The ice sheets were believed to be the largest water reservoir on Earth. But scientists need to get back to that now.

in the sheet Geophysical Research Letters researchers come up with a new estimate of the amount of deep groundwater. Deep under our feet – about 2 to 10 kilometers below the earth’s surface – there is an estimated 20 million cubic kilometers of water. That’s enough to fill the Grand Canyon 4800 times.


If we then add the amount of groundwater that is less deep, we arrive at about 44 million cubic kilometers of groundwater. And with that, there is more water under our feet than in the ice caps. Quite surprising, according to researcher Jennifer McIntosh. “Prior to this study, ice sheets were thought to be the largest water reservoirs on the continents. We now show that there is more water stored underground than in the ice caps.”

Deep groundwater

In their study, scientists only focus on the so-called deep groundwater. This deep groundwater differs in a number of ways from the much more familiar groundwater that is located in the top two kilometers of the soil and is frequently used as drinking water or to irrigate crops. For example, the deep groundwater – as the name suggests – is much deeper: at a depth of between 2 and 10 kilometers. In addition, it is not fresh, but salty (about 10 times saltier than seawater) and has been locked up in rock – sometimes for billions of years.


To get an idea of ​​how much deep groundwater there is, researchers need to have a good idea of ​​the rock that is located at a depth of 2 to 10 kilometers. You need to know (approximately) where which (sedimentary) rocks are located and how porous they are – in other words, how much space they offer for water. “We used a new sediment thickness database for the study,” said researcher Jennifer McIntosh “And that combined with data on the porosity of different types of rocks at different depths.”


And that leads to a new estimate of the amount of deep groundwater: 22 million cubic kilometers. It’s an estimate. And it is only as good as the data on which it is based. Unfortunately, that data is scarce. “Porosity measurements at depths over 2 kilometers aren’t that many,” admits McIntosh. “More research, for example involving very deep drilling, is needed to shed better light on the deep underground.” For now we have to make do with what we have and McIntosh’s estimate – based on the latest data – is the best we have.

Although we cannot directly use the deep groundwater – partly due to the fact that it is difficult to access and very salty – it is important, according to McIntosh, to know how much water there is deep underground. “Deep groundwater systems can be used for the extraction of energy and minerals, the storage of alternative energy sources (hydrogen for example) and the storage of anthropogenic waste products (such as spent nuclear fuel and CO2). In addition, deep groundwater systems can also harbor microbial life and provide more insight into how life can survive in extreme environments here on Earth, but also on other planets.”