Most of us have had the pleasure of seeing the expression on a child’s face when the child receives a Christmas or birthday gift that far exceeds their expectation. This past August, geophysicists had a similar experience. An interdisciplinary, international team of 11 scientists led by Grant Ferguson at the Global Institute for Water Security at the University of Saskatchewan published a paper in which they announced the discovery of a huge reservoir of terrestrial water.1 This underground water discovery adds evidence for the design of Earth’s deep water cycle.

Previously Known Terrestrial Water Reservoirs
Lakes and rivers comprise the most obvious sources of terrestrial water, followed by glaciers and icefields. What may not be so obvious are the crucial roles that the rapid melting of enormous ice sheets, ice fields, and glaciers from the last ice age played in creating the present rich abundance of lakes and rivers—the greatest that has ever existed during the past 2.6 million years.

A known reservoir of terrestrial water that we do not see but nevertheless is one of humanity’s most important sources of potable water is groundwater in the upper 2 kilometers (6,560 feet) of Earth’s continental crust. A research team led by Tom Gleeson determined that 22.6 million cubic kilometers (5.4 million cubic miles) of groundwater exists in this part of Earth’s crust.2 Spread out over all the world’s landmasses, this quantity of water would be 3 meters (10 feet) deep. It ranks larger than all other components of the active hydrologic cycle.

Discovery of a New Terrestrial Water Reservoir
Ferguson’s team examined detailed measurements of the distribution of sedimentary and crystalline rocks at crustal depth and applied porosity-depth relationships to these measurements. This analysis enabled them to calculate the volume of what they called “deep groundwater,” water present in crustal depths between 2 and 10 kilometers. Their calculations showed that the quantity of groundwater at crustal depths of 2–10 kilometers is equivalent to the quantity of groundwater in the top 2 kilometers of Earth’s crust.

Economic Implications
This newly discovered reservoir of terrestrial water will not be a significant source of mineable freshwater. Nearly all of it is saline. Therefore, with rare exceptions, this water would need to be desalinated before it could be delivered to households, offices, and factories. The desalination of seawater would be a less expensive source of potable water than the desalination of deep groundwater.

Not all the porous parts of Earth’s deep crust are filled with water. Furthermore, most of these porous parts are completely isolated from the upper crust. Therefore, Ferguson and his colleagues pointed out in their paper that these porous parts of the deep crust could be used to isolate and safely dispose of waste and toxic fluids that cannot be easily recycled.

Design Implications of Deep Groundwater
Ferguson’s team also demonstrated that we need not worry too much about deep groundwater contaminating upper groundwater reservoirs. Though they are large and extensive, virtually all deep groundwater reservoirs are disconnected from the active hydrologic cycle. Hydrothermal activity and seismicity occasionally bring the upper groundwater and deep groundwater reservoirs into contact. However, in most cases it is upper groundwater being driven into the deep groundwater and desalinated water from the mantle being driven to the surface (geysers). Only in a few instances does saline water from deep groundwater mix with the upper groundwater.

While deep groundwater plays only a minuscule role in the active hydrologic cycle, it plays a major role in the deep water cycle. A year ago, I wrote an article in which I described six different ways the deep water cycle is fine-tuned to make advanced life on Earth’s surface possible.3 The assumption by geophysicists at that time was that the tectonic subduction of deep ocean water was virtually the sole contributor to the deep water cycle. Ferguson’s team’s research has established that deep groundwater also makes an important contribution. Their discovery implies that less violent tectonic activity is needed to sustain the deep water cycle at the level required for sustaining advanced life.

The recent determination of the size and extent of the deep groundwater reservoirs affirms all the deep water cycle fine-tuning. Future measurements of deep groundwater reservoirs will yield more precise quantifications of Earth’s deep geobiochemical cycles, which may reveal even more fine-tuned designs in the deep water cycle.

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Endnotes

1. Grant Ferguson et al., “Crustal Groundwater Volumes Greater Than Previously Thought,” Geophysical Research Letters 48, no. 16 (August 28, 2021): id. e2021GL093549, doi:10.1029/2021GL093549.
2. Tom Gleeson et al., “The Global Volume and Distribution of Modern Groundwater,” Nature Geoscience 9, no. 2 (February 2016): 161–167, doi:10.1038/ngeo2590.
3. Hugh Ross, Earth’s Deep Water Cycle Is Designed for Life, Today’s New Reason to Believe (blog), Reasons to Believe, July 13, 2020.