Redox processes on a grain scale, the possible ...

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XVI International Conference on Computational Methods in Water Resources (CMWR-XVI) Ingeniørhuset

Redox processes on a grain scale, the possible effect of organic matter in stagnant inter/intra-granular voids
Author:Rasmus Jakobsen <> (Institute of Environment and Resources)
Presenter:Rasmus Jakobsen <> (Institute of Environment and Resources)
Date: 2006-06-18     Track: Special Sessions     Session: Pore-Scale Modelling: New Developments And Applications

The presence of stagnant zones in porous media is often used to explain the observations of self-contradictory redox chemistry once the possibilities of borehole mixing are ruled out. As an example, methane is found in very small amounts in many suboxic groundwaters, though methanogenesis requires very reduced conditions (high H2 levels) in order to be thermodynamically feasible. This feature has been produced in a previous model using a simple dual-porosity 1-D PHREEQC model (Jakobsen, 2002), implying a stagnant domain containing organic matter diffusively exchanging with water moving by. In other words the diffusive exchange occurs on the scale of the model. This paper tries to asses whether actual inter/intra granular stagnant zones containing organic matter can sustain adequately reduced conditions in a system closer to reality, given the importance of diffusive exchange on the pore scale. The model tool PHAST is used to set up a simple 2-D flow system of active and non-active dead-end channels and pores in a pore scale grid. Simple Darcian flow is assumed, implying a simple constant velocity across a pore. Dead-end pores and channels are set up to contain solid organic matter. Fe-oxides are placed in the active channels with flow and suboxic water containing sulfate enters the model flow system. The partial equilibrium model used by Jakobsen (2002) is used to control the redox processes, implying that the reduced equivalents released by adding the organic matter in the stagnant domains are used for the processes that release enough energy for sustaining microbial activity. This also implies that methane formed can only be reoxidized if the energy available for the process is adequate. Using this setup the model can produce a system where sulfate reduction and Fe-oxide reduction occurs in the parts with active flow and methanogenesis occurs in the stagnant zones, and the water in the water leaving the system contains small amounts of methane. This indicates that detoxifying processes, such as the dechlorination of vinylchloride, requiring rather reduced conditions, may take place in stagnant intergranular voids, of an apparently not adequately reduced system. The process may occur slowly in this manner, but may still produce the conditions necessary for hosting the microorganisms capable of carrying out the process, implying that they are present if a reductant is added as a remedial measure. Jakobsen R. (2002) Including stagnant zones in order to model overlapping redox processes occurring in a shallow sandy aquifer. Geochim. Cosmochim. Acta 66 (15A): A362-A362 Suppl. 1 AUG 2002