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

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