Groundwater arsenic in the Red River delta, Vietnam ... - Fiva

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Postma et al., submitted to Geochim. Cosmochim. Acta river mud is so much higher than in the sand deposits. The occurrence of such a pool of highly reactive, poorly crystalline, iron oxide seems not really consistent with long distance sediment transport in the Red River and points rather towards a more local source. Weathering of primary minerals like silicates or sulfide minerals in the river environment could be an option. Another more likely explanation could be that it is the result of reoxidation of the Fe(II) contained in the groundwater that is discharged into the river. As described by Larsen et al. (2008) for the Red River at Dan Phuong, and by Polizzotto et al. (2008) for the Mekong, there is a large flux of groundwater, high in Fe(II) and As, discharging from the floodplain aquifers into the river. When the discharging anoxic groundwater mixes with the oxic river water, presumably poorly crystalline iron oxides will precipitate that may incorporate arsenic. The resulting highly reactive small particles of iron oxyhydroxides will easily adhere to the clay particles with a high surface area that are suspended in the river water. In times with a high river stage the muddy sediment will deposit on the floodplain, on the surface of fields and in particular accumulate in ponds and ox-bows. For a field site at the shores of the Mekong river in Cambodia, Polizzotto et al. (2008) calculated that the flux of As discharging with the groundwater into the river is more or less balanced by the flux of As from the river to the floodplain by sedimentation. The mud sediment contains both a highly reactive pool of iron oxide as well as very reactive organic matter as was demonstrated by unamended sediment incubations (Fig. 8). These microcosms became anoxic in a matter of days and both arsenic and iron were readily released. This combination of a highly reactive iron oxide phase, containing arsenic, being present together with reactive organic carbon makes it an extremely potent source for arsenic contamination. After deposition in topographic 25
Postma et al., submitted to Geochim. Cosmochim. Acta depressions, like ponds and along channels, the mud will quickly turn anoxic and start to release Fe and As which upon subsequent infiltration through these mud layers may yield high arsenic groundwaters in good agreement with the model proposed by Polizzotto et al (2008) for plumes of high arsenic groundwater, emitted from ponds and spreading through the aquifer. However, when infiltration proceeds through fractured overbank clay deposits (Larsen et al., 2008; Postma et al., 2007) downwashing of high As clay does not appear to play a major role, since the oxidized aquifer sediments are low in arsenic (Fig. 5) 5. CONCLUSIONS Goethite and hematite were identified as the only Fe-oxides occurring in the Red River and adjacent aquifer sediments by Mössbauer spectroscopy. Line broadening indicates that goethite is present in a disordered form. Kinetic analysis shows the river mud to contain highly reactive Fe-oxides, equivalent to ferrihydrite, while in the river sand and oxidized aquifer sediment the Fe-oxides had a reactivity comparable to a range from disordered goethite to hematite. The release of As from the sediments is directly coupled to the reduction of iron oxides and, except for the river mud, the rate of arsenic release is limited by availability of reactive organic carbon. In the reduced aquifer sediment an unidentified authigenic Fe(II)-phase accumulates which apparently also contains As(III). The Red River mud contains both very reactive Fe-oxides with a high As content, and reactive organic carbon. Potentially this constitutes an extremely strong source for arsenic contamination. After deposition of the mud, the sediment will quickly turn anoxic and start to release both Fe and As. Deposition on the flood plain may occur 26
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Groundwater arsenic in the Red Rive
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Søren Jessen Groundwater arsenic i
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organic matter may prevent As mobil
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Resumé Naturligt frigivet arsen (A
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efterfølgende forsøgt at opstille
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Preface This PhD thesis deals with
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Table of contents 1 INTRODUCTION: T
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The PhD research presented in this
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educed form as As(III), which is al
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A more complex and intimate relatio
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sedimentary As content (Postma et a
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(Anawar and Mihaljevi, 2009), and g
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3 Regional As distribution in the R
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in large part by changes in the eus
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In the Red River delta, the Pleisto
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the Red River delta, which are not
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5 The mitigation potential of bank
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Noteworthy is, that the ratio of Fe
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trend in the distribution of the RF
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2008) or the Hach or Merck test kit
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Badloe C., Nguyen T. P. T. and Nguy
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Driehaus W., Seith R. and Jekel M.
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Islam F. S., Boothman C., Gault A.
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Mondal D. and Polya D. A. (2008) Ri
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A. Ali and Z. Adeel). ITN Centre, B
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Ahmed K. M. (2006) A transect of gr
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Arsenic in groundwater of the Red R
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In short, there is a consensus amon
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The local geology shows mainly sand
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ane concentration and P CO2 values
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Fig. 7. The distribution of arsenic
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H3AsO3 þ H2O $ H2AsO4 þ 3H þ þ
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Fig. 12. The relative composition o
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of the Fe-oxides in the upper 3 m c
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found that while As(V) coprecipitat
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desh: further evidence of decouplin
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Controlling geological and hydrogeo
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Here the results of the geological
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sediments. In general, the coarser
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Fig. 5. Thickness of the clay-rich
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Jessen et al., manuscript to be sub
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Table 1
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Fig. 1
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Fig. 3
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Fig. 5
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Fig. 7
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Fig. 9
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Fig. 11
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Groundwater arsenic in the Red River delta, Vietnam ... - Fiva

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