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

5058 D. Postma et al. / Geochimica et Cosmochimica Acta 71 (2007) 5054–5071
we have confined ourselves to a summary to provide the
background for the chemical data.
2.6. Geochemical modeling
Speciation calculations and reactive transport modeling
were done using the code PHREEQC (Parkhurst and
Appelo, 1999). The employed database was for aqueous
As based on the compilation of Langmuir et al. (2006).
3. RESULTS
3.1. Water chemistry in the aquifer
Table 2 lists the water composition in the Red River as
compared to a typical composition of the water in the
Holocene aquifer. The groundwater is a CaHCO 3–
MgHCO 3 type of water, anoxic and enriched in methane
and ferrous iron. The river water is a much more dilute oxic
CaHCO3 type of water. The substantial difference between
the composition of the oxic river water and the water in the
anoxic part of the aquifer indicates that sediment-water
interactions have a large impact on the groundwater composition.
Table 2 also contains the logP CO2 and saturation
index (SI) for calcite as calculated with PHREEQC.
3.1.1. Redox conditions
Fig. 4 shows the distribution of the main redox sensitive
components in the aquifer. The upper 2 m of the saturated
zone contains some O2 but the concentration is significantly
lower than the concentration of 0.26 mM (27 °C) expected
for equilibrium with the atmosphere. At slightly greater
depth nitrate and manganese are found at elevated concen-
Table 2
Water composition in the Red River and in the groundwater of
borehole H51 at elevation 0.3 m (Fig. 2)
River water Ground water
EC (lS/cm) 290 880
Temp. (°C) 30 26.4
O2 (mmol/L) 0.3 0
pH 7.18 6.95
As total (lmol/L)