New estimate makes groundwater, not ice caps, the largest reservoir of water on Earth

New research more than doubles the estimated volume of ancient salty groundwater stored deep in the earth’s crust.

About 24 million cubic kilometers (5.8 cubic miles) of groundwater is found in the first two kilometers (1.2 miles) of the earth’s crust. This shallow groundwater is what we use for drinking and irrigation, and it’s mostly fresh water. But below are vast brine reservoirs, some of which are hundreds of millions to over a billion years old, enclosed in the rocks. The question was: how many are there?

A new study estimates that there are about 20 million cubic kilometers of deep groundwater, enough to fill about 4,800 Grand Canyons. Combined with previous estimates of shallower groundwater, the new research reveals that groundwater is the largest reservoir of water on earth, measuring 44 million cubic kilometers and exceeding the volume of Earth’s ice caps.

“This estimate expands our conceptual and practical understanding of how much water the Earth contains, and it adds a whole new dimension to the hydrologic cycle,” said Grant Ferguson, hydrogeologist at the University of Saskatchewan and lead author of the new study. in AGU magazine Geophysical research letters, which publishes high-impact, short reports with immediate implications covering all Earth and space sciences.

Although this deep groundwater cannot be used for consumption or irrigation, accurate estimates of the volume and connectivity of deep groundwater are needed to safely plan other underground activities, such as the production of water. hydrogen, nuclear waste storage and carbon sequestration. Potential storage sites should be large enough and isolated from surface aquifers to avoid contamination of usable and shallow groundwater.

Because these deep reservoirs can be disconnected from shallow aquifers, in some places the brine has been trapped during geological periods. In addition to offering insight into past conditions on the Earth’s surface, these ancient waters can also support microbial ecosystems that are still active today. Such deep underground biological communities inform mission planning to explore potential habitable areas elsewhere in the solar system.

Deep and salt water

Scientists can estimate the volume of deep groundwater by calculating how much water can hold different types of rocks, which have different porosity (amount of empty space). Previous estimates of deep groundwater between two and 10 kilometers focused only on crystalline rocks with low porosity, such as granite. The new study added the volume of buried sedimentary rocks, which are more porous than crystalline rocks, which they estimated to be around 8 million cubic kilometers. This is about 339 times the volume of Lake Baikal.

Because much of this groundwater is so deep and often contained in rocks with very low permeability, water cannot easily circulate or drain to the surface, severely cutting it off from the planet’s hydrologic cycle. Salt water can be about 25% denser than sea water, making it very difficult to “flush” the system. But high elevation areas near low elevation can have pressure differences that allow shallow water to flow to greater depths, such as the water pressure created by storing water in a water tower. Surface water that may have circulated more than 2 kilometers deep has only been documented in a few places in North America, with the deepest circulation being near the Rocky Mountains in northwestern Wyoming and the southern Alberta.

Although vast, this deep groundwater will not solve the world’s water shortages. It is not possible to rely on desalination of this brine and use it as a source of drinking water or irrigation, according to the study’s authors.

“We still have this precious and limited volume of groundwater on planet Earth that we need to protect,” said hydrologist Scott Jasechko of the University of California-Santa Barbara, who was not involved in the new study. .

Life finds a way

Deep groundwater is important for the storage of waste fluids from oil and gas production and for carbon sequestration. By better quantifying the size of these deep reservoirs, as well as their disconnection from shallower groundwater, scientists can determine which are the safest to use for long-term underground storage.

The new findings could also aid the search for alien life by allowing scientists to study environments similar to those where microbial communities might exist on other planets. Microbial life can survive in a variety of complex environments, from extremely acidic conditions to high temperatures, and the depths of the Earth’s crust are no exception: microbes have been found up to 3.6 kilometers (2.2 miles). ) in the continental crust.

For Jennifer Biddle, a microbiologist at the University of Delaware who was not involved in the study, doubling the deep groundwater estimates also means doubling the potential size of the enigmatic deep microbial biosphere.

“If you have liquid water, there’s a good chance there are germs,” Biddle said. Subterranean organisms mostly survive on water and rocks rather than water and sunlight. “In these microbial systems, they are able to live on [chemicals]. As long as there are chemicals around that they can put together to make energy, microbes can use it for life. ”

This flexibility means that Martian microbes could be lurking in the deep groundwater of the Earth’s crust on the Red Planet, if they are found there.

“If there is deep groundwater on Mars, it’s entirely possible that if Mars was inhabited in the past, that deep groundwater could potentially contain residual microbes,” much like ancient water on Earth, Biddle said. “So deep groundwater habitats could be excellent analogues for other planetary bodies like Mars or Enceladus – a moon of Saturn – which certainly has deep water.”