Fate of Arsenic during Red River Water Infiltration into Aquifers

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Fate of arsenic during Red River water infiltration into aquifers beneath Hanoi, Vietnam. Dieke Postma, Nguyen Thi Hoa Mai, Vi Mai Lan, Pham Thi Kim Trang, Helle Ugilt Sø, Pham Quy Nhan, Flemming Larsen, Pham Hung Viet, and Rasmus Jakobsen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05065 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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Environmental Science & Technology

Revision submitted to EST

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Fate of arsenic during Red River water

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infiltration into aquifers beneath Hanoi,

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Vietnam.

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Dieke Postma*1, Nguyen Thi Hoa Mai2 , Vi Mai Lan2, Pham Thi Kim Trang2, Helle Ugilt Sø1,

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Pham Quy Nhan3, Flemming Larsen1, Pham Hung Viet2, Rasmus Jakobsen1

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Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen, Denmark.

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Research Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi

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University of Science (VNU), Hanoi, Vietnam.

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* corresponding author [email protected] phone +45 30273702

Dept. of Hydrogeology, Hanoi University of Mining and Geology (HUMG), Hanoi, Vietnam.

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ABSTRACT

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Recharge of Red River water into arsenic contaminated aquifers below Hanoi was investigated. The

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groundwater age at 40 m depth in the aquifer underlying the river was 1.3 ± 0.8 year, determined by

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tritium-helium dating. This corresponds to a vertical flow rate into the aquifer of 19 m/yr. EC and

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PCO2 indicate that water recharged from the river is present in both the sandy Holocene and gravelly

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Pleistocene aquifers and is also abstracted by the pumping station. Infiltrating river water becomes

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anoxic in the uppermost aquifer due to the oxidation of dissolved organic carbon. Further

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downward, sedimentary carbon oxidation causes the reduction of As-containing Fe-oxides. Because

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the release of arsenic by reduction of Fe-oxides is controlled by the reaction rate, arsenic entering

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the solution becomes highly diluted in the high water flux and contributes little to the groundwater

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arsenic concentration. Instead the As concentration in the groundwater of up to 1 µM is due to

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equilibrium controlled desorption of arsenic, adsorbed to the sediment before river water started to

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infiltrate due to municipal pumping. Calculations indicate that it will take several decades of river

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water infiltration to leach arsenic from the Holocene aquifer to below the WHO limit of 10 µg/l.

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INTRODUCTION

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Many large Asian cities rely on groundwater for their water supply and the abstraction of

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groundwater has commonly caused the development of large depression cones1-5. In addition, some

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of these cities, exemplified by Kolkata6,7, and Hanoi8,9 are situated on top of aquifers that are

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seriously contaminated by geogenic arsenic. Generally the uppermost Holocene aquifers are most

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contaminated by arsenic while underlying Pleistocene aquifers in most cases are low in arsenic10,11.

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Because groundwater abstraction for municipal water supply occurs from the deeper Pleistocene

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aquifers, with groundwater low in arsenic, the change in groundwater flow patterns may cause the

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migration of arsenic rich groundwater from upper aquifers into the underlying low arsenic

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aquifers8,9,12,13. The effect of municipal groundwater abstraction pumping also influences the water

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quality on a regional scale because depression cones extend far beyond the city limits5,14 .

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Both Hanoi and Dhaka are located along major rivers and hydrogeological studies

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have shown1,2,15,16 groundwater abstraction to cause a very significant infiltration of river water into

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the underlying aquifers. Recharge of aquifers by losing rivers, often called bank infiltration,

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enhances the available water resource and managed and monitored bank infiltration is a strategy

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that has been used extensively particularly in Europe17-20. Apart from increasing the quantity of the

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groundwater resource, bank infiltration also may improve the quality of the infiltrating surface

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water. During bank infiltration, processes taking place at the surface water-groundwater interface

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can remove nutrients, organic carbon, including organic contaminants, and pathogens such as

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microbes and viruses21,22. However, the effect of Red River bank infiltration on the water quality of

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the aquifers below Hanoi is completely unknown even though it is considered as the main source of

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groundwater recharge below the city2.



