Even the deepest, oldest bodies of groundwater show signs of modern pollution, a new paper from the University of Calgary reports.

That’s one spooky well.
Image credits Sid Litke.

There’s a lot more water hidden in the bowels of the Earth than you’d believe — an estimated 100 times more water is stored deep underground in fissures and pores than what we see on the surface. And some of it is incredibly old, so old in fact that we refer to it as ‘fossil water’. For both these reasons, it’s been believed that fossil water wouldn’t be impacted by the same elements that act on surface waters, such as chemical pollutants.

But it seems that this isn’t the case, and fossil, as well as groundwaters, are still able to mix with potentially contaminated surface waters. Which is bad news for the billions of people who rely on these resources for agriculture, washing, and drinking.

“Groundwater is an immense resource,” says Scott Jasechko, an assistant professor of geography at the University of Calgary and lead author on the new study. “About a third of human water uses are derived from groundwater, so it’s really an important and precious resource that’s already widely used.”

Jaschko and his team analyzed groundwater samples from 6,455 wells around the world and used carbon dating to determine the age of water levels intercepted by the wells. They found that fossil groundwater makes up anywhere between 42-85% of the water in aquifer bodies within the top kilometer of the crust. They further showed that fossil water which has been trapped underground for at least 12,000 years makes up most of the water volume pumped from 250m or deeper wells.

The really worrying find is that half of the fossil groundwater-heavy wells used in the study also contained “detectable levels of tritium”. Tritium is a radioactive hydrogen isotope which scientists use to spot recently recharged water — during the 1950s nuclear testing, there was a massive increase in tritium levels. Finding the isotope in fossil waters indicates that water recharged in the past six decades was mixed in. This would suggest that modern pollutants can also find their way into fossil aquifers by piggybacking on water circulation through deep wells.

“We conclude that water quality risk should be considered along with sustainable use when managing fossil groundwater resources,” the authors write.

The paper “Global aquifers dominated by fossil groundwaters but wells vulnerable to modern contamination” has been published in the journal Nature Geoscience.