An 800,000-year-old water system with low-salinity water stored in sediments beneath the seafloor has been revealed by an international research team in Gulf of Corinth. At a time when challenges from limited water supplies are many, groundwater mapping is valuable information for the future.

Researchers from universities and research centers in Malta and Italy focused on the Gulf of Corinth, “one of the most well-studied areas of the Mediterranean in terms of tectonic activity and sediment deposition”as the lead author of the research, Senai Horozal, explains to APE-MPE. “This is a geologically active rift basin with a high rate of sediment accumulation that has experienced significant sea-level fluctuations over the past hundreds of thousands of years, making it an ideal natural laboratory for studying how low-salinity coastal groundwater systems are formed and maintained.”he adds.

According to the study, published in the Journal of the International Union of Hydrogeologists “Journal of Hydrogeology”the team identified and mapped low-salinity coastal waters stored in sediments beneath the Gulf of Corinth. As it turned out, the great drops in sea level during the ice ages allowed rainwater and river water to infiltrate coastal and submarine sediments, and some of it has been preserved to this day.

Low salinity water is found at depths from about 20 to 600-700 meters below the seabed in the central basin of the Corinthian Gulf and from 15 to 150 meters below the seabed in the eastern Alcyonides area. Lateral layers of sediments in the basin can contain up to about 250 cubic kilometers of this groundwater.

The team applied a multidisciplinary approach combining high-quality seismic data, drill cores from previous international programs such as the International Ocean Discovery Program (IODP) Mission 381, in which the Hellenic Center for Marine Research participated among others, as well as hydrogeological modeling, to investigate the presence and distribution of coastal water of low salinity below the sea floor and changes in water salinity over time. These methods combined reveal where water is most likely to be stored and how it has moved over time.

Computational simulations of the past 800,000 years have tracked changes in the salinity of water within the sediments over time and confirm that the water was deposited there mainly during glaciations, when sea levels were much lower and large areas of the continental shelf were exposed. “During these periods, rainwater and river water could infiltrate the sediments and be stored. The presence of laterally extensive, low-permeability sedimentary layers has helped to maintain these bodies of water for 800,000 years, which acted as reservoirs that were continuously replenished during the period when the sea level was low. Although the stored fresh water has not remained completely unchanged for 800,000 years, the system as a whole has been stabilized at low salinity over the long term by mixing with seawater during multiple rises and falls of sea level,” highlights Ms. Horozal.

Greece, like many Mediterranean countries, is facing increasing pressure on water resources, so mapping where this groundwater exists and understanding how the geological structures protect it can give the state a clearer picture of the subsoil. Senai Horozal emphasizes that “in the context of climate change and increasing demand for fresh water, knowing where these low-salinity water bodies are located and how much water they may contain is critical for long-term water management strategies.” However, he points out that “any potential exploitation should be approached very carefully, given the environmental sensitivity of coastal systems”.