H-2 CONCENTRATIONS IN A LANDFILL LEACHATE PLUME (GRINDSTED, DENMARK) - IN-SITU ENERGETICS OF TERMINAL ELECTRON-ACCEPTOR PROCESSES

Empirical H-2 concentration ranges are currently related to specific r edox processes, assuming steady-state conditions at which only one mic robiologically mediated redox process occurs due to competetive exclus ion of others. Here the first H-2 data from a landfill leachate plume are presented, and an alternative partial equilibrium approach is used . The approach implies that TEAPs (terminal electron-accepting process es) occur at negative Delta G(r) values, close to thermodynamic equili brium, and that the fermentative H-2 production is overall rate limiti ng. It eliminates the steady-state prerequisite and may explain the oc currence of concomitant TEAPs. Concentrations of H-2 and redox process reactants and products were measured in 52 sampling points, downgradi ent of the Grindsted Landfill (Denmark), and used to calculate in situ Delta G(r) values of TEAPs, assuming partial equilibrium. H-2 general ly ranged from 0.004 to 0.88 nM, with most values around 0.2 nM. Fe re duction was, according to the empirically defined ranges, the most pro minent TEAP, but concomitant methanogenesis and sulfate reduction occu rred as well. This indicated a need for an alternative approach to exp laining the H-2 distribution, and the measured H-2 concentrations are viewed as being controlled by a partial equilibrium. A derived theoret ical relation between H-2 concentrations and temperature indicates tem perature effects to be more important than currently appreciated. Calc ulated in situ Delta G(r) values can, combined with a threshold value, predict which TEAPs can occur via H-2 oxidation. For our samples, Del ta G(r) for methanogenesis was always >-7 kJ/mol, and CO2 reduction sh ould only occur in stagnant porewater at higher H-2 concentrations or by directinterspecies transfer. In contrast, sulfate and Fe reduction occur close to or slightly below a threshold of -7 kJ/mol H2 and may o ccur concomitantly at partial equilibrium.