A new report shows that less than 6% of groundwater in the upper two kilometres of the Earth’s landmass is renewable within a human lifetime.
Project leader Tom Gleeson of Australia’s University of Victoria says the findings are a shock.
“We already know that water levels in lots of aquifers are dropping,” he says. “We’re using our groundwater resources too fast – faster than they’re being renewed.”
The data-driven estimate of the Earth’s total supply of groundwater is the first since a back-of-the-envelope calculation of the global volume of groundwater was attempted in the 1970s.
Using multiple datasets including data from close to a million watersheds, and more than 40,000 groundwater models, the study estimates a total volume of nearly 23 million cubic kilometres of total groundwater of which 0.35 million cubic kilometres is younger than 50 years old.
The researchers say it is important to differentiate old from modern groundwater as they are fundamentally different in how they interact with the rest of the water and climate cycles.
Old groundwater is found deeper and is often used as a water resource for agriculture and industry. Sometimes it contains arsenic or uranium and is often more salty than ocean water. In some areas, the briny water is so old, isolated and stagnant it should be thought of as non-renewable, says Gleeson.
The volume of modern groundwater dwarfs all other components of the active water cycle and is a more renewable resource but, because it’s closer to surface water and is faster-moving than old groundwater, it’s also more vulnerable to climate change and contamination by human activities.
The study’s maps show most modern groundwater in tropical and mountain regions.
Some of the largest deposits are in the Amazon Basin, the Congo, Indonesia, and in North and Central America running along the Rockies and the western cordillera to the tip of South America.
High northern latitudes are excluded from the data because satellite data doesn’t accurately cover these latitudes, but this area is largely under permafrost with little groundwater. The least amount of modern groundwater is not surprisingly in more arid regions such as the Sahara.
In a 2012 study published in the journal Nature, Gleeson mapped global hot spots of groundwater stress, charting rates of precipitation compared to the rates of use through pumping, mostly for agriculture. Some of these hot spots are northern India and Pakistan, northern China, Iran, Saudi Arabia, and parts of the US and Mexico.
“Since we now know how much groundwater is being depleted and how much there is, we will be able to estimate how long until we run out,” Gleeson says.
If all the Earth’s modern groundwater was pooled above ground, how deep would it be? (Graphic by Karyn Ho)