Geochemistry at the sulfate reduction-methanogenesis transition zone in an anoxic aquifer-A partial equilibrium interpretation using 2D reactive transport modeling

The study addresses a 10 m deep phreatic postglacial sandy aquifer of vertically varying lithology and horizontally varying infiltration water chemistry, displaying calcite dissolution, ion-exchange, and anaerobic redox processes. The simple variations in lithology and infiltration combine into a complex groundwater chemistry, showing ongoing Fe-oxide reduction, sulfate reduction and methanogenesis. Rates of sulfate reduction, methanogenesis and methane oxidation were measured directly using radiotracers. Maximum rates were 1.5 mM/yr for sulfate reduction, 0.3 mM/yr for methanogenesis, and only 4.5 μM/yr for methane oxidation. The overlap of sulfate reduction and methanogenesis was very small. The important intermediates formed during the degradation of the organic matter in the sediment, formate and acetate, had concentrations around 2 μM in the sulfate reducing zone, increasing to 10 and 25 μM in the methanogenic part. The concentration of H₂ was around 0.25 nM in the Fe-reducing zone, 0.4 nM in the sulfate reducing zone, and increased to 6 nM in the methanogenic zone. Using in situ concentrations of products and reactants the available energies for a range of different reactions could be calculated. The results of the calculations are in accordance with the observed distribution of the ongoing redox processes, implying that the system is well described using a partial equilibrium approach. A 2D numerical PHAST model of the system based on the partial equilibrium approach, extended by implementing specific energy yields for the microbial redox processes, could explain most of the observed groundwater geochemistry as an expression of a closely coupled system of mineral equilibria and redox processes occurring at partial equilibrium.