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Download fileFate of Arsenic during Red River Water Infiltration into Aquifers beneath Hanoi, Vietnam
journal contribution
posted on 2016-12-13, 00:00 authored by Dieke Postma, Nguyen Thi Hoa Mai, Vi Mai Lan, Pham Thi Kim Trang, Helle Ugilt Sø, Pham Quy Nhan, Flemming Larsen, Pham Hung Viet, Rasmus JakobsenRecharge
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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water fluxgroundwater arsenic concentrationVietnam RechargeRed River Water Infiltration1 μ Mriver water infiltrationCO 2Infiltrating river waterarsenic-contaminated aquifersRed River waterWorld Health Organization limitgravelly Pleistocene aquifersP CO 240 m depthflow rateAs-containing Fe-oxidescarbon oxidation causesriver waterreaction rateHolocene aquiferequilibrium-controlled desorptiongroundwater age