According to a new study in the journal Nature Geoscience, the total groundwater volume in the upper 2 km of the Earth’s landmass is approximately 22.6 million cubic km, of which only around 0.35 million cubic km is younger than 50 years old.
The global distribution of modern groundwater as a depth if it was extracted and pooled at the land surface like a flood. Image credit: Karyn Ho.
The study shows that less than 6% of the groundwater in the uppermost portion of Earth’s landmass is modern.
“This has never been known before. We already know that water levels in lots of aquifers are dropping,” said Dr Tom Gleeson of the University of Victoria in Canada, lead author on the study.
“We’re using our groundwater resources too fast – faster than they’re being renewed.”
“Young and old groundwater are fundamentally different in how they interact with the rest of the water and climate cycles,” Dr Gleeson added.
“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.”
Using multiple datasets, and more than 40,000 groundwater models, the study estimates a total volume of 22.6 million cubic km of total groundwater.
The study’s maps show most modern groundwater in tropical and mountain regions.
The different volumes of water stored in the global water cycle. Global volumes of young (less then 100 years old), modern (less then 50 years old) and total groundwater to 2 km depth compared with other fresh water volumes stored in the atmosphere, in surface waters (that is, wetlands, rivers and lakes), within plants or in soils. Image credit: Tom Gleeson et al.
“Although modern groundwater represents a small percentage of the total groundwater on Earth, the volume of modern groundwater is equivalent to a body of water with a depth of about 3 m spread over the continents,” the researchers said.
“This water resource dwarfs all other components of the active hydrologic cycle.”
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.
The least amount of modern groundwater is not surprisingly in more arid regions such as the Sahara.
“Intuitively, we expect drier areas to have less young groundwater and more humid areas to have more, but before this study, all we had was intuition,” said co-author Dr Kevin Befus of the U.S. Geological Survey.
“Now, we have a quantitative estimate that we compared to geochemical observations.”
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Tom Gleeson et al. The global volume and distribution of modern groundwater. Nature Geoscience, published online November 16, 2015; doi: 10.1038/ngeo2590