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The city of Hanoi has a leaky sewerage system as well as open channels carrying

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wastewaters and untreated sewage water through the city, both of which are leaking into the

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underlying aquifers12,23,24. In addition the aquifers beneath Hanoi are contaminated by arsenic with

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concentrations exceeding 300 µg/L8,9. In contrast the water of the Red River running along the city

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of Hanoi is very low in dissolved arsenic12,25,26. Therefore one could hope that the large scale

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infiltration of river water2 can improve the water quality in the aquifers and also flush out the

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arsenic. The time scale needed for flushing arsenic from the aquifer is currently unknown but must

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depend on infiltration rates, groundwater flow patterns and residence times as well as the

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geochemical processes controlling the groundwater arsenic content. The latter includes the

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mobilization of arsenic by reductive dissolution of Fe-oxides and the adsorption and desorption of

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the mobilized arsenite12,25,27. The objective of the present study is to investigate the rate of

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infiltration of Red River water into the underlying aquifer, clarify the geochemical processes

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occurring between the infiltrating water and the aquifer sediment and predict the effect of river

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water infiltration on the groundwater arsenic content.

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MATERIALS AND METHODS

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Field Area Description. Our field locality at Nam Du (20o58´25.43”N, 105o54’09.73”E) is located

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on the eastern boundary of the city of Hanoi, Vietnam, directly on the shore of the Red River. The

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geology of the area has been investigated extensively in relation with the establishment of the Nam

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Du well field in 200412. Generally, the geological sequence in the area consist of a top clay layer

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underlain by a up to 40 m thick Holocene aquifer consisting of fine to medium grained sand,

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comprising some more fine or coarse grained layers. The sandy aquifer is underlain by an aquitard

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of variable thickness (1-10 m) followed by a coarse grained sand to gravel aquifer of Pleistocene


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age12. Remote sensing shows the Nam Du area to belong to the meander belts of the Red River28. A

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detailed geological profile of our study area, based on natural gamma logging of boreholes and

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sediment description is shown in Fig.1. Uppermost there is an up to 11 m thick layer of clay and

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silty clay which is absent close to the river. Below follows an up to 40 m layer of fine to medium

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sized sand. Underneath there is an up to 20 m thick layer of coarse sand and gravel. Between the

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two sand formations a clay layer is found in some borings but not in all. Finally at the bottom there

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is a layer of claystone-siltstone. In accordance with the geological description of the area12, the

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upper aquifer with fine grained sand is considered to be of Holocene age while the underlying

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coarse sand-gravel layer is interpreted as being of Pleistocene age. The intermediate clay lenses are

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probably remnants of the aquitard separating the Holocene and the Pleistocene aquifers which

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elsewhere is found as a more continuous layer12. The claystone-siltstone corresponds to the

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lithology of the Neogene in the area.

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The Red River at this location is about 400 m wide. The depth of the river channel at

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Nam Du varies between 13.5 and 17.0 m below the ground level at the nest of boreholes on the

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shore (Fig.1). The mid channel depth is 15.3 m and is situated within the Holocene aquifer. The

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groundwater below Hanoi has been exploited since 1894 and groundwater abstraction presently

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amounts to > 900,000 m3/day27. Groundwater is being pumped mainly from six major well fields

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distributed throughout the city2. As a result, large coalescing depression cones have developed

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below the city of Hanoi in both the Holocene and Pleistocene aquifers which are partially

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connected8,12. The well field at Nam Du comprises a gallery of pumping stations close to the river

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(Norrman 2008). The pumping station included in our field site (Fig 1) is part of this gallery of

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pumping stations but the flow towards the pumping station is the result of the drawdown caused by

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the entire pumping activity below Hanoi12. The hydrostatic pressure in our wells screened in the

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Pleistocene aquifer is about 6 m lower than those in the nest screened in the Holocene aquifer (Fig. 5 ACS Paragon Plus Environment


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1), indicating a strong downward gradient induced by the pumping. The pumping combined with

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the less permeable Holocene deposit on top of the high permeable Pleistocene causes

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predominantly vertically downward groundwater flow, providing the shortest passage through the

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Holocene. The pressure difference between the Holocene and Pleistocene aquifers is also found

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elsewhere in the city but its magnitude varies from locality to locality2,12.

