Redox microniches in groundwater: A model study on the geometric and kinetic conditions required for concomitant Fe oxide reduction, sulfate reduction, and methanogenesis

A pore-scale model using PHAST is used to study the distribution of redox processes and other geochemical processes in intergranular and intragranular microniches in a groundwater system. The goal is to determine the geometric and kinetic conditions that may give rise to the presence of methane under Fe oxide and sulfate reducing conditions. The model includes FeS precipitation, calcite dissolution and precipitation, and an extended partial equilibrium description of the redox processes: Fe oxide reduction, sulfate reduction, and methanogenesis/ methane oxidation, which takes the microbial energy requirements into account. The model indicates that a separation of redox processes within the pore space is possible, if the stagnant pores are deep and narrow and the rate of organic matter decomposition is fast. However, in most aquifers the organic matter reactivity will be so low that isolated lumps or actual layers of organic matter rather than particles or intragranular coatings of organic matter are required in order to produce methanogenic conditions; otherwise sulfate reduction will take place in the stagnant parts. In the model, the redox processes lead to localized secondary processes occurring at the grain scale; for example, the oxidation of organic matter in a microniche releases CO(2) which dissolves calcite, which is reprecipitated where the Fe oxides are being dissolved and reduced because of the locally increased pH.