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

Jessen et al., manuscript to be submitted to Geochim. Cosmochim. Acta
sediment at a dissolved As concentration of 3.3 μM (Fig. 4). The length of the
equilibration time, indicated by the symbols in Fig. 4, is not related to the adsorbed or
solute As(III) concentration, suggesting a fast process. Points obtained by desorption
subsequent to an adsorption step (connected by dashed lines, Fig. 4) show that the
As(III) sorption reaction is reversible.
3.2. Field experiment
3.2.1. Hydrogeology
The forced gradient experiment was conducted in a transect at the bank of a side
channel of the Red River (Fig. 1 and 2). The Holocene aquifer consists of fine sand to
pebbles underneath a surface cover of silt-clay to fine sand (Fig. 2). Drillings in the
channel bottom (Fig. 2) reached a sand layer after penetrating the silty-clayey unit,
suggesting that the sandy aquifer extends beneath the channel bottom. In the drilling
furthest away from the well field, a sandy sequence with only few thin clay layers was
observed (Fig. 2).
The surface water-groundwater interaction at the field site is described by Larsen
et al. (2008). During the dry season the Red River stage is at a minimum, and the
channel is disconnected from the Red River (Fig. 1a). The water body in the channel is
perched, causing a downward infiltration to the Holocene aquifer, and the channel
almost dries out at the end of the dry season (Fig. 2). A general northeastward
groundwater flow direction with discharge to the Red River is observed during the dry
season. During the rainy season the Red River stage rapidly increases and the channel
and the Red River become connected. The downward flow from the channel to the
underlying aquifer persists during the initial part of the rainy season. As the Red River
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