What Is Fossil Water? Geology and Renewability

Fossil water refers to underground reservoirs formed during the last glacial ice age.  Also known as petrowater or paleowater, fossil water borrows its name from paleontology. Like a traditional fossil, fossil water is trapped in rock—sometimes for millennia. 

These aged aquifers acquired their water from melting ice and prehistoric lakes that pooled into subterranean layers of water-porous rocks, silt, and sand. Over time, those layers were covered with more sediment, effectively sealing off the water from Earth’s surface for tens of thousands of years. 

Just 70 years ago, farmers began drilling into fossil aquifers, pumping water to previously dry climates worldwide. Today, billions of people rely on fossil aquifers for their drinking water and as irrigation to grow staple food crops.

Geography and Geology

Fossil water appears all over the globe in humid, semi-arid, and arid regions as well as areas of permafrost.

Lake Vostok, for example, is one of more than 70 subglacial lakes trapped beneath Antarctica's ice sheet. The Nubian Sandstone Aquifer System is the biggest fossil aquifer in the world and runs beneath the four northeast African nations. The Arabian peninsula, eastern Australia, the Parisian basin in France, the Great Plains of the U.S., and parts of Mexico and California all rely on paleowater for agriculture, sanitation, and consumption.

Regardless of the current climate of these places, sometime before the Holocene epoch (the geologic age in which we now live), the planet was much wetter. These more humid climate conditions allowed aquifers—water-bearing layers of semi-porous rock and other materials—to form.

Fossil aquifers are frequently contained by layers of impermeable rocks and clay, preventing the aquifer from absorbing any precipitation. Some petrowater not encased in a hard outer layer is so far beneath the ground that it's often only detectable by airborne electromagnetic surveys. Understandably, these aquifers are nearly impossible to replenish. Other fossil water sits beneath deserts where there’s not enough annual precipitation to refill the aquifers. 

Today, billions of people in some of the world’s most arid regions—such as in North America, Iran, India, and Africa—rely almost entirely on petrowater. Research published in 2017 found that fossil water accounts for up to 85% of all groundwater in the top meter of the Earth’s crust. Most well water drawn from sources 250 meters (820 feet) and lower also comes from fossil aquifers.

Sustainability

If left untouched by human activities, paleowater would remain in equilibrium. But as the need to grow crops (often to feed livestock) expanded around the world in the last half of the 20th century, many farmers turned to fossil aquifers. These free water sources provided enormous advantages for communities in dry climates, turning previously inhospitable areas into lush farming zones.

Unfortunately, once fossil water has been pumped to the surface, it can take thousands of years to replenish. Experts estimate, for example, that once the Ogallala aquifer in the High Plains has been fully mined, it would take over 6,000 years to restore it to its original levels.

As an aquifer is depleted, pumping water becomes more difficult and more expensive. The water brought to the surface can become brackish, requiring desalination, especially if it’s used as drinking water. Deplete an aquifer too quickly and it can collapse on itself, further trapping existing supplies.

The Future of Fossil Water

Because fossil water is non-renewable, aquifers around the world risk soon being mined dry: The Ogallala aquifer, which supplies nearly one-third of the groundwater used for irrigation in the U.S., may have only 30 to 50 years of remaining productivity. NASA satellite studies confirm irreparable aquifer declines in India. Experts have spent the last decade warning that Yemen is running out of water. In Africa, the non-renewable Nubian aquifer will have to support an estimated 731 million people by 2050.

Experts already predict that more than 60% of people in the world will experience water scarcity by 2025, and thanks to global warming, increasing temperatures will only make matters worse. These stark realities understandably spark fear in the hearts of people in arid climates who depend on fossil water to cultivate resource-intensive crops like wheat and rice.

In order to support a more sustainable future, farmers in fossil water-dependent regions must use less water, grow alternative drought-friendly crops, and rely on imports for staple foods that previously underpinned the agricultural economy.