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

2 Source and mobilization of As
The Red River delta, and other South East Asian deltas, ultimately derive their
sediments from the Himalayas (Datta and Subramanian, 1997; Stummeyer et al.,
2002; Stanger, 2005; Charlet and Polya, 2006; Guillot and Charlet; 2007).
Arsenic is sorbed to neo-formed iron oxides in the sediment, and hence codeposited
on the delta plain in channels, point bars and overbank deposits which
eventually form new aquifers (Nickson et al., 1998; Stummeyer et al., 2002;
Raymahashay and Khare, 2003). Elevated groundwater concentrations of As
occur when the As is released from the sediments. Hence the source of the As
contaminating the groundwater in large areas of the South East Asian deltas is
natural in origin.
2.1 Arsenic release from channel bed sediments
The mechanisms of the release of As from newly deposited sediments was
investigated as part of this thesis (Jessen et al., manuscript). A river bed sediment
was incubated with river water, to simulate the above described sequence of
sediment deposition followed by burial into an aquifer. Both As and Fe 2+ was
concomitantly released from a muddy Red River bed sediment within a few days
(Fig. 1). Addition of acetate as an energy source available to microbes, lead to an
increased release of Fe 2+ and As, suggesting that the release of As is linked to
microbial reductive dissolution of As-bearing Fe(III) oxides.
As shown in Fig. 1, the As released in the incubation was almost entirely in the
Fig. 1. Release of As and Fe from an incubated muddy Red River bed riverbed sediment.
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