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

3120 S. Jes sen et al. / Applied Geochemistry 23 (2008) 3116–3126
and Alam, 2004). The two estu a rine sed i ment units (Holocene
in the SW, and Pleis to cene in the NE part of the transect)
are clay-rich, con sis tent with the ele vated gammaray
lev els of up to 140–150 API. The gamma-ray lev els in
the Holo cene sand depos its are rel a tively high, 110–120
API, con firm ing the silty-clayey con di tions described in
the lith o log i cal logs. The lithol ogy of the sed i ments will be
described fur ther in Sec tion 4.1.
3.2. Cross sec tion hydrol ogy
Fig. 4A shows the dis tri bu tion of 18 O in the water
sam ples col lected by the depth-spe cific sam pling in the
11 mon i tor ing wells (Fig. 1B). The value of 18 O ranges
from ¡12.2 to ¡6.3‰, and rel a tively 18 O-rich ground water
is gen er ally found in the Holo cene depos its while the
ground wa ter in the Pleis to cene depos its is more depleted
in 18 O with 18 O-val ues lower than ¡8.6‰. The ver ti cal
dis tri bu tion of hydrau lic head (not shown) gen er ally indicated
a down ward migra tion of water. In the NE part of
the tran sect (dis tance 20–50 km), a ver ti cal hydrau lic head
dif fer ence of 2.4–2.7 m water col umn was observed across
the estu a rine depos its, while a less pro nounced ver ti cal
hydrau lic head dif fer ence (