Fossil groundwater vulnerable to modern contamination

Study shows that over half of global groundwater is over 12,000 years old

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 Dr. Scott Jasechko (University of Calgary) and co-authored by an international team of researchers including Professor Richard Taylor (UCL Geography & UPGro GroFutures), is published online today (April 25) 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, Taylor and his co-researchers dated groundwater from over 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. The team 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.

Until now, the scientific community has generally believed that fossil groundwater is safe from modern contamination but this study has proved otherwise.

“Deep wells mostly pump fossil groundwater but many still contain some recent rain and snow melt, which is vulnerable to modern contamination,” says Jasechko.

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 contemporary rain and snow melt can mix with deep fossil groundwater and, in turn, potentially contaminate this ancient water.

According to Taylor, this discovery has important ramifications that should influence the way humans use groundwater in the future,

“Our results reveal not only current use of fossil groundwater but also the potential risks to water quality associated with the use of deep wells. Indeed, we need to better understand how the construction and pumping of deep wells themselves may connect fossil groundwater to the present-day water cycle.”