Microbially mediated kinetic sulfur isotope fractionation: reactive transport modeling benchmark
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Microbially mediated kinetic sulfur isotope fractionation: reactive transport modeling benchmark Yiwei Cheng 1 & Bhavna Arora 2 & S. Sevinç Şengör 3 & Jennifer L. Druhan 4 & Christoph Wanner 5 & Boris M. van Breukelen 6 & Carl I. Steefel 2 Received: 8 January 2019 / Accepted: 24 June 2020 # Springer Nature Switzerland AG 2020
Abstract 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 has been utilized as a proxy for the onset of sulfate reduction in subsurface systems such as oil reservoirs and aquifers undergoing heavy metal and radionuclide bioremediation. Reactive transport modeling (RTM) of kinetic sulfur isotope fractionation has been applied to field and laboratory studies. We developed a benchmark problem set for the simulation of kinetic sulfur isotope fractionation during microbially mediated sulfate reduction. The benchmark problem set is comprised of three problem levels and is based on a large-scale laboratory column experimental study of organic carbon amended sulfate reduction in soils from a uranium-contaminated aquifer. Pertinent processes impacting sulfur isotopic composition such as microbial sulfate reduction and iron-sulfide reactions are included in the problem set. This benchmark also explores the different mathematical formulations in the representation of kinetic sulfur isotope fractionation as employed in the different RTMs. Participating RTM codes are the following: CrunchTope, TOUGHREACT, PHREEQC, and PHT3D. Across all problem levels, simulation results from all RTMs demonstrate reasonable agreement. Keywords Reactive transport modeling . Benchmark . Microbial sulfate reduction . S isotopes . Kinetic isotope fractionation
1 Introduction
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10596-020-09988-9) contains supplementary material, which is available to authorized users. * Yiwei Cheng [email protected] 1
Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA
2
Energy Geosciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA
3
Now at Department of Environmental Engineering, Middle East Technical University, Ankara, Turkey
4
Department of Geology, University of Illinois Urbana-Champaign, Urbana, USA
5
Institute of Geological Sciences, University of Bern, Bern, Switzerland
6
Department of Water Management, Delft University of Technology, Delft, the Netherlands
Dissimilatory sulfate reduction (DSR) occurs when sulfatereducing microorganisms (SRM) oxidize organic matter anaerobically using sulfate as electron acceptor. Isotopic fractionation is typically characteristic of this anaerobic process, since sulfate-reducing bacteria (SRB) favor the reduction of the lighter sulfate isotopologue (32SO42−
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