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Fossil groundwater vulnerable to modern contamination, geologist finds

Study led by Scott Jasechko shows that over half of global groundwater is more than 12,000 years old


University of Calgary hydro-geologist Scott Jasechko led a team that discovered ancient groundwater is not immune to modern-day pollution. Photo by Riley Brandt

By Heath McCoy
April 25, 2017

Most of the groundwater in the world that is accessible by deep wells is fossil groundwater, stored beneath the earth's surface for more than 12,000 years — and that ancient water is not immune to modern contamination, as has been widely assumed.

This study, led by University of Calgary hydro-geologist Scott Jasechko and co-authored by an international team of researchers, is published online today in Nature Geoscience.

Groundwater is the water stored beneath the earth's surface in soil pore spaces and within the fractures of rock formations. It provides drinking and irrigation water for billions of people around the world.

Jasechko and his co-researchers dated groundwater from more than 6,000 wells around the globe. By measuring the amount of radioactive carbon in the water, the team was able to determine the age of the groundwater. They discovered that the majority of the earth's groundwater is likely fossil groundwater, derived from rain and snow that fell more than 12,000 years ago. Jasechko and his team have determined that this fossil groundwater accounts for between 42 to 85 per cent of total fresh, unfrozen water in the upper kilometre of the earth's crust.

Study shows fossil groundwater not immune to modern contamination

Until now, the scientific community has believed that this fossil groundwater was safe from modern contamination, but Jasechko's study has proved otherwise.

"The unfortunate finding is that even though deep wells pump mostly fossil groundwater, many still contain some recent rain and snow melt, which is vulnerable to modern contamination," says Jasechko. "Our results imply that water quality in deep wells can be impacted by the land management decisions we make today."

Jasechko explains rain and snow that fell after the 1950s contains tritium, a radioactive isotope that was spread around the globe as a result of thermonuclear bomb testing. Disturbingly, traces of tritium were found in deep well waters, which indicates that contaminated rain and snow melt of today may be able to mix in with deep fossil groundwater and, in turn, potentially contaminate that ancient water, thought to be pure. 

Discovery suggests fossil groundwater protection is important to maintain water quality

"We're using the analogy of grandkids visiting their grandparents," says Jasechko. "Imagine fossil groundwaters are the grandparents and that younger groundwaters are the grandkids. We're finding that groundwater grandkids often visit groundwater grandparents deep underground, and, unfortunately, sometimes these grandkids have the flu. These young groundwaters may carry contaminants down with them, impacting deep groundwater once considered immune to modern contamination."

This discovery has important ramifications that should influence the way humans use groundwater in the future, Jasechko says.

"The upshot is that when we use fossil groundwater we should consider water quality risk in addition to sustainable use,” he says. "We may do well to develop land management plans that protect fossil groundwaters from pollutants so that these resources are available for future generations.”