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Groundwater salinity influenced by Holocene seawater trapped in incised valleys in the Red River delta plain

Nature Geoscience volume 10, pages 376–381 (2017)Cite this article

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Abstract

Salty and brackish groundwater has been observed at least 100 km inland in some aquifers contained within Quaternary delta plains. This phenomenon limits access to fresh groundwater resources, particularly in the densely populated deltas of Southeast Asia. However, the causes of inland salinity are unclear. Here we present borehole and geophysical data that show that in the Red River delta plain of Vietnam, salty and brackish groundwater primarily occurs in incised valleys that were formed during sea-level lowstands during the Pleistocene. During the mid-Holocene, these valleys were filled with fine-grained marine deposits containing trapped seawater. We conduct groundwater flow simulations that show that the age, thickness, and permeability of the marine sediments are the primary controls on the leaching of salty porewater into the freshwater aquifer. We find that salty groundwater originating from this trapped seawater is still present in Holocene-aged sediments with low permeability, and affects groundwater salinity in adjacent aquifers. In contrast, trapped seawater from all Pleistocene-aged sediments has been leached. We identify a number of brackish to saline delta aquifers elsewhere in Asia and throughout the world that have a similar sedimentary history, and thus are likely to be influenced by this leaching process.

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Figure 1: The Quaternary geology of the RRDP.
Figure 2: Subsurface geology and groundwater types based on TEM soundings.
Figure 3: Saltwater and freshwater in RRDP aquifers.
Figure 4: Simulated salinity distributions in the RRDP sediments.
Figure 5: Geology and water composition in exploratory borehole VA1.

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Change history

  • 12 May 2017

    In the version of this Article originally published, the titles of the Supplementary Movies, which described the hydraulic conductivities of the clay materials, were not included. This has been corrected in the online versions of this Article.

  • 17 May 2017

    The titles of the Supplementary Movies have been updated to give more detailed descriptions of the hydraulic conductivities.

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Acknowledgements

The project received financial support as a research capacity building grant from the DANIDA research council (Grant 8-075-KU). P. T. K. Trang and her students from Hanoi University of Science are thanked for the chemical laboratory work and J. R. Ineson (GEUS) for linguistic support.

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Authors and Affiliations

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

    Flemming Larsen

  2. Department of Hydrogeology, Hanoi University of Mining and Geology, Hanoi, Vietnam

    Long Vu Tran & Hoan Van Hoang

  3. Department of Geology, Hanoi University of Science, Hanoi, Vietnam

    Luu Thi Tran

  4. Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, 8000 Aarhus, Denmark

    Anders Vest Christiansen

  5. Hanoi University of Natural Resources and Environment, Hanoi, Vietnam

    Nhan Quy Pham

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  1. Flemming Larsen
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  5. Anders Vest Christiansen
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Contributions

F.L. and N.Q.P. conceived the project. L.T.T. interpreted the geologic data. L.V.T., H.V.H. and L.T.T. carried out the geophysical field data. A.V.C. carried out the geophysical data processing and interpretation. L.T.T. estimated the effective diffusion coefficient of the sediments. L.V.T., N.Q.P. and F.L. did the hydrogeological modelling. F.L., L.T.T., H.V.H. and N.Q.P. analysed and interpreted the data. F.L. wrote the paper.

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Correspondence to Flemming Larsen.

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Larsen, F., Tran, L., Van Hoang, H. et al. Groundwater salinity influenced by Holocene seawater trapped in incised valleys in the Red River delta plain. Nature Geosci 10, 376–381 (2017). https://doi.org/10.1038/ngeo2938

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