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

On our field site in Dan Phuong, 30 km upstream the Red River from Hanoi, As
becomes mobilized both surficially in channel bed sediment (Larsen et al., 2008)
and within the reducing aquifer sediments (Postma et al., 2007; Postma et al.,
submitted). At the Dan Phuong field site, the variation of the stable isotopic
composition of water in the Red River and in the local precipitation provides a
signature by which surface water recharge can be distinguished from
precipitation recharge (Larsen et al., 2008). Infiltrated surface water had an As
concentration of 75 to 300 μg/L, suggesting, since the As concentration in the
surface water is negligible, a release of As within the channel bed (Larsen et al.,
2008). At another location in the Dan Phuong field site, precipitation recharges
the Holocene aquifer through fractures in the clay cover. Here the concentration
of As gradually increases from below the limit of detection at the groundwater
table, to more than 500 μg/L at 10 m depth below the water table. 3 H/ 3 He-dating
indicated a downward velocity component of the groundwater flow of 0.5
m/year, yielding an As release rate of roughly 20 μg/L/year, in good agreement
with previously published results from Bangladesh (Stute et al., 2007).
The studied Holocene aquifer in Dan Phuong is very young, ~400 years (Larsen
et al., 2008), compared to the 2,000 to 4,000 years old Bangladeshi aquifer
studied by Harvey et al. (2002) and the 6,000 years old aquifer studied by
Polizzotto et al. (2008). Probably the reactivity of the organic carbon in our
young aquifer is higher than that of these older aquifers, facilitating the As
release at depth in Dan Phuong. Indeed, As concentrations increased upon the
injection of organic carbon to the Bangladeshi aquifer (Harvey et al., 2002),
indicating that mobilization of As from the aquifer sediment is limited by the
supply of reactive organic carbon from the surface.
Hence, in the South East Asian deltas high levels of As mobilization under
natural conditions has been observed, but alterations in the hydraulic regime
and/or the supply of carbon or As source material will affect the groundwater As
concentrations (Polizzotto et al., 2008; Postma et al., 2007; Larsen et al., 2008;
Berg et al., 2008). Where sedimentation has not ceased due to dike construction
and flow regulations of the major rivers, the deposition of As-loaded sediments
carried by the rivers results in a continuous flux of As to the delta aquifers
(Polizzotto et al., 2008). Alternatively, where flooding control has caused
sediment deposition to cease, mass balance estimates indicate that decades to
millennial timescales are required before aquifers are flushed for their
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