Fate of Arsenic during Red River Water Infiltration into Aquifers beneath Hanoi, Vietnam

Abstract

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 (P_CO2) 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.

Figure 1

Hydrogeological setting at the Nam Du field site. The upper layers of clay and fine–medium-grained sand belong to the Holocene, the coarse sand–gravel to the Pleistocene, and the silt–claystone to the Neogene period. Water levels in the different boreholes are indicated by the ▼ symbol. The low measured electrical conductivity (EC) values (200–270 μS/cm) indicate the presence of river water in the whole system. Arsenic is low to absent in the river water and upper 17 m of the groundwater but is mobilized further down.

Figure 2

Red River water infiltration into the aquifer: tritium–helium groundwater ages, plotted in duplicate at all depths, show the presence of young water down to 40 m depth. The electrical conductivity (EC) in the groundwater is close to the value of 205 μS/cm in the river water. The partial pressure of CO₂ is 10^–3.3 in river water and 10^–2.3 to 10^–2.8 in the groundwater.

Figure 3

Effect of redox processes on the groundwater chemistry in the Holocene aquifer at Nam Du. Symbols designate field data, and lines indicate reactive transport model predictions with adsorption of As(III) disabled. In the model, the upper cell consumes 0.27 mM DOC, which reduces O₂ and NO₃ in the infiltrating water. Further down, the oxidation of organic carbon and reduction of As-containing Fe-oxides are controlled by rate equations in the model of Postma et al. Model lines are given for 60 years but are nearly identical for other times <100 year.

Figure 4

Leaching and retardation of As(III) from the Holocene aquifer. The symbols are field data. Lines are leaching profiles calculated using a K′_d of 12.6 L/kg and a flow velocity of 19 m/year, derived from tritium–helium dating. The calculations were carried out for an initial groundwater concentration of 1 μM As(III).