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Groundwater sampling and analysis. To investigate the hydrogeological and geochemical

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processes we have collected three sets of data (Fig. 1); A) Detailed water sampling of the sediment

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porewater performed on a sandbank in the river. B) Water sampling in a nest of boreholes located

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on the river shore with screens installed at different depths in the Holocene aquifer, C) Water

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sampling in boreholes and the nearest pumping station screened in the Pleistocene aquifer.

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To sample the interface between river and groundwater we used a 1 cm diameter

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drive-point piezometer, and a 50 mL syringe for extracting the water. Sampling was carried out in

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shallow water, approximately 25 cm deep, on a sandbank in the river located about 50 m from the

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shore, with a depth resolution of 0.2-0.5 m until 3 m below the surface (Fig. 1). At each depth the

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internal volume of the piezometer was emptied at least three times before taking the sample. On the

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shore a nest of 11 wells (Fig 1) was installed in the depth range 4.9-40 m. Additionally, four wells

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ranging in depth from 40 to 60 m were installed in the Pleistocene aquifer between the pumping

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station and the nest at the shore (Fig. 1). The wells were equipped with Ø60 mm PVC-casings, a 1

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m long screen and 1 m sand trap. A quartz sand filter pack was installed, and the well was sealed

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using bentonite. The top end of the PVC-casing was sealed to prevent the entrance of surface water

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during flooding. To further avoid leakage along the casing, the top of the wells were embedded in a

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concrete platform, each well within a protective steel casing and steel screw cap. Directly after

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completion the wells were pumped to remove the water affected by the drilling operation. Sampling


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of the wells was done more than three months after their installation in 2010-2011. The pumping

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well is screened 48-65 m below the surface.

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Groundwater was sampled from the boreholes using a Grundfos MP1 submersible

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pump. Five borehole volumes were flushed before taking the sample. O2, pH, and electrical

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conductivity (EC) were measured with probes in a flow cell mounted directly on the sampling tube.

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Samples for other parameters were collected in 50 mL syringes and filtered through 0.2 µm

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Sartorius Minisart cellulose acetate filters. Aqueous As(V) and As(III) were separated by passing

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the water through a disposable anion exchange cartridge29. The cartridge was together with the

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filter, carefully flushed with N2 before use. Arsenite was determined as the As concentration in the

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water filtered through a cartridge, and As(V) was calculated as the difference between the total As

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and As(III) concentrations. In the field Fe(II) was measured spectrophotometrically and alkalinity

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determined by Gran-titration. Samples for cations were preserved with 2% of a 7 M HNO3 solution

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and refrigerated until analysis in the laboratory. Samples for NH4+, Cl- and SO42- were collected in

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20-mL polypropylene vials and frozen immediately after sampling. In the laboratory cations were

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analyzed by flame atomic absorption spectrophotometry. Arsenic was determined on the same

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instrument using a HVG hydride generator and a graphite furnace. Anions were analyzed by ion

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chromatography using a Shimadzu LC20AD/HIC-20A Super. Detection limits were; As 0.013 µM,

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Mn 0.91 µM, Ca 0.50 µM, NH4 5.6 µM, PO4 1.1 µM, NO3 3.2 µM, SO4 2.1 µM. For further details

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on procedures and methods consult Postma et al.25.

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Samples for tritium-helium dating were taken in duplicate in clamped copper tubes in

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2015 and analyzed by the Institut für Umweltphysik, Universität Bremen. 3H concentrations were

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determined by the 3He in growth method. The accuracy of the dates is given by the standard

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deviation as ± 0.8 years (Jürgen Sültenfuß, pers. comm.)



