Water is vital for the survival of the human race, but thanks to overuse of our water supplies and the hazards presented by man-made climate change, there is a distinct lack of it in many parts of the world. A new study published in Nature Geoscience details the most accurate atlas of the world’s supply of subterranean water – and reveals that no more than 6 percent is “useful” to humans.
Water held within geological formations is known as groundwater, which often exists in reservoirs called aquifers. Ancient water is contained in these aquifers for many thousands of years before various geological processes cause it to percolate up to the surface, where it will flow in rivers, end up as clouds, and eventually make its way back into deep rock formations once again.
Creating a map of the Earth’s groundwater is vital – without aquifer supplies, our exponentially rising global population would be in dire straits, particularly those in more arid regions. The current volume of aquifers can be calculated by taking three measurements: their capacity, how quickly they recharge over geological time, and the amount that humans extract from them.
This is an extremely difficult task to undertake, but the team of scientists led by the University of Victoria in Canada looked through large pre-existing datasets and combined their calculated quantities with numerical modeling, which allowed them to look at the changes in aquifer volumes over time.
They found that in the upper two kilometers (1.24 miles) of the Earth’s crust, there is 22.6 million cubic kilometers (5.42 million square miles) of groundwater. That’s roughly 4,600 Lake Michigans-worth of water.
A small proportion of this groundwater is extractable by humans and used to significantly supplement our surface supply of freshwater. This fraction is estimated by the study to be around 6 percent of the total groundwater supply. In addition, just 347,180 cubic kilometers (83,300 cubic miles) is considered “modern,” in that it is less than 50 years old – it is this water that is both most useful to humans and most vulnerable to global change.
This modern water is identifiable by looking for tritium, a mildly radioactive element mostly formed around 50 years ago following the beginning of the thermonuclear weapon testing prevalent during the Cold War. All water younger than 50 years can be traced.
Image credit: The groundwater atlas. Dark blue represents quickly renewed water; light blue shows more ancient water – most of which is non-renewable. Gleeson et al./Nature Geoscience.
As the authors of the study point out, our supply of extractable, modern groundwater is our most important source of water. In comparison, the total useful surface water on the planet is only 100,000 cubic kilometers (24,000 cubic miles): just over a quarter of the groundwater supply available to us.
“This has never been known before,” researcher Tom Gleeson, lead author of the study and a hydrologist at the University of Victoria, said in a statement. “We already know that water levels in lots of aquifers are dropping. We’re using our groundwater resources too fast — faster than they’re being renewed.” Hopefully recent advancements in seawater filtration technologies will help to plug this widening gap somewhat.
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