Microbially Mediated Kinetic Sulfur Isotope Fractionation: Reactive Transport Modeling Benchmark

Wednesday, 17 December 2014
Christoph Wanner1, Jennifer L Druhan2, Yiwei Cheng3, Richard T Amos4, Carl I Steefel5 and Jonathan Blair Ajo Franklin5, (1)Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley, CA, United States, (2)Stanford University, Geological and Environmental Sciences, Stanford, CA, United States, (3)Lawrence Berkeley National Lab, Marietta, GA, United States, (4)University of Waterloo, Department of Earth and Environmental Sciences, Waterloo, ON, Canada, (5)Lawrence Berkeley National Laboratory, Berkeley, CA, United States
Microbially mediated sulfate reduction is a ubiquitous process in many subsurface systems. Isotopic fractionation is characteristic of this anaerobic process, since sulfate reducing bacteria (SRB) favor the reduction of the lighter sulfate isotopologue (S32O42-) over the heavier isotopologue (S34O42-). Detection of isotopic shifts have been utilized as a proxy for the onset of sulfate reduction in subsurface systems such as oil reservoirs and aquifers undergoing uranium bioremediation. Reactive transport modeling (RTM) of kinetic sulfur isotope fractionation has been applied to field and laboratory studies. These RTM approaches employ different mathematical formulations in the representation of kinetic sulfur isotope fractionation. In order to test the various formulations, we propose a benchmark problem set for the simulation of kinetic sulfur isotope fractionation during microbially mediated sulfate reduction. The benchmark problem set is comprised of four problem levels and is based on a recent laboratory column experimental study of sulfur isotope fractionation. Pertinent processes impacting sulfur isotopic composition such as microbial sulfate reduction and dispersion are included in the problem set. To date, participating RTM codes are: CRUNCHTOPE, TOUGHREACT, MIN3P and THE GEOCHEMIST’S WORKBENCH. Preliminary results from various codes show reasonable agreement for the problem levels simulating sulfur isotope fractionation in 1D.