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RESULTS AND DISCUSSION

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River Water Infiltration. Tritium-helium dating of water sampled from the boreholes in the nest

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screened in the Holocene aquifer (Fig. 1) is shown in Fig.2. The groundwater is extremely young in

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screens at all depths indicating fast infiltration of river water into the underlying aquifer. The oldest

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groundwater age of 8 years is found at 30 m depth. This piezometer is located in one corner of the

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well cluster and here a clay layer (not shown in Fig. 1) was encountered during drilling from 26 to

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28 m depth which was not present in the other drillings just a few meters away. Below this clay

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layer, the groundwater apparently has a larger residence time yielding a higher groundwater age.

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The 30 m depth screen also shows a maximum in the dissolved As(III) concentration (Fig. 3) and in

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the NH4 concentration (not shown) reflecting that the longer groundwater residence time allows

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more As(III) and NH4 to build up. The remaining screens yield groundwater ages ranging from less

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than one year to 1.3 year in the screen at 40 m depth. Duplicate measurements are, except for the 30

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m screen (Fig. 2), within 0.05 year indicating a high precision. However, the estimated accuracy of

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± 0.8 year is high compared to the measured ages. In addition there is uncertainty as to whether

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infiltration occurs from the deeper or the more shallow parts of the channel. Using the vertical

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distance between the mean channel depth of 15.3 m and the groundwater age of 1.3 ± 0.8 years at

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40 m yields a vertical transport rate of 19 m/yr with an uncertainty range of 12-49 m/yr.

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A further indicator for river water intruding into the aquifer is the electrical

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conductivity (EC) of the groundwater (Fig. 2). Shallow groundwater on the shores of the Red River

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30 km NW of Hanoi, unaffected by pumping, has an EC in the range 750-1000 µS/cm25. River

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water, however, has an EC of around 200 µS/cm and the difference is due to mineral weathering

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mainly occurring in the soil zone. The strong contrast in EC between river water and normal


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groundwater makes the EC a good indicator for intrusion of river water into aquifers. As shown in

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Fig. 1 and 2, river water, groundwater in the Holocene and Pleistocene aquifers, as well as water of

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the pumping station all have an EC in the range 200-320 µS/cm indicating rapid recharge from the

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river to be the dominant source of water in the aquifer.

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An additional useful tracer parameter is the partial pressure of CO2. River water has a

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low PCO2 = 10-3.3, close to that in the atmosphere (PCO2 = 10-3.4). However water infiltrating through

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soils, or through organic rich sediment, is exposed to root respiration and organic decay that

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generate CO2. At our field site 30 km upstream along the Red River, the groundwater therefore has

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a PCO2 in the range 10-1.0 - 10-1.4 ref 25. Fig 2 shows how all the water in the aquifer below the river

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has a PCO2 in the range 10-2.3 to 10-2.8. These low partial pressures of CO2 also indicate the recharge

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of river water into the aquifer without being exposed to extensive organic matter decay or

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respiration. This is consistent with fast recharge of water through the bottom of the river.

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Redox Processes. The river water has an O2 concentration of 0.24 mmol/L, only slightly below the

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concentration of 0.27 mmol/L in equilibrium with the atmosphere30. In addition river water contains

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28 µmol/L of nitrate. At a depth of a few meters below the surface, both oxygen and nitrate have

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disappeared from the groundwater (Fig. 3). At the same time the river water, and one sample at 1 m

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depth, contain about 0.6 mmol/L dissolved organic carbon (DOC), while the remaining

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groundwater has a DOC concentration near 0.1 mmol/L. As the river water infiltrates, the DOC

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appears to be filtered by the uppermost sediment. Stoichiometric calculation suggest that the

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oxidation of 0.27 mmol/L DOC would be sufficient for the complete removal of dissolved oxygen

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and nitrate from the infiltrating river water. There is no evidence for Fe-oxides functioning as

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electron acceptor for DOC oxidation in the top sediment layers because Fe(II) appears in the

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groundwater at greater depth. This pattern of DOC filtration by the uppermost sediment layer and

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concomitant removal of dissolved O2 and nitrate, was also observed at other bank infiltration 9 ACS Paragon Plus Environment


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sites18,19,21,31. Below 10 m depth the Fe(II) concentration starts to increase in the infiltrating water

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(Fig. 3), probably because the degradation of sedimentary organic carbon causes reductive

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dissolution of Fe-oxides. Similar Fe(II) increases attributed to reductive dissolution of Fe-oxides

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have been observed at other sites with ongoing bank infiltration18.21.

