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

Jessen et al., manuscript to be submitted to Geochim. Cosmochim. Acta
and N4 on day 9 has natural 18 O-values of -8.7‰ to -8.5‰, consistent with our
previous study at the field site (Larsen et al., 2008). On day 34, the values of 18 O have
increased to -5.9‰ to -5.0‰, respectively, whereas on day 54 the 18 O-values have
returned to -9.0‰ to -8.9‰. A sample taken from OB4, positioned upgradient and
inland from the pumping wells (Fig. 1b and 2), shows a similar 18 O-value of -9.0‰ on
day 54. Contrasting to these values, the 18 O-value of the channel water decreases
successively from -9.4‰ on day 9, to -10.9‰ on day 54, resembling the isotopic
signature of the water in the Red River main channel (Larsen et al., 2008).
3.2.2. Water chemistry
The variation of the electrical conductivity (EC) and of major cations in groundwater
and channel water during the pumping period is shown in Fig. 6 for the observation
wells N1 and N4 and the channel water. Fig 6a shows an initial EC of the groundwater
of 765 μS/cm, while the EC of the channel water is at 180 μS/cm. On day 9 after the
start of pumping the EC begins to decrease, indicating the first appearance of water
derived from the channel. The breakthrough curves for EC are shifted by ca. five days
for the well closest to the channel (N1) and the well closest to the pumping well (N4).
On day 34, when the flow direction reverses, a minimum EC of 340 μS/cm is observed
in both N1 and N4. After day 34, the EC increases again, but this time first in N4 and
then in N1. On day 54, the EC in the two observation wells has returned to the initial
(pre-pumping) values. In the channel the EC increases after day 34, and approaches that
of the groundwater. This indicates a discharge of groundwater to the channel subsequent
to day 34, when the hydraulic head of the aquifer exceeds the channels stage (Fig. 5a).
During the following year, with no pumping, the EC in N1 remained at 930 ± 11 μS/cm,
15