Most of the groundwater that is accessible by deep wells is old, but still vulnerable to modern contamination, reports a study published online in Nature Geoscience this week. The world’s groundwater provides drinking water and irrigation for billions of people around the world. Some of this groundwater is young and easily affected by pollution and changes in climate. But potentially much more groundwater is old having been stored beneath the Earth’s surface for many thousands of years. Old groundwater is more difficult to sustainably harvest, but it is largely protected from climate variability and - it had been widely assumed - immune to contamination from human activity. Scott Jasechko and colleagues date groundwater from over 6,000 wells around the globe. They find that so-called fossil groundwater - stored for more than 12,000 years - accounts for between 42-85% of total aquifer storage in the upper kilometre of the Earth’s crust, and that it comprises the majority of groundwater pumped from wells deeper than 250 metres. However, they also detect traces of tritium - a radioactive isotope of hydrogen - in over half of the wells that they analyse. They note that this is significant because tritium was spread around the globe by nuclear testing conducted in the 1950s, and its presence shows that at least some of the groundwater in the wells post-dates 1950. These findings imply that fossil groundwater in wells is often mixed - either in the wells or in the aquifer itself - with much younger water and whatever contaminants it carries. The authors conclude that, as the old groundwater is recycled over thousands of years, any contamination of this crucial water resource will persist on human timescales. This research will be presented at a press conference at the General Assembly of the European Geosciences Union on Tuesday, 25 April at 0800 BST. More information can be found here, and a live stream of the press conference can be viewed here.
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