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

Jessen et al., manuscript to be submitted to Geochim. Cosmochim. Acta
level decreases later in the rainy season, the head in the Holocene aquifer exceeds the
channel stage, causing upward discharge of groundwater to the channel. Towards the
end of the rainy season, the groundwater head gradually lowers, the channel becomes
again disconnected, and in the initial part of the following dry season the channel stage
again exceeds the head of the Holocene aquifer.
Fig. 5 shows the water level in the channel and in the two observation wells N1
and N4 (Fig. 1 and 2) from 25 days before the start of pumping, followed by 55 days of
continuous pumping (Fig. 1b and 2). From day -25 to day 34, the head in the channel is
above that observed in N1 and N4, but on day 34 the channel head decreases to below
the well heads. Pumping was initiated on 7 July 2007 (day zero), when the channel
became connected to the Red River, flooding first the flat sandy channel bottom and
later also the bank slope (Fig. 2). Fig. 5b shows the gradient between the two
observation wells N1 and N4. Unfortunately, the observations in Fig. 5b have a large
and possibly systematic uncertainty, but still the change in flow direction induced by the
pumping is significant. During day zero to around day 35, the head difference (Fig. 5b)
between N1 and N4 infers a flow from the channel towards the pumping wells (Fig. 1b
and 2). When the channel stage drops after 30 days of pumping, the gradient reverses
(Fig. 5b), inferring flow in the opposite direction. Hence, the natural hydraulic gradient
between the groundwater head and the channel head overrides the effect of the sustained
pumping.
3.2.1.1. Isotopic composition. In order to identify more directly the source of the water
drawn to the transect by pumping, we analyzed 18 O and 2 H in groundwater and
channel water samples of day 9, 34 and 54 (Fig. 9). The groundwater sampled from N1
14