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

Jessen et al., manuscript to be submitted to Geochim. Cosmochim. Acta
4.3. Sulphate reduction and organic matter degradation
Previous studies at the H transect (Fig. 1a) suggests that As is released within the
aquifer by reductive dissolution of As-containing Fe oxides (Postma et al., 2007). A
reactive transport model for the H transect by Postma et al. (2007), was optimized to an
organic carbon degradation rate of 0.15 mmol C/L/yr and a solid As/Fe mole-ratio of
2.5‰. Using these values together with a residence time of 30 days would yield a
release of 0.1 μM of As, which would not show in the observed data shown in Fig. 7.
The reduction of SO4 2- is evident from the increased HS - concentration and the
elevated SIFeS(ppt) (Figs. 8f and i). While all other reactions seem to have occurred up-
gradient of N1, the reduction of SO4 2- apparently occurs within the aquifer, because in
Fig. 8e, SO4 2- arrives 10 days later than predicted by the model (Fig. 8f). The quantity
of SO4 2- that becomes reduced along the travel path between N1 and N4 can be
estimated from the difference between the modelled and observed SO4 2- in N4 on day
34, and amounts to ~20 μM. This translates to a sulphate reduction rate of ~4
μmol/L/day, applying the five days travel time from N1 to N4.
The sulphate may become reduced by organic carbon (OC), as represented by
reaction 4 in Table 2, or by CH4. The reduction of one mole of SO4 2- to HS - requires
eight e - equivalents, while each mole of OC will donate four e - equivalents. Therefore, if
OC drives the SO4 2- reduction, it will correspond to an OC degradation rate of 8 μmol
C/L/day. This rate is reasonable for a young Holocene aquifer (Appelo and Postma,
2005), but 30 times higher than the rate applied by Postma et al. (2007) who noted that
the H transect model was extremely sensitive to the rate of OC degradation.
Alternatively, SO4 2- may be reduced by anaerobic CH4 oxidation according to the
overall reaction:
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