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The distribution of arsenic in the river and groundwater is displayed in Fig. 1 and in

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detail for the Holocene aquifer in Fig. 3. The river water and the shallow groundwater are free of

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arsenic, while it is present in the deeper groundwater of the Holocene, the Pleistocene aquifers and

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the water of the pumping station. This distribution indicates that the infiltrating river water is

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mobilizing arsenic from the aquifer sediment. Postma et al.32,33 found the rate of arsenic

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mobilization from the sediment into the groundwater to depend on the burial age of the aquifer

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sediment with the youngest sediments being most reactive. The geochemical processes controlling

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the groundwater arsenic content were quantified in a 1-D reactive transport model, up to a burial

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age of 6000 years, using the code PHREEQC-334. In the model the oxidation of organic matter

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causes the reduction of arsenic-containing Fe-oxides, resulting in the release of Fe(II) and As(III) to

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the groundwater. Both organic carbon and Fe-oxides become consumed over time and less reactive

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as well. The oxidation of organic matter and the reduction of Fe-oxides are controlled by separate

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rate equations that quantify their decreasing rates over time (Supporting Information Eqn. 1 and 2).

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The rate equations were calibrated using the groundwater chemistry of four sites on the Red River

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floodplain 30 km NW of Hanoi33, where both the burial age of the Holocene aquifer sediment and

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the vertical flow rate of the groundwater are known.

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The application of the model developed for the field area 30 km N of Hanoi to the

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Nam Du field site assumes that floodplain sediment is similar at the two sites. This appears a

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reasonable assumption given that remote sensing28 shows both sites to belong to the Red River

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meander belts and they also have comparable Holocene sediments. Accordingly we have applied 10 ACS Paragon Plus Environment

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the model to the Nam Du data (see Supporting Information for the PHREEQC input file). The main

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difference between the two field sites is the much faster vertical flow rate at Nam Du of 19 m/yr

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compared to 0.5 m/yr in the undisturbed aquifers north of Hanoi32. The burial age of aquifer

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sediments adjacent to the river at Nam Du is not known but as end-member case we assume the

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aquifer to be very young (< 100 yr). With that assumption the organic carbon and iron oxides have

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the highest possible reactivity and calculated release rates of Fe(II) and As(III) will accordingly be

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maximum rates. Any older sediment would consequently show lower Fe(II) and As(III) release

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rates. The composition of the solution entering into the 1-D model is that of river water. The results

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calculated by the model for 70 years of infiltration are shown as the solid lines in Fig 3 but would

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be near identical for other times < 100 yr. To simulate in addition the consumption of DOC in the

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uppermost sediment we have added a constant rate of organic carbon degradation, of 5.1 mM C/yr

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in the upper layer of the model, matching the removal of O2 and NO3 in the incoming water by

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DOC oxidation (Fig. 3). Below the top layer the oxidation rate of sedimentary organic carbon, is

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calculated from the rate equation and the main electron accepting process is the reductive

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dissolution of Fe-oxides. Fig. 3 shows the modelled release of Fe(II) to the groundwater which

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corresponds to a rate of 35 µM Fe/yr. In the bottom 4 m of the profile a small amount of Fe(II)

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becomes precipitated as siderite but higher up in the profile all Fe(II) stays in solution. Comparison

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with field data shows the modelled values to be slightly higher than the field data but the

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assumption that the Holocene aquifer at Nam Du is young appears reasonable. Otherwise the

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modelled Fe(II) concentration would be much higher than the measured Fe(II) values because the

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Fe(II) release of an old low reactive sediment would be significantly lower.

