Offshore fresh groundwater reserves as a global phenomenon

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The flow of terrestrial groundwater to the sea is an important natural component of the hydrological cycle. This process, however, does not explain the large volumes of low-salinity groundwater that are found below continental shelves. There is mounting evidence for the global occurrence of offshore fresh and brackish groundwater reserves. The potential use of these non-renewable reserves as a freshwater resource provides a clear incentive for future research. But the scope for continental shelf hydrogeology is broader and we envisage that it can contribute to the advancement of other scientific disciplines, in particular sedimentology and marine geochemistry.

At a glance


  1. World map of topography and bathymetry showing known occurrences of fresh and brackish offshore groundwater.
    Figure 1: World map of topography and bathymetry showing known occurrences of fresh and brackish offshore groundwater.

    Bathymetry from ref. 98. The occurrence of vast meteoric groundwater reserves (VMGRs) proven by direct observational data are shown in red. Offshore groundwater that is not necessarily fresh, but for which a freshwater mixing component has been inferred on the basis of pore-water composition is shown in green. Significant meteoric offshore groundwater with indirect evidence based on onshore palaeo-groundwater is shown in blue. Although no coordinates for the sample locations were provided, it seems probable that the Pattani sedimentary basin in Thailand96 and the delta sediments of the Mahakam river in Indonesia97 correspond to the definition of a VMGR that is used here. BMB, Beaufort-Mackenzie Basin

  2. Global overview of inferred key metrics and cross sections of well-characterised vast meteoric groundwater reserves.
    Figure 2: Global overview of inferred key metrics and cross sections of well-characterised vast meteoric groundwater reserves.

    Data sourced from refs 28, 30, 32, 35, 36, 37, 48, 53. The location of each cross-section is indicated by a red line. In the cross-sections, the blue contour lines indicate total dissolved solid (TDS) concentrations (g l−1); distance (km) and elevation (m) relative to mean sea level are indicated along the horizontal and vertical axis, respectively; vertical grey lines indicate well locations where salinity is inferred from water samples and borehole logs; crystalline bedrock or low-permeability sedimentary rocks containing salt groundwater are shown in brown; the black, sub-horizontal lines denote faults; undifferentiated continental shelf sediments are in pale green; and sea water is pale blue. Within the Nantucket and Greenland cross sections, salinity contours are based on numerical model results48, 53 and well data. The inferred widths, lengths and volumes per kilometre of coastline pertain to the groundwater with a TDS concentration less than 10 g l−1 (Box 1).

  3. The geology and the key groundwater flow, and dissolved salt transport processes below the continental shelf.
    Figure 3: The geology and the key groundwater flow, and dissolved salt transport processes below the continental shelf.

    Lower sea levels during glacial periods promote further penetration and recharge of groundwater below continental shelves, whereas incised rivers provide a driving force for topographic flow systems, and saline groundwater retreats seaward. When the shelves are flooded during interglacials, intruded seawater (red arrows) migrates landward as well as downward, while the flow of fresh water (blue arrows) stagnates.


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  1. School of the Environment, Flinders University, PO Box 2100, Adelaide SA 5001, Australia.

    • Vincent E.A. Post
  2. National Centre for Groundwater Research and Training, GPO Box 2100, Adelaide SA 5001, Australia.

    • Vincent E.A. Post
  3. VU University Amsterdam, Faculty of Earth and Life Sciences, Critical Zone Hydrology Group, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands.

    • Jacobus Groen &
    • Henk Kooi
  4. Acacia Water, Jan van Beaumontstraat 1, 2805 RN, Gouda, the Netherlands.

    • Jacobus Groen
  5. New Mexico Tech, Department of Earth & Environmental Science, 801 Leroy Place, Socorro, NM 87801, USA.

    • Mark Person
  6. University of Colorado, Department of Geological Sciences, Boulder, Colorado 80309, USA

    • Shemin Ge
  7. University of Oxford, School of Geography and the Environment, South Parks Road, Oxford OX1 3QY, UK.

    • W. Mike Edmunds

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