In the past, it was only in arid regions that freshwater was viewed as a precious asset. Now, however, the lakes, rivers, springs and wells in many of the Earth‘s coastal regions are also drying up. The reasons for this are many and varied. In some places it is raining less as a result of climate change. In others the precipitation is no longer regularly spaced through time, but occurs episodically as heavy rain events. During these extreme precipitation events most of the water runs off the surface because the soil cannot absorb it fast enough. At the same time, human demands upon freshwater resources are increasing because more people are moving to the coastal regions or taking their vacations there, while farmers are watering larger areas. In some places, the inland surface waters and groundwater reservoirs are being senselessly polluted, for example through over-fertilization or the excessive application of pesticides. In response to the increasingly frequent and severe water shortages, researchers have long been searching for new freshwater reservoirs. Their explorations have become strongly concentrated on areas beneath the sea. It has been known for a number of years that untapped groundwater reserves exist below the sea floor near the coasts, and that this is presumably the case on all the continents. Most of the offshore reservoirs of this kind discovered so far are on the east coast of the USA, the northwest coast of Europe and the west coast of Australia. It is estimated that all of the known reservoirs beneath the sea together store around one million cubic kilometres of freshwater. This amount would theoretically be enough to fill the Black Sea twice or, to give a more practical example, to supply the population of Germany with drinking water for more than 192,000 years.
Occurrences of groundwater below the sea can originate in different ways. Experts currently distinguish between five fundamentally different formation processes. Some reservoirs are formed by the natural breakdown of gas hydrates in the sea floor, during which low-salinity water is released. In other places, water accumulates in the subsurface as a result of physical and chemical processes during the induration of sediments. Geologists refer to this kind of rock formation process as diagenesis. Occurrences in coastal regions previously covered by glaciers can be attributed to meltwater from the former large ice masses penetrating into the sea floor and collecting there (subglacial/proglacial injection). Some known offshore reservoirs are also fed from the land, for example by precipitation on land percolating downward and then being carried underground toward the sea by deep-lying rock layers (meteoric recharge 1).
Most of the groundwater reservoirs found under the sea, however, originated during past cold intervals, such as the last glacial period around 20,000 years ago. During that period the ice sheets in the Arctic and Antarctic regions grew. Due to the great amount of water that was thus bound in the ice sheets, the global sea level dropped by more than 100 metres compared to today. As a result, the shallow coastal waters around the world retreated and the shelf areas of the continental margins dried up. When it rained or snowed on these areas, the water percolated into the soil and collected in hard, porous limestones, where it was stored in a manner similar to being absorbed by a sponge. This process is known as meteoric recharge 2. At the end of the glacial period, as sea level began to rise again, the shelf areas were flooded once more. Since then, the groundwater reservoirs have been located underwater beyond the coasts, and have become especially interesting for countries with limited water resources such as South Africa, Mexico, New Zealand and Malta.
In the waters of Malta, studies led by German scientists have revealed that the water-bearing limestones in the region lie around 400 metres below the sea floor. Off the New Zealand coast of Canterbury (eastern part of the southern island), by contrast, the researchers only had to drill 20 metres into the sea floor to find freshwater-bearing rocks. This is one of the shallowest groundwater reservoirs in the world. It extends to as far as 60 kilometres from the coastline, and is thought to hold up to 200 cubic kilometres of water. By comparison, Germany’s largest inland lake, Lake Constance, holds 48 cubic kilometres of water. The volume of groundwater discovered off the coast of New Zealand is around four times as large.
Researchers have been able to obtain such detailed knowledge of the freshwater systems beneath the sea recently through a combination of various geophysical and geochemical research methods. With the help of marine electromagnetics, they can measure electrical resistivity below the seabed. Using these measurements, it is possible to determine whether the rocks in the subsurface have saltwater or freshwater stored in their pores. Saltwater is an excellent conductor, while freshwater has three times the electrical resistance.
In order to determine the salinity of the pore water with a great degree of accuracy and, furthermore, to estimate the volume of the groundwater, the geologists then combine the electromagnetic data with seismic profiles of the sea floor layers. This integration is essentially a statistical-mathematical process that links numerical models with machine learning algorithms. The method puts scientists in a position to characterize and map offshore freshwater systems in extraordinary detail. Strictly speaking, this research field is actually a bit more mathematics than geology and hydrology.
5.18 > Groundwater reservoirs have now been discovered in the coastal areas of all continents. They are formed by the breakdown of gas hydrates, by the consolidation of sediments (diagenesis), through the input of glacial meltwater, and by rainwater input from the land, but most commonly by the formation of groundwater reservoirs during past glacial periods.