GT + RT ln
HCO3- + 4H2 + H+ h CH4 + 3H2O
∆Gr ) ∆GT + RT ln
[Fe2+]2 [H2][H+]4 [HS-] [H2]4[SO2-4][H+] [CH4] [H2] [H+][HCO3-] 4
∆G 281.15 (kJ/mol)
∆Hr° (kJ/mol)
-178.5 -157.5
-198.7
-262.4
-235
-229.8
-237.8
a R is the gas constant, T is the absolute temperature in K, [ ] indicate species activities. Thermodynamic data on solutes are from ref 15. The ∆Gr° values for iron oxide reduction corresponds to the reduction of the least (∆G°f ) -472.8 kJ/mol) and most stable (∆G°f ) -483.3 kJ/mol) lepidocrocites (derived from ref 25) the ∆Hr° values are calculated using ∆Hf° ) -559.4, which is for goethite since there is no available value for lepidocrocite. The error introduced by this approximation is minor because the effect of ∆H is not high for small temperature differences, and the temperature effect on ∆GT is small. The major effect comes from the second term of the equation.
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TABLE 2. Concentrations of Redox-Sensitive Species Used in the Calculations of Gibbs Free Energies for H2 Oxidizing TEAPs, at the 52 Sampling Points Downgradient of the Landfill dist. (m)a
masl (m)a
O2 (µM)
Mn (µM)
Fe2+ (mM)
SO42(µM)
S(-II)b (µM)
CH4 (mM)
alkal. (mequiv/L)
pH
H2 (nM)
0 0 0 0 0 0
36.20 35.20 33.20 32.20 31.20 30.20
