Redox zoning, rates of sulfate reduction and interactions with Fe-reduction and methanogenesis in a shallow sandy aquifer, Rømø, Denmark

Fe-oxide reduction, sulfate reduction and methanogenesis, have been studied in a shallow aquifer with the main focus on sulfate reduction. Direct measurements of sulfate reduction rates have for the first time been applied in an aquifer system. Rates were much lower than reported in other anoxic environments - three orders of magnitude lower than in marine settings and one order of magnitude lower than in lacustrine environments, and varied substantially mainly due to differences in the reactivity of the organic matter. At the extremely low substrate levels in the aquifer, sulfate reduction rates are not primarily limited by low sulfate concentrations. The produced sulfide forms framboidal pyrite via a FeS precursor, with elemental sulfur as an intermediate. Fe-oxide reduction rates were comparable to sulfate reduction rates, but appeared to depend more on Fe-oxide reactivity than organic matter reactivity. Low sulfate concentrations, combined with low-reactivity Fe(III), in the aquifer sediment, has led to an increased appreciation of the existence of concomitant redox processes. This indicates that competitive exclusion is not always effective, and raises questions as to what H₂ data reflect in such a system. Calculations of the in situ energy yield for Fe- and sulfate reduction via H₂ oxidation are ~2.5 kcal/mol H₂, indicating that thermodynamic equilibrium is approached. The calculated available energy yield for methanogenesis was very low indicating that CH₄ production must occur in micro-environments where higher H₂ concentrations prevail. The system may be described as being in a state of partial equilibrium, where the overall rate of organic matter oxidation is controlled by the rate of fermentation of the organic matter, and terminal electron acceptor processes occur at close to equilibrium conditions. This partial equilibrium depends on other processes in the system, in this case an increase in pH due to calcite dissolution appears to induce a shift from predominantly Fe-reducing to predominantly sulfate reducing conditions by changing the energy available to Fe-oxide reduction. Numerical modeling using the partial equilibrium approach was successful in modeling this complex of interacting processes.