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

Jessen et al., manuscript to be submitted to Geochim. Cosmochim. Acta
quality (Stuyfzand, 1989; Hiemstra et al., 2003). For bank infiltration, an assessment of
the mobility of As in the reduced aquifer is needed.
Here we report the results of a forced gradient field experiment and an arsenite
adsorption/desorption laboratory batch experiment using reduced Holocene aquifer
sediments. The effect of competitive sorption is first examined by the two layer model
for ferrihydrite (Dzombak and Morel, 1990). However, as the stability of the iron oxides
in the aquifer can be assumed to correspond to that of goethite or hematite (Akai et al.,
2004; Postma et al., 2007; Postma et al., submitted), subsequently, the charge
distribution-multisite complexation (CD-MUSIC) model for goethite is employed,
because recent model development indicates that the database is adequately consistent
for a first assessment by this model (Stachowicz et al., 2008). Arsenite was used as the
predominant As species in both the laboratory and in the modelling, because dissolved
As is predominantly arsenite in the reducing Holocene aquifer (Postma et al., 2007).
2.1. Sediment sampling
2. METHODS
The sediment sample was collected in January 2005 in Dan Phuong, 30 km upstream
the Red River from Hanoi, Vietnam (Fig. 1a). This sampling site has been described in
detail by Postma et al. (2007). The core was recovered from the reduced part of the
Holocene aquifer, 15.2-15.5 mbs, by pushing a stainless steel tube into the bottom of the
borehole, drilled by a bailer to the desired depth of the sediment sampling. The section
of the core affected by the drilling operation was removed, and the core was sealed by
plastic end caps and Al-tape and put into an N2-flushed Al-laminate bag, and in this
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