H2 Concentrations in a Landfill Leachate Plume (Grindsted, Denmark): In Situ Energetics of Terminal Electron Acceptor Processes

Abstract

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 ΔGr values, close to thermodynamic equilibrium, and that the fermentative H2 production is overall rate limiting. It eliminates the steady-state 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 ΔGr 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 ΔGr values can, combined with a threshold value, predict which TEAPs can occur via H2 oxidation. For our samples, ΔGr 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.