The river–groundwater interface as a hotspot for arsenic release

Abstract

Geogenic groundwater arsenic (As) contamination is pervasive in many aquifers in south and southeast Asia. It is feared that recent increases in groundwater abstractions could induce the migration of high-As groundwaters into previously As-safe aquifers. Here we study an As-contaminated aquifer in Van Phuc, Vietnam, located ~10 km southeast of Hanoi on the banks of the Red River, which is affected by large-scale groundwater abstraction. We used numerical model simulations to integrate the groundwater flow and biogeochemical reaction processes at the aquifer scale, constrained by detailed hydraulic, environmental tracer, hydrochemical and mineralogical data. Our simulations provide a mechanistic reconstruction of the anthropogenically induced spatiotemporal variations in groundwater flow and biogeochemical dynamics and determine the evolution of the migration rate and mass balance of As over several decades. We found that the riverbed–aquifer interface constitutes a biogeochemical reaction hotspot that acts as the main source of elevated As concentrations. We show that a sustained As release relies on regular replenishment of river muds rich in labile organic matter and reactive iron oxides and that pumping-induced groundwater flow may facilitate As migration over distances of several kilometres into adjacent aquifers.

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Fig. 1: Field site, observation bores and approximate groundwater flow field in the study area.
Fig. 2: Conceptual model of As plume evolution at Van Phuc.
Fig. 3: Simulated concentrations of 3H, 3Hetri and Astot (1960–2010) along the cross-section from the Red River (RR) towards the northwest.
Fig. 4: Observed and simulated concentrations and ECs versus apparent groundwater age.
Fig. 5: Observed and simulated depth profiles of concentrations, EC and pH.
Fig. 6: Model-computed sensitivities of As plume formation at biogeochemical reaction hotspots.

Data availability

The geochemical data analysed during this study are included in this article in the supplementary information in Supplementary Tables 1 and Supplementary Tables 2. The groundwater age data analysed during this study has been published and is available in van Geen et al.7 and Stahl et al.10 (Supplementary Table 1). The solid phase chemistry data at the site was available from Eiche et al.32 and Eiche21.

Code availability

All codes used as part of this study are publicly available and can be accessed freely. The USGS flow model MODFLOW37 (https://www.usgs.gov/software/software-modflow) was used to perform the groundwater flow simulations, whereas the reactive multi-component transport model PHT3D38 was used to simulate solute and reactive transport processes (http://www.pht3d.org/). PHT3D couples the 3D transport simulator MT3DMS39 with the USGS geochemical model PHREEQC-240. The PEST++ software suite41 was employed for model calibration and uncertainty analysis (http://www.pesthomepage.org/).

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Acknowledgements

This study was supported by the Swiss National Science Foundation (SNSF grant no. IZK0Z2_150435/ IZK0Z2_150435/1 and SNSF grant no. 167821) and the German Research Foundation (DFG grant no. 320059499). M. O. Stahl (Union College), B. Bostick and A. van Geen (Columbia University) contributed to this work through helpful discussions on previous work at the field site. P. Ortega prepared Fig. 1.

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R.K., M.B., I.W. and H.P. conceived the study. M.B. and R.K. provided hydrochemical and tracer data and contributed to the groundwater age, hydraulic and hydrogeochemical interpretation. I.W. and H.P. carried out the flow and reactive transport modelling and J.S., M.B., R.K, I.W. and H.P. contributed to the development of the geochemical conceptual model underpinning the numerical model. A.J.S. undertook flow and solute transport model calibration and contributed to model uncertainty analysis. All authors contributed to writing and editing the paper.

Corresponding author

Correspondence to Ilka Wallis.

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Extended data

Extended Data Table 1 Conceptual and numerical model variants
Extended Data Table 2 Measured and modelled initial (that is, native groundwater and river water) concentrations of aqueous components. Concentrations in mol/L except temperature in [°C], EC in [µS/cm] and pH, pe with bd = below detection limit
Extended Data Table 3 Measured and modelled initial concentration of minerals, arsenic solid phase and organic matter. Units are in [mol/L of bulk aquifer volume]. Sorbed initial concentrations are not predefined in the model but the product of initial dissolved concentrations (extended data Table 2) and iron oxide concentration in the sediment (extended data Table 3) (nd = not determined; n/a = not applicable)

Supplementary information

Supplementary Information

Supplementary Tables 1–6 and Figs. 1–12.

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Wallis, I., Prommer, H., Berg, M. et al. The river–groundwater interface as a hotspot for arsenic release. Nat. Geosci. 13, 288–295 (2020). https://doi.org/10.1038/s41561-020-0557-6

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