Fresh groundwater under the sea: a potential source of water?

Published: 10 January 2022

There are large volumes of fresh groundwater below the seafloor in many coastal areas around the world. These may be used as a temporary source of water in water stressed coastal regions.

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 off the coastline. The total volume of these resources is estimated to be 1.1 million km3. This is roughly three times more than estimated previously and about 10% of fresh groundwater that can be found on land.

These huge volumes of fresh offshore groundwater date from times that the sea level was almost 120 m lower than today, such as was the case during the last ice age. The low sea levels during these times left the bottom of the shallow shelf seas next to the global coasts exposed to the atmosphere, filling the underlying groundwater bearing layers (called aquifers) with fresh water from rainfall. Later, parts of these layers got sealed off by impermeable clays, such that the fresh groundwater in these layers was preserved, even when the sea level rose to the current elevation.

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 for a total of 116 coastal regions. These simulations ran from the beginning of the last ice age until now. First author Daniel Zamrsky of Utrecht University comments: “For each profile, we built a separate model, where we included as much as possible detailed information 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, and could only be done on a super computer”.

This map shows the estimated volumes of below seafloor (offshore) fresh groundwater (OFG; blue ribbons) and the water demand for drinking water, irrigation and industries onshore (green to red ribbons). Clearly, in many regions with high onshore water demand, large volumes of offshore groundwater can be found.

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

The scientists confronted the resulting map of offshore fresh groundwater volumes with present and future estimates of water demand in the coastal zone. “This clearly shows that in many regions with large onshore water demands, such as South-East Asia and the East 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 these volumes can be found at shallow depths and may last for at least hundreds of years. In areas where oil rigs are already present, they may be readily exploited 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