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Fe-oxides being reduced in the model contain 1.2 mmol As(V) per mol of Fe, a value

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based on analysis of Fe-oxides by reductive dissolution in sediment samples from four sites with a

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different burial age of the aquifer sediment on the Red River floodplain33. During the reduction of



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Fe-oxides in the aquifer, As(V) becomes reduced to As(III) and is released to the groundwater.

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Using the model we can evaluate how much of the As measured in the groundwater could possibly

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be derived from reductive dissolution of As-containing Fe-oxides from sediment that is considered

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to be < 100 yr old. For any older sediment the arsenic release rate would be lower. To keep all

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As(III) released from reduced Fe-oxides in solution we have disabled the adsorption of As(III) in

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the model. Figure 3 compares the measured As(III) in the groundwater with the model prediction

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assuming reductive dissolution of As-containing Fe-oxide being the only source of As(III) in the

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water. The modelled As profile in Fig. 3 corresponds to an As(III) release rate of 0.052 µM

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As(III)/yr. For the 19 m/yr vertical flow rate, the modelled As(III) concentration at 40 m depth is

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0.11 µM while the uncertainty range of 12-49 m/yr in the vertical flow rate corresponds to a As(III)

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concentration range of 0.04-0.17 µM.

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Fig. 3 clearly shows the modelled As concentration, released by reductive dissolution

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of Fe-oxides in 70 year old sediment, to be much lower than the As concentration measured in the

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aquifer. Any older sediment would have a lower reactivity and therefore a lower As(III) release

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rate. Therefore most of the As(III) present in the groundwater cannot be released by reductive

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dissolution of As-containing Fe-oxides. Note that the same release rate of 0.052 µM As(III)/yr

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yields a modelled groundwater arsenic concentration of up to 4 µmol As(III)/L at the Dan Phuong

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locality upstream along the Red River33. Because the reduction of Fe-oxides, and thereby the release

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of As, is kinetically controlled the resulting groundwater concentration will depend on the

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groundwater flow rate35. At the Dan Phuong site the downward groundwater velocity is 0.5 m/yr25

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while at Nam Du it is 19 m/yr. The same As(III) release rate therefore produces groundwater with a

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As(III) concentration that is almost 40 times higher at Dan Phuong than at Nam Du! The

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importance of reaction kinetics on arsenic mobilization is also evident from the much higher arsenic

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concentration found in porewater in sediment along the shores of the Red River, 6 km further


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downstream at Van Phuc26. Vertical flow rates here are 0.1-3.1 m/yr26, which is an order of

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magnitude lower than the vertical flow rate we derive for the bottom of the river channel, and

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therefore result in a higher arsenic concentration. In fact, the main role of redox processes for

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arsenic mobility in the Nam Du aquifer is to maintain anoxic conditions in the aquifer, thereby

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keeping arsenic in its As(III) form. If the oxidation of arsenic to As(V) should occur it would cause

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a much stronger arsenic retardation because As(V) adsorbs much stronger to the aquifer sediment36.

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As(III) retardation. Groundwater must have been high in As(III) before water supply pumping by

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the city of Hanoi induced river water infiltration into the aquifer, because high As(III)

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concentrations are seen everywhere else on the Red River floodplain in aquifers contained in young

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sediments25,32. Dissolved As(III) in the groundwater is in equilibrium with As(III) adsorbed onto the

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sediment and the results of a forced gradient experiment37 and push pull tests38 indicate that arsenic

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sorption rapidly responds to changes in the water chemistry. The infiltration of low arsenic river

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water is therefore expected to cause desorption of arsenic from the aquifer sediment. The

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distribution of As(III) in the groundwater over depth, with low As(III) in the upper 15 m and higher

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As(III) further down (Fig. 3), is consequently interpreted as a leaching front and its position will be

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determined by the number of pore volumes of groundwater flushed through the aquifer and the

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adsorption isotherm.

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In a previous study36 we experimentally investigated the adsorption of As(III) onto the

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Holocene aquifer sediment at the Nam Du field site using a sediment sample from 5.5-6 m depth.

