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Arsenic release metabolically limited to permanently water-saturated soil in Mekong Delta

Nature Geoscience volume 9, pages 70–76 (2016)Cite this article

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Abstract

Microbial reduction of arsenic-bearing iron oxides in the deltas of South and Southeast Asia produces widespread arsenic-contaminated groundwater. Organic carbon is abundant both at the surface and within aquifers, but the source of organic carbon used by microbes in the reduction and release of arsenic has been debated, as has the wetland type and sedimentary depth where release occurs. Here we present data from fresh-sediment incubations, in situ model sediment incubations and a controlled field experiment with manipulated wetland hydrology and organic carbon inputs. We find that in the minimally disturbed Mekong Delta, arsenic release is limited to near-surface sediments of permanently saturated wetlands where both organic carbon and arsenic-bearing solids are sufficiently reactive for microbial oxidation of organic carbon and reduction of arsenic-bearing iron oxides. In contrast, within the deeper aquifer or seasonally saturated sediments, reductive dissolution of iron oxides is observed only when either more reactive exogenous forms of iron oxides or organic carbon are added, revealing a potential thermodynamic restriction to microbial metabolism. We conclude that microbial arsenic release is limited by the reactivity of arsenic-bearing iron oxides with respect to native organic carbon, but equally limited by organic carbon reactivity with respect to the native arsenic-bearing iron oxides.

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Figure 1: Total aqueous As and Fe concentrations for batch incubations of Permanent Wetland sediments as a function of time.
Figure 2: Total aqueous As and Fe concentrations for batch incubations of Seasonal Wetland sediments as a function of time.
Figure 3: CBD-extractable As and Fe for pre-incubated (solid bars) and post-incubated As-ferrihydrite (chequered bars) and glucose + As-ferrihydrite (striped bars).
Figure 4: Pore-water As and Fe concentrations at 1.1 m depth in the Seasonal Wetland.
Figure 5: Diagram (not to scale) depicting the processes controlling the partitioning of As within the three distinct zones of Mekong sediment.

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Acknowledgements

This work was financially supported by a US EPA STAR Graduate Fellowship awarded to J.W.S., the Stanford Woods Institute for the Environment, and a National Science Foundation Graduate Research Fellowship Program Grant no. DGE-114747 awarded to M.V.S. Portions of this work were also supported by the National Science Foundation (grant number EAR-0952019), the Stanford NSF Environmental Molecular Science Institute (NSF-CHE-0431425), the EVP programme of Stanford’s Woods Institute, and by the US Department of Energy, Office of Biological and Environmental Research, Terrestrial Ecosystem programme (award number DE-FG02-13ER65542). We are indebted to the staff at Resource Development International for logistical and field support, including A. Shantz, T. Makara, K. Dina and P. Nuon. We are grateful to G. Li for laboratory assistance and statistical consultation, D. Turner and M. Keiluweit for laboratory assistance, and A. Adelson, K. Boye and J. Dittmar for help with fieldwork. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

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Authors and Affiliations

  1. Department of Earth System Science, Stanford University, Stanford, California 94305, USA

    Jason W. Stuckey, Michael V. Schaefer, Benjamin D. Kocar & Scott Fendorf

  2. Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    Benjamin D. Kocar

  3. Department of Geosciences, Boise State University, Boise, Idaho 83725, USA

    Shawn G. Benner

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  1. Jason W. Stuckey
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Contributions

S.F., J.W.S. and B.D.K. conceived the experiments, which were carried out by J.W.S., M.V.S. and B.D.K.; S.F., B.D.K., J.W.S. and S.G.B. performed site selection, and M.V.S. provided logistical support; J.W.S. performed data analyses; J.W.S., S.F. and S.G.B. performed data interpretation and co-wrote the manuscript.

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Correspondence to Scott Fendorf.

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Stuckey, J., Schaefer, M., Kocar, B. et al. Arsenic release metabolically limited to permanently water-saturated soil in Mekong Delta. Nature Geosci 9, 70–76 (2016). https://doi.org/10.1038/ngeo2589

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