New research involving a University of Arizona faculty member reveals that the volume of ancient, salty groundwater stored deep within the Earth's crust is more than double previous estimates.

Around 24 million cubic kilometers (5.8 cubic miles) of groundwater reside within the top 2 kilometers (1.2 miles) of Earth's crust. This shallow groundwater is what we use for drinking and irrigation, and it's mostly freshwater. But below that are vast reservoirs of brine, some of it hundreds of millions to more than a billion years old, locked away in the rocks. The question was: How much is there?

A new study estimates there are around 20 million cubic kilometers of deep groundwater, or enough to fill around 4,800 Grand Canyons. Combined with previous estimates of shallower groundwater, the new research finds underground water is the largest reservoir of water on land, measuring 44 million cubic kilometers and surpassing the volume of Earth's ice sheets.

"This estimate expands our conceptual and practical understanding of the amount of water that Earth holds, and it adds a whole different dimension to the hydrologic cycle," said Grant Ferguson, a hydrogeologist at the University of Saskatchewan and lead author of the new study in Geophysical Research Letters.

While this deep groundwater cannot be used for drinking or irrigation, accurate estimates of deep groundwater volume and connectivity are necessary.

"We know so little about the volume of water 1 to 2 kilometers beyond Earth's surface, yet our estimates of microbial biomass, carbon sequestration, production of critical elements and gases, and ability to store waste, like spent nuclear fuel, depends on it," said study co-author Jennifer McIntosh, a professor in the UArizona Department of Hydrology and Atmospheric Sciences. "I worked closely with Dr. Ferguson to calculate the volume of water using new global maps of sedimentary rock thicknesses and porosity relationships with depth."

Potential waste storage sites need to be big enough and sealed off from surface aquifers to avoid contaminating usable, shallow groundwater.

Because deep reservoirs can be disconnected from shallow aquifers, in some places the brine has been trapped for geologic spans of time. In addition to offering insights into past conditions on Earth's surface, these ancient waters may also support microbial ecosystems still active today.

Deep, Salty Water

Scientists can estimate deep groundwater volume by calculating how much water different rock types, which have different porosity, can hold. Previous estimates of deep groundwater between 2 and 10 kilometers only focused on crystalline rocks with low porosity, like granite. The new study added the volume from much more porous, buried sedimentary rocks, estimated at around 8 million cubic kilometers. That's roughly 339 times the volume of Lake Baikal in Russia, the deepest lake in the world.

Because much of this groundwater is so deep and often within rocks with very low permeabilities, the water can't easily circulate or flow to the surface, largely cutting it off from the planet's hydrologic cycle. The salty water can be around 25% denser than seawater, which makes it very difficult to "flush out" the system. But areas with high elevation near low elevation can have pressure differences that let shallow water flow to greater depths, like the water pressure created by storing water in a water tower. Surface waters likely to have circulated deeper than 2 kilometers have only been documented in a few places in North America, with the deepest circulation near the Rocky Mountains in northwestern Wyoming and southern Alberta.

While vast, this deep groundwater will not solve the world's water shortages.

"The amount of fresh groundwater is still limited," McIntosh said. "Groundwater extends to 10 kilometers, but most of the water beyond 1 kilometer is too saline to use for water supply."

It is not feasible to rely on desalinating this brine and using it as a source of water for drinking or irrigation, according to the study's authors.

Life Finds a Way

Deep groundwater is important for storing waste fluids from oil and gas production and for carbon sequestration. By better quantifying how large these deep reservoirs are, as well as how disconnected from shallower groundwater they are, scientists can determine which are safest to use for long-term subsurface storage.

The new findings could also aid the search for extraterrestrial life by letting scientists study environments similar to where microbial communities could exist on other planets. Microbial life can survive in a variety of complex environments, from extremely acidic conditions to high temperatures, and deep in Earth's crust is no exception; microbes have been discovered as deep as 3.6 kilometers (2.2 miles) in the continental crust.

"This discovery means there may be more microbial life underground than previously estimated. Previous estimates of subsurface microbial biomass depended on the amount of water. Our results, which show there's more water than previously thought, can help to refine and possibly increase those estimates." McIntosh said.