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The results showed a linear adsorption isotherm up to an aqueous concentration of at least 10 µM

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As(III) corresponding to a K’d of 12.6 L/kg. The adsorption of As(III) was also found to be

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completely reversible. For the measured aqueous concentration of 1 µM As(III) (Fig. 3) and a

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porosity of 0.439 the isotherm predicts an adsorbed concentration of 59 µmol As(III) per liter of

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contacting groundwater corresponding to a retardation of 60. Accordingly it will take the flushing 13 ACS Paragon Plus Environment


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of 60 pore volumes of groundwater through the Holocene aquifer to desorb all adsorbed As(III).

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Starting with an initial concentration of 1 µM As(III) in the groundwater, close to what is measured

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in the groundwater, and the corresponding adsorbed As(III) concentration of 59 µmol/L, we have

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calculated how the As(III) versus depth profile would look like at different times and compared it to

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the measured As(III) concentration (Fig. 4). As the water moves downward the As(III)

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concentration increases along the isotherm until it reaches the concentration that was present in the

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aquifer before river water infiltration began. Because the adsorption isotherm is linear, the shape of

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the leaching front will be independent of the As(III) concentration. In the model, the vertical flow

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rate is 19 m/yr and the leaching front moves downward with 0.32 m/yr and after 70 years of

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modelled river water infiltration there is a reasonable correspondence with the field data (Fig. 4).

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Using the uncertainty in the infiltration rate from 12 to 49 m/yr the range in front progression is

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0.20 to 0.82 m/yr and the time for the leaching front to reach the same depth from 112 to 27 years.

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Although groundwater abstraction in Hanoi started 100 years ago8 we do not know when river

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water started to infiltrate into the aquifer at Nam Du. Van Geen et al.27 constructed a flow model for

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Van Phuc, on the next bend of the river 6 km further south. Here the pumping of the Hanoi water

307

supply has caused a flow reversal from a gaining to a losing river. The timing of the flow reversal

308

was estimated to be between 1951 and 1971. Our estimate of 70 ± 42 years of river water

309

infiltration is within the time space of groundwater abstraction by the Hanoi water supply but

310

clearly associated with considerable uncertainty.

311

Environmental Perspective. Groundwater flow modeling2 suggest that infiltrating Red River water

312

is the main source of groundwater recharge into the aquifers beneath Hanoi which are heavily

313

pumped by the municipal water supply. Infiltrating river water may be beneficial for the

314

groundwater quality in at least two ways. First of all, the river water contains no arsenic and one


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315

may hope that its infiltration will lower the groundwater arsenic content over time. The second

316

effect will be dilution of the sewage leaking into the aquifer23,24.

317

It is of major importance to understand the hydrogeological and geochemical

318

processes occurring at the river-groundwater interface to assess the future water quality in the

319

aquifers below Hanoi. Our data from Nam Du suggest that very fast recharge is taking place from

320

the river channel. However another recent study at the Van Phuc site26 located 6 km downstream

321

from our site at Nam Du suggests much slower infiltration to occur along the shores of the river. In

322

future work it would be important to delineate the relative importance of the two modes of recharge,

323

particularly because they seem to result in a different groundwater arsenic content. While we find at

324

Nam Du that due to the high infiltration the kinetically controlled reductive dissolution of As-

325

containing Fe-oxides only contributes insignificantly to the groundwater arsenic content, the study

326

at Van Phuc suggest that the slow infiltration rates allow much higher arsenic concentrations to

327

build-up in the groundwater26. An additional reason for the much higher arsenic concentrations

328

found in porewater of the shoreface sediment could also be that Van Phuc is located just

329

downstream of the channel at Yen My which drains most of the untreated urban wastewater from

330

the city of Hanoi into the Red River24, providing an ample source of reactive carbon.

