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

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3124 S. Jes sen et al. / Applied Geochemistry 23 (2008) 3116–3126 pro mote the flush ing of labile As from the sed i ment. An enhanced flux of oxy gen ated water and the unsat u rated con di tions will also pro mote oxi da tion of sed i men tary organic mat ter and immo bi lize As in re crys tal lized, more sta bile Fe-oxides. There fore, in the Ben gal delta the oxidized Pleis to cene sed i ments has a smaller As con tent (and a smaller oxa late extract able pro por tion of the total As) than the Holo cene sed i ments (Ahmed et al., 2004; Swartz et al., 2004; Stol len werk et al., 2007). How ever, in some cases a high As con tent in Pleis to cene sed i ments has also been reported (Shah, 2008). Because of the sed i men to log ical sim i lar i ties, a com pa ra ble pal ae o-hydro log i cal con trol on the As con tent of ground wa ter in the Red River delta might be expected. The deep est ele va tion of the bot tom of the incised val ley in the pres ent tran sect is only about ¡54 m, and the lower chan nel slope dur ing the sea level low, when the shore line moved east ward to the shelf edge south of Ha inan island (inset in Fig. 1A), makes it diffi cult to argue that the incised val ley in the tran sect was much deeper than 50–60 m. Con di tions required for a thick unsat u rated zone to develop are a com bi na tion of a low recharge and a high trans mis siv ity of the sed i ments. The thick coarse grained depos its in the NE part of the tran sect have a high trans mis siv ity, and these depos its have a regional exten sion in the cen tral part of the delta (Ma thers and Zal asiewicz, 1999). Low mete oric recharge is likely under the cli matic arid con di tions which prevailed in parts of the cold gla cial phases (Liew et al., 1998; Rost, 2000; Zheng and Li, 2000; Jian et al., 2001). Therefore, in the Red River delta, ground wa ter flow may have occurred towards the high trans mis siv ity regions, rather than towards incised pal ae o-chan nels, giv ing rise to deep atmo spheric oxi diz ing con di tions in the cen tral part of the delta. This may have caused the observed deep yel lowish col our ing of the Pleis to cene depos its in the NE part of the tran sect (see Sec tion 4.1). Alter na tively, the allu vial depos its have been oxi dized from the start, in a sed i mento log i cal regime fea si ble for co-depo si tion of only lim ited and low-reac tiv ity organic mat ter, in that way pre serv ing the oxi dized col our ing. 4.5. Seques tra tion of As Fig. 7 sug gests ongo ing pre cip i ta tion of iron sulp hides which in estu a rine, sul phi dic envi ron ments has a large As Fig. 9. The mea sured con cen tra tion of ground wa ter As plot ted against the cal cu lated con cen tra tion of miss ing SO 4 (see text). Note the log a rithmic axis used for As. sorp tion capac ity (Bo stick et al., 2004). The con cen tra tion of sea wa ter derived SO 4 in a sam ple is cal cu lated as the Cl con cen tra tion in the sam ple mul ti plied by the SO 4/Cl ratio in oce anic sea wa ter (29.5 mM/566 mM = 0.052). The result of this cal cu la tion is shown in Fig. 8, where the grey and the black dots indi cate, respec tively, the sea wa ter derived SO 4 and the mea sured ground wa ter SO 4 con cen tra tions. Except for the sam ple from the shal low screen in Q131, all sam ples have a defi cit of SO 4 (Fig. 8), which must be due to the reduc tion of SO 4 to sul phide. Fig. 9 shows the con cen tra tion of As vs. ‘miss ing SO 4’, the lat ter cal cu lated as the dif fer ence between the measured SO 4 and sea wa ter derived SO 4. The As con cen tra tion decreases when the amount of miss ing SO 4 increases, suggest ing As seques tra tion in Fe sulp hides (Fig. 9). This trend is not fol lowed by the sam ple from Q85 (61 mbs) which had an excep tion ally high CH 4 con cen tra tion (Fig. 5E) and appears to be an out lier (see Sec tion 4.3). In Fig. 9, sam ples that plot close to the 0.013 M detec tion limit for As and which are inde pen dent of miss ing SO 4 (open dia monds) are from the Pleis to cene aqui fer in the NE part of the tran- Fig. 8. The dis tri bu tion of the cal cu lated sea wa ter derived SO 4 con cen tra tion (grey) and the mea sured (pres ent) con cen tra tion of SO 4 (black) (Seaw. = seawa ter).
sect and from shal low screens, rep re sent ing super fi cial ground wa ter which may have a lower As con cen tra tion (c.f., Post ma et al., 2007) (For the dis tri bu tion of groundwa ter As see Fig. 6). It should be noted, that because most of the sea wa ter derived SO 4 has in fact been reduced, the rela tion ship between As and ‘sea wa ter derived SO 4’ is sim i lar to that shown in Fig. 9 for As and ‘miss ing SO 4’, infer ring that a con trol on ground wa ter As might exist even with out the pre cip i ta tion of Fe sulp hides. Also, the Stiff dia grams (Fig. 4C) for the Holo cene aqui fer indi cate fresh wa ter sig na tures and Cl (and SO 4) in these sam ples may be derived from sea spray from the SE mon soon. The Cl con cen tra tions in the fresh wa ter in the tran sect is gener ally an order of mag ni tude higher than in fresh groundwa ter from upstream Ha noi (
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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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Page 63 and 64: In short, there is a consensus amon
Page 65 and 66: The local geology shows mainly sand
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Page 69 and 70: Fig. 7. The distribution of arsenic
Page 71 and 72: H3AsO3 þ H2O $ H2AsO4 þ 3H þ þ
Page 73 and 74: Fig. 12. The relative composition o
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Page 77 and 78: found that while As(V) coprecipitat
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Page 82 and 83: Controlling geological and hydrogeo
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Page 86 and 87: sediments. In general, the coarser
Page 88 and 89: Fig. 5. Thickness of the clay-rich
Page 90 and 91: a b c The groundwater head distribu
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Page 96 and 97: that the situation in Dan Phuong co
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Postma et al., submitted to Geochim
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The mobility of arsenic in a Red Ri
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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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