Recharge
of Red River water into arsenic-contaminated aquifers
below Hanoi was investigated. The groundwater age at 40 m depth in
the aquifer underlying the river was 1.3 ± 0.8 years, determined
by tritium–helium dating. This corresponds to a vertical flow
rate into the aquifer of 19 m/year. Electrical conductivity and partial
pressure of CO2 (PCO2) indicate that water recharged from the river is present in both
the sandy Holocene and gravelly Pleistocene aquifers and is also abstracted
by the pumping station. Infiltrating river water becomes anoxic in
the uppermost aquifer due to the oxidation of dissolved organic carbon.
Further downward, sedimentary carbon oxidation causes the reduction
of As-containing Fe-oxides. Because the release of arsenic by reduction
of Fe-oxides is controlled by the reaction rate, arsenic entering
the solution becomes highly diluted in the high water flux and contributes
little to the groundwater arsenic concentration. Instead, the As concentration
in the groundwater of up to 1 μM is due to equilibrium-controlled
desorption of arsenic, adsorbed to the sediment before river water
started to infiltrate due to municipal pumping. Calculations indicate
that it will take several decades of river water infiltration to leach
arsenic from the Holocene aquifer to below the World Health Organization
limit of 10 μg/L.
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