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EGU 2017: Much groundwater is ‘surprisingly old’ but may be contaminated

Groundwater stores 100 times more water than all the world’s lakes put together and supplies around 40% of the water for irrigated agriculture. And most of that groundwater is “surprisingly old”, according to James Kirchner of ETH Zurich and Swiss Federal Research Institute WSL in Switzerland, speaking at a press conference at the European Geosciences Union general assembly in Vienna.

The bulk – 42-85% – of this groundwater in aquifers in the upper 1 km of the Earth’s crust is “fossil” groundwater that’s more than 12,000 years old, Kirchner, Scott Jasechko and colleagues have estimated, so it was around “when mammoths roamed”. There’s roughly twice as much of this ancient water by volume as modern groundwater that collected within the last 60 years.

What’s more, previously scientists had assumed that this ancient water was isolated from modern contamination such as pesticides. But Kirchner and the team found that around half of the wells dominated by fossil groundwater contained modern water too. So it’s not safe to assume that the older groundwater is free from manmade contaminants like fertilizers or chlorinated hydrocarbons.

“It’s like going to a giant old folks’ home and finding little kids running round,” said Kirchner. “It’s great unless the kids have ‘flu.”

Kirchner stressed that it’s not that half of the world’s groundwater is now known to be contaminated but that we cannot exclude the possibility that it may include contaminants.

To age the groundwater, the researchers assembled existing data on the age of groundwater from thousands of wells based on its contents of isotopes of carbon and tritium. A hydrogen isotope, tritium was released into the atmosphere in large quantities by thermonuclear testing that began in the early 1950s so it acts as a tracer for more recently recharged groundwater. Carbon-14 has a half life of nearly 6,000 years so if water is depleted in C-14 it hasn’t “seen the atmosphere” for at least 12,000 years.

The fossil and modern groundwater may mix either in the well or in the aquifer beneath; it’s not yet clear which. Wells deeper than about 250 metres mainly pump fossil groundwater, Kirchner explained. Many of these fossil sources may be non-renewable on human timescales. The High Plains aquifer in the US, for example, contains rain that mainly fell in the Pleistocene but its water level has dropped more than 100 metres in the last few decades and it would take 6,000 years to replenish.

The paper was published in Nature Geoscience yesterday.

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