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

Jessen et al., manuscript to be submitted to Geochim. Cosmochim. Acta
The overall chemical variation of the channel water along the infiltration flow path
to the aquifer can be described by the three types of reactions shown in Table 2: (i)
electron transfer, (ii) weathering and (iii) cation exchange (Jacobs et al., 1988). Most of
the reactions in Table 2 cause a change in the alkalinity and/or in the total inorganic
carbon (TIC), and therefore the contribution of each reaction to the overall change in
solution chemistry can be traced in an AlkCO -TIC diagram (Jacobs et al., 1988). In Fig.
2
10, the reactions occurring during the infiltration of channel water to the aquifer are
represented by vectors laid end-to-end, with references to the reaction numbers in Table
2. For the calculation, the composition of the infiltrated channel water was set equal to
the composition in the sample from N1 on day 34; the channel water composition was
set to average parameter values during day zero to day 34. Fig. 10 shows that the total
increase in Alk CO is covered by dissolution of calcite (reaction 6) and biotite (reaction
2
7), as calculated from the increase in Ca 2+ and Si, respectively, between the channel
water and the composition in the sample from N1 on day 34. The contributions from
reactions 1-4 are minor, due to the generally low concentrations of O2 (decreases by
0.22 mM), and especially SO4 2- and NO3 - (Fig. 8a and e). The concentration of Fe 2+
increases by 0.076 mM (Fig. 8d), resulting in a small alkalinity increase by 0.15 meq/L
(reaction 3). The remaining TIC deficit of 1.5 mM must be due to O2 diffusion into the
channel bed during the dry season and its reaction with organic matter (reaction 1) and
methanogenesis (reaction 5) with the escape of CH4 to the atmosphere, both processes
probably accompanied by carbonate dissolution (reaction 6).
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