What is Groundwater?

Groundwater is all the water that exists beneath Earth’s surface. On land, groundwater is used by billions of people as a source of fresh drinking water. We also use groundwater to water our plants and grow our food. Groundwater generally forms and renews when it rains or snows, as the water slowly trickles down into the soil. We call this process of water trickling into the ground “recharge” of the groundwater. Actually, 99% of the liquid freshwater on our planet (everything not stored in ice caps and glaciers) is found underground, in the tiny spaces between grains of sand and other sediments [1]. This means that all the freshwater you can see on the surface, for example in rivers or lakes, makes up only 1% of the Earth’s liquid freshwater!

Geological formations called aquifers store and transport groundwater underneath Earth’s surface (Figure 1). Within these aquifers, groundwater slowly flows downwards, generally from the mountains toward the oceans, just like rivers. By drilling wells into aquifers, we can pump groundwater back to the surface and use it for drinking, irrigation, or other purposes. Often, aquifers are blocked at the top and bottom by geological layers that are very dense, like rock or clay, and where the groundwater cannot move through easily. These flow-blocking layers are called aquitards (Figure 1).

What is Offshore Freshened Groundwater?

Offshore groundwater describes all the water that is stored in the ground beneath the seafloor. Since the oceans are very salty, the groundwater within the seafloor is generally also salty and cannot be used for drinking or cooking. In some regions though, very large volumes of freshwater can be found beneath the ocean. In fact, these regions can extend up to 90 km off the coast and contain more freshwater than all the water that has ever been removed from aquifers on land [2]. Because the water in these offshore aquifers is sometimes fresh but might be mixed with some saltwater, we call these water bodies offshore freshened groundwater.

Where Does Offshore Freshened Groundwater Come From?

Groundwater forms when rain and melting snow slowly soaks into the soil and trickles down through the rocks. But no rain or snow falls on the seafloor, right? Rain and snow fall on top of the ocean—so how can there be freshwater beneath our salty oceans? Two main processes are responsible for groundwater beneath the seafloor [3].

First, aquifers that are recharged on land can extend beneath the oceans. Often, aquifers are capped at the top and the bottom by aquitards and the water cannot escape. So, when it rains on land and the aquifer fills with water, it will run “downhill” within the aquifer and continue past the coastline, so that the freshwater ends up under the ocean (Figure 1). When a lot of rain falls on land, the pressure in the aquifer builds up. Then the groundwater starts to seep out from the seafloor, like fountains of freshwater under the ocean. We call this submarine groundwater discharge (SGD) (Figure 1). SGD happens in many regions, most of the time relatively close to land, where it often results in holes in the seafloor [4].

A second reason that freshwater can be found beneath the seafloor is that it has been there for thousands of years. During past ice ages, a lot of water was frozen at the North and South Poles. This caused sea levels to drop about 120 m lower than they are today. When the sea level dropped, large areas that are currently covered by the ocean were dry land. During this time, rain was trickling into the ground, building up large volumes of groundwater. By the end of the ice ages, the sea level rose again and these areas became covered with water, as they are today. However, in many places, the groundwater is still there, under the ocean (Figure 2). Since these areas are now covered by seawater, which is slightly heavier than freshwater, the seawater slowly sinks down into the ground, forming salty groundwater beneath the ocean. Eventually, the offshore freshened groundwater will be replaced by seawater.

Why is Offshore Freshened Groundwater Important?

Humans can pollute the groundwater in coastal areas due to farming or release of industrial wastes and sewage. When this groundwater seeps out from aquifers beneath the ocean, it often carries large amounts of nutrients into the ocean water. These nutrients can cause harmful overgrowth of tiny plants called algae. Such algal blooms can damage the marine ecosystem. In other areas where groundwater seeps out under the ocean, certain ecosystems thrive because the water is not as salty as the surrounding ocean. Some fish, corals, and plants grow better when the water is less salty and carries more nutrients [5]. So, to protect these environments in the future, it is important to understand where this groundwater discharge occurs, the quality of the groundwater, and how the groundwater affects the surrounding ecosystems.

Freshwater is a valuable resource. In many regions worldwide, people do not have access to fresh, clean drinking water. With the advance of climate change and an increase in droughts, the groundwater in some regions is being increasingly depleted. When the normal water reserves are no longer enough for the coastal population, offshore groundwater might be a unique resource to help with freshwater shortages in some regions [2]. In the future, we could be pumping groundwater from offshore aquifers back onto land, to provide people in coastal areas with enough water to drink and water their plants.

Where and How Do We Find Offshore Groundwater?

Offshore freshened groundwater has been found along many coastlines around the world (Figure 3). On land, it is relatively easy to drill a groundwater well and to explore aquifers, but it is a lot more complicated and expensive in the ocean. We cannot just use a snorkel and some goggles to dive down to the seafloor and start digging! The freshwater is far too deep under the ground, and the water is much deeper than in swimming pools. Because it is so expensive to drill a well in the ocean, it is very important for us to know exactly where offshore groundwater and SGDs are before we try digging. When the groundwater seeps out into the ocean, the freshwater mixes with the saltwater very quickly. That makes it quite difficult to find the regions where SGDs occur.

To find offshore groundwater, we use our research vessels to take special kinds of measurements. One of these methods measures the electrical conductivity of the seafloor. We use a battery and a long cable to send electricity into the ground, to see how well the seafloor conducts the electric current. Saltwater is a very good electrical conductor, so the electric current can easily pass through it. Freshwater, on the other hand, cannot conduct electricity very well. Therefore, we look for regions where the seafloor and the ground below show low electrical conductivity. This gives us clues about where we might find offshore freshened groundwater.

We can also look at the properties of water samples that we collect from the ocean. Groundwater can not only carry large amounts of nutrients but can also sometimes have very small amounts of radioactive elements like radon that normally do not occur in the ocean. So, when we find elevated concentrations of radon in our water samples from the ocean, it is likely that groundwater is coming out of the seafloor in these regions.

Conclusions

It is difficult to find freshwater beneath the ocean, but better techniques are being invented and more and more scientists are searching for these water sources. Several normal Earth processes can generate freshwater beneath the oceans, and a better understanding of these processes could help us to more accurately predict where offshore freshwater might occur and where we should search. The use of offshore freshened groundwater can potentially help us meet our increasing need for freshwater in the future.

Glossary

Aquifer: ↑ An underground layer of porous rock or soil that collects and transports water that people can pump out and use.

Aquitard: ↑ An underground layer of rock or sediment that slows down or permits the flow of groundwater between aquifers above and below it.

Nutrients: ↑ A substance that provides nourishment. Nutrients are contained in food and are essential for life and for the growth of biology.

Algal Blooms: ↑ Dense layers of tiny green plants (algae) that occur on the surface of the ocean when there is an overabundance of nutrients.

Electrical Conductivity: ↑ The ability of a material to allow electric currents to pass through it.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This work was funded by the Marie Curie action KARST, under the EU H2020 program, project number 101027303.