First global groundwater balance of the earth

Only six percent of groundwater is potentially renewable

Groundwater is one of our most important water sources on this planet © Jan Mocnak / freeimages
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A 180-meter-high layer of water: If you were to pump all the groundwater at once, the earth would be a water planet. But of this gigantic amount, only six percent are less than 50 years old and thus, so to speak, "renewable". This shows the first global mapping of Earth's groundwater. This means that the underground wet is a limited and also rather unevenly distributed resource, as researchers report in the journal "Nature Geoscience".

Groundwater is one of the most important water resources on our planet. But this supply is not infinite: a large part of the groundwater is fossil groundwater and thus water, which was already millions of years ago in the underground. But this means that it can not be refilled so quickly. The increasing demand for water from agriculture, industry and cities, however, ensures that more and more of the precious water is pumped out of the ground.

"We already know that levels fall in many aquifers, " says Tom Gleeson of the University of Victoria in Canada. "We use our groundwater too fast - faster than it can renew itself." Recently, researchers estimated that about one third of all groundwater reservoirs are already overused. It is therefore all the more important to know exactly how much groundwater can be sustainably extracted.

How much "modern" groundwater is there?

But until now it was unclear how much "modern" groundwater exists on earth - groundwater that was formed only in the last 50 years and thus is a potentially renewing resource. Gleeson and his colleagues have now determined this with the help of several methods. First, they used more than 3, 700 measurements of tritium in groundwater from 55 countries and regions.

The map draws the distribution and amount of "modern" groundwater on the ground © Gleeson et al. / University of Victoria

Because this radioactive trace element in the 1950s in the 1950s by the atomic bomb tests in large quantities into the atmosphere and has since slowly declined, reveals its concentration in the groundwater, when this seeped under the ground. In addition, researchers used geological data from more than 900, 000 watersheds to compute the distribution and abundance of groundwater in different regions of the world using computer modeling. display

Only six percent are renewable

Their result: In principle, our planet has a gigantic groundwater supply: in the upper two kilometers of the earth's crust, there are 22.6 million cubic kilometers of water. "If you pumped this up, that would be enough to cover the entire surface of the earth 180 meters high with water, " the researchers explain. "So groundwater is a resource that far surpasses all other water sources on our planet."

Unfortunately, only a small part of this water supply can be reached and used by us and an even smaller part is renewed during our lifetime. According to Gleeson and his colleagues, this modern groundwater accounts for only 0.35 million cubic kilometers, or six percent of the total stock. If this resource were to be distributed on the earth's surface, only a three-meter-high layer of water would result.

After all, this is still more than what is stored in the atmosphere in the form of water vapor, as ice in the glaciers or in the surface waters of the earth. "Modern groundwater surpasses all other components of the water cycle, " explain the researchers.

Unevenly distributed

Unfortunately, this potentially renewable groundwater is unfortunately not evenly distributed. The distribution maps produced by the researchers show that most of modern groundwater is in the tropics and mountainous regions. Thus, some of the largest deposits in the Amazon Basin, in the Congo, in Indonesia and along the Rocky Mountains in North America and the Cordilleras in South America. Not surprisingly, the Sahara and other desert areas are poor on this geologically young groundwater.

"The groundwater, which is renewed within the human lifetime of 25 to 100 years, is a finite, limited resource with a spatially very heterogeneous distribution, " the scientists summarize. Knowing where and how much is there is therefore crucial. Because only then can one predict where and when the groundwater will be scarce, but also where it is particularly at risk of being polluted. (Nature Geoscience, 2015; doi: 10.1038 / ngeo2590)

(University of Victoria / Nature, 17.11.2015 - NPO)