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

Fe-oxide reduction, sulfate reduction and methanogenesis, have been studied in a shallow aquifer with the main focus on sulfate reduction. Direct meas urements of sulfate reduction rates have for the first time been applied in an aquifer system. Rates were much lower than reported in other anoxic env ironments - three orders of magnitude lower than in marine settings and one order of magnitude lower than in lacustrine environments, and varied subst antially mainly due to differences in the reactivity of the organic matter. At the extremely low substrate levels in the aquifer, sulfate reduction ra tes are not primarily limited by low sulfate concentrations. The produced s ulfide forms framboidal pyrite via a FeS precursor, with elemental sulfur a s an intermediate. Fe-oxide reduction rates were comparable to sulfate redu ction rates, but appeared to depend more on Fe-oxide reactivity than organi c matter reactivity. Low sulfate concentrations, combined with low-reactivi ty Fe(III), in the aquifer sediment, has led to an increased appreciation o f the existence of concomitant redox processes. This indicates that competi tive exclusion is not always effective, and raises questions as to what H-2 , data reflect in such a system. Calculations of the in situ energy yield f or Fe- and sulfate reduction via H-2 oxidation are similar to 2.5 kcal/mol H-2 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 H-2 concentration s prevail. The system may be described as being in a state of partial equil ibrium, where the overall rate of organic matter oxidation is controlled by the rate of fermentation of the organic matter, and terminal electron acce ptor processes occur at close to equilibrium conditions : This partial equi librium depends on other processes in the system, in this case an increase in pH due to calcite dissolution appears to induce a shift from predominant ly Fe-reducing to predominantly sulfate reducing conditions by changing the energy available to Fe-oxide reduction. Numerical modeling using the parti al equilibrium approach was successful in modeling this complex of interact ing processes. Copyright (C) 1999 Elsevier Science Ltd.