In a recently published study, scientists from Utrecht University and Deltares estimated the volumes of fresh groundwater in geological layers that are currently below the seafloor. Fresh groundwater can be found up to 100 km offshore. The total volume of these resources is estimated at 1.1 million km3, roughly three times more than estimated previously and about 10% of the amount of fresh groundwater that can be found on land.

These huge volumes were formed back when the sea level was almost 120 m lower than today, as was the case during the last ice age. The low sea levels at the time left the bottom of the shallow shelf seas next to the global coasts exposed to the atmosphere, and the underlying, groundwater-bearing, layers (known as ‘aquifers’) were filled with fresh water from rainfall. Parts of these layers were sealed off later by impermeable clays, trapping the fresh groundwater even when the sea rose to the current level.

In order to arrive at their estimates, the scientists used a large number (2784) of computer models that simulate the evolution of fresh and salt groundwater for profiles perpendicular to the coast in a total of 116 coastal regions. These simulations ran from the beginning of the last ice age until the present. First author Daniel Zamrsky of Utrecht University comments: “We built a separate model for each profile, including as much detailed information as possible about the geological layering under land and under the seafloor. Running all these computer models over such a long time span was a huge effort that was only possible with a super computer”.

Estimated volumes of offshore groundwater
This map shows the estimated volumes of offshore fresh groundwater (OFG; blue ribbons) and demand for drinking water, irrigation and industry onshore (green to red ribbons). It is clear to see that large volumes of offshore groundwater can be found in many regions with high onshore water demand.

Co-author Gualbert Oude Essink of Deltares adds: “This is not the first time that volumes of offshore groundwater have been estimated. However, this study is a first in that it includes almost the entire global coast in a consistent and quantitative manner. However, it should be pointed that we did not include areas with limestone layers and so our volume estimates may even be on the low side”.

The scientists compared the resulting map of offshore fresh groundwater volumes with present and future estimates of water demand in the coastal zone. “That clearly showed that, in many regions with large onshore water demand such as Southeast Asia and the Eastern Coast of the USA, large volumes of offshore groundwater are present,” says co-author Marc Bierkens of Utrecht University. “And quite a large proportion of that water can be found at shallow depths, where it may stay present for at least hundreds of years. In areas where oil rigs are already present, the water could be readily accessed and used as a temporary source of fresh water in water-stressed areas while they seek for pathways towards sustainable water use”.

Publication: Zamrsky, D., Oude Essink, G.H.P., Sutanudjaja, E.H., van Beek, L.P.H and Bierkens, M.F.P. (2021). Offshore fresh groundwater in coastal unconsolidated sediment systems as a potential fresh water source in the 21st century. Environmental Research Letters, 17

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