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Mekong River Delta sediment bears arsenic that is released to groundwater under anaerobic conditions over the past several thousand years. The oxidation state, speciation, and distribution of arsenic and the associated iron bearing phases are crucial determinants of As reactivity in sediments. Peat from buried mangrove swamps in particular may be an important host, source, or sink of arsenic in the Mekong Delta. The total concentration, speciation, and reactivity of arsenic and iron were examined in sediments in a Mekong Delta wetland by X-ray fluorescence spectrometry (XRF), X-ray absorption spectroscopy (XAS), and selective chemical extractions. Total solid-phase arsenic concentrations in a peat layer at a depth of 6 m below ground increased 10-fold relative to the overlying sediment. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that arsenic in the peat was predominantly in the form of arsenian pyrite. Arsenic speciation in the peat was examined further at the micron-scale using μXRF and μX-ray absorption near-edge structure (XANES) spectroscopy coupled with principal component analysis. The multiple energy μXRF mapping and μXANES routine was repeated for both iron and sulfur phase analyses. Our μXRF/μXANES analyses confirm arsenic association with pyrite – a less reactive host phase than iron (hydr)oxides under anaerobic conditions. The arsenian pyrite likely formed upon deposition/formation of the peat in a past estuarine environment (∼ 5.5 ka BP), a process that is not expected under current geochemical conditions. Presently, arsenian pyrite is neither a source nor a sink for aqueous arsenic in our sediment profile, and under present geochemical conditions represents a stable host of As under the reducing aquifer conditions of the Mekong Delta. Furthermore, organic carbon within the peat is unable to fuel Fe(III) reduction, as noted by the persistence of goethite which can be reduced microbially with the addition of glucose.

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NOTICE: This is the author’s version of a work that was accepted for publication in Geochimica et Cosmochimica Acta. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Geochimica et Cosmochimica Acta, (In Press) doi: 10.1016/j.gca.2014.10.021