331

While the infiltrating river water may readily displace the original groundwater as the

332

result of pumping, our results indicate that a decrease in the arsenic concentration in the pumped

333

water will take much longer. Our data indicates that it probably will take another 60 ± 36 years to

334

flush the adsorbed As(III) from the Holocene aquifer. To reach the screens of the groundwater

335

abstraction wells the groundwater thereafter flows through Pleistocene deposits. Van Geen et al.27

336

determined the retardation of As(III) in Pleistocene sands to be 16-20 which is a factor 3-4, lower

337

than we find in the Holocene. In addition many Pleistocene sediment layers consist of highly

338

permeable gravels that probably have an even lower retardation. Flushing of arsenic from the 15 ACS Paragon Plus Environment


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339

Pleistocene aquifers is therefore expected to proceed much faster than from the Holocene aquifers.

340

Thus bank infiltration of Red River water into the Holocene and Pleistocene aquifers will likely

341

cause the groundwater arsenic concentration to decrease but it will take several decades.

342

343

SUPPORTING INFORMATION

344

Description of the reactive transport model and model input file

345

346

ACKNOWLEDGEMENT

347

This research has been funded by the European Research Council under the ERC Advanced Grant

348

ERG-2013-ADG. Grant Agreement Number 338972.

349

350

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FIGURE CAPTIONS

521

522

Figure 1. Hydrogeological setting at the Nam Du field site. The upper layers of clay and fine-

523

medium grained sand belong to the Holocene, the coarse sand-gravel to the Pleistocene and the silt-

524

claystone to the Neogene period. Water levels in the different boreholes are indicated by ▼. The

525

low measured electrical conductivity (EC) values (200-270 µS/cm) indicate the presence of river

526

water in the whole system. Arsenic is low to absent in the river water and upper 17 m of the

527

groundwater but is mobilized further down.

528

529

Figure 2. Red River water infiltration into the aquifer: Tritium-Helium groundwater ages, plotted in

530

duplicate at all depths, show the presence of young water down to 40 m depth. The electrical

531

conductivity (EC) in the groundwater is close to the value of 205 µS/cm in the river water. The

532

partial pressure of CO2 is 10-3.3 in river water and 10-2.3 to 10-2.8 in the groundwater.

533

534

Figure 3. The effect of redox processes on the groundwater chemistry in the Holocene aquifer at

535

Nam Du. Symbols designate field data and lines reactive transport model predictions with

536

adsorption of As(III) disabled. In the model the upper cell consumes 0.27 mM DOC which reduces

537

O2 and NO3 in the infiltrating water. Further down the oxidation of organic carbon and reduction of

538

As-containing Fe-oxides are controlled by rate equations in the model of Postma et al. (2016).

539

Model lines are given for 70 years but are near identical for other times < 100 yr.

540


Page 25 of 30


541

Figure 4. Leaching and retardation of As(III) from the Holocene aquifer. The symbols are field

542

data. Lines are leaching profiles calculated using a K’d of 12.6 L/kg and a flow velocity of 19 m/yr

543

derived from tritium-helium dating. The calculations were carried out for an initial groundwater

544

concentration of 1 µM As(III).



Figure 1. ACS Paragon Plus Environment

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EC µS/cm 0

200

400

0

10

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30

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Figure 2.

ACS Paragon Plus Environment HeT_GEUSVietnam2015_26C (3)_dip_nam du.xls

Nam_Du_1_0510_Total_dp_newCH4_mod_v12.xls


O2 mmol/L

DOC mmol/L

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0

0.5

1

0

0

10

0

0.1

0.2

NO3 µmol/L 0.3

0

20

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Fe(II) µmol/L 40

0

50

As(III) µmol/L 100

0

0

0

0

10

10

10

10

20

20

20

20

20

30

30

30

30

30

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2

Figure 3.

ACS Paragon Plus Environment Nam_Du_1_0510_Total_dp_newCH4_mod_v14.xls

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As(III) µmol/L 0

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2

0

60 yr

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70 yr 80 yr

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30

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Figure 4.

ACS Paragon Plus Environment Nam_Du_1_0510_Total_dp_newCH4_mod_v13.xls


47x26mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Fate of Arsenic during Red River Water Infiltration into Aquifers

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