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April 26, 2017

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Key water source not as safe as thought

HUMAN activity risks contaminating pristine water locked underground for millennia and long thought impervious to pollution, said a study yesterday that warned of a looming threat to the crucial resource.

Even at depths of more than 250 meters under the Earth’s surface, so-called “fossil” groundwater — more than 12,000 years old — has been found to contain traces of present-day rainwater, they said.

This suggests deep wells, believed to bring only unsullied, ancient water to the surface, are “vulnerable to contaminants from modern-day land uses,” said the study’s co-author Scott Jasechko.

Groundwater is rain or melted ice that filters through Earth’s rocky layers to gather in aquifers underground, a process that can take thousands, even millions, of years. It is the largest store of unfrozen fresh water on the continents.

Groundwater is pumped to the surface by deep wells for drinking and irrigation, and supplies about a third of all human water needs.

For the latest study, presented at a European Geosciences Union meeting in Vienna, Jasechko and a team set out to determine how much of Earth’s groundwater was more than 12,000 years old.

New groundwater has more radioactive carbon because it was more recently exposed to Earth’s atmosphere and shallow soil, tainted by nuclear tests since about the 1950s.

Half of the fossil groundwater wells they studied contained detectable levels of tritium — a radioactive isotope of hydrogen found in younger waters.

“This observation questions the common perception that fossil groundwaters are largely immune to modern contamination,” concluded the study, published in Nature Geoscience.


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