Scientists believe the groundwater below the seafloor off the Maltese islands could provide potable water for 75 years.

However, most of this offshore groundwater appears to be hosted in low permeability layers, making its extraction complicated and expensive.

The discovery was made by a team of scientists as part of the MARCAN project, led by Prof Aaron Micallef from the Department of Geosciences at the University of Malta.

The international team estimated the volume of freshened groundwater resources offshore of the Maltese islands at 1km³, which could potentially provide an alternative supply of potable water for 75 years.

Offshore freshened groundwater is groundwater with salinity below that of seawater that is stored in sediments and rocks below the seafloor.

To reach this estimate, the scientists first developed a detailed 3D geological model of the Maltese Islands and the adjacent seafloor. They then used advanced numerical models to simulate how the groundwater system of the archipelago would evolve over time.

The simulation results show that 18,000 years ago, during the Ice Age, the groundwater system extended across the entire continental shelf of the islands, which was exposed at the time.

As a result of the ensuing sea-level rise, the continental shelf was drowned, and only a quarter of the original groundwater volume was preserved.

At present, offshore freshened groundwater is expected to predominantly occur between Malta and Gozo, and along the coast between Ċirkewwa and Valletta, with the largest extensions offshore of St Paul's Bay, Salini and St Julian's.

The groundwater body is up to 100m thick and 3km from the coast.

In a statement, the university said there were two issues with the potential exploitation of these offshore freshened groundwater resources. Firstly, most of the groundwater appears to be hosted in low permeability layers, which would mean that its extraction could be complicated and expensive.

Secondly, a decrease in precipitation is predicted for the coming 100 years as a result of regional climate change - this is expected to diminish the extent of offshore freshened groundwater by 40%.

The study has just been published in the international journal Frontiers in Water.

The team involved in this study includes scientists from Italy, the USA and Finland.