India (SciDev) By Madhukara Putty
Contemporary pollutants can reach deep wells that tap fossil aquifers, says a study by an international team of researchers.
More than half of some 6,000 wells studied around the world by the researchers showed traces of tritium, a radioactive isotope of hydrogen that has been marking water since 1953 when nuclear weapons testing became widespread. As tritium released by nuclear tests shows up in rainfall, it has become a handy tool in studying hydrological cycle.
“This is a matter of concern because fossil groundwater is non-renewable and won’t be recharged in human time scales.”
Yoshihide Wada, International Institute for Applied Systems Analysis, Austria
The discovery of tritium traces in deep fossil aquifers, which were originally recharged by precipitation more than 12,000 years ago, is being attributed by the researchers to contamination by ‘younger’ groundwater closer to the earth’s surface.
“In the vast majority of wells we examined, tritium does not exist at levels high enough to present a human health risk,” clarifies Scott Jasechko, assistant professor, department of geography, University of Calgary, Canada and lead author of the study published on 25 April in Nature Geoscience.
Though fossil waters—common in wells deeper than 250 metres—are found in major aquifers, their global extent and depth are not clearly understood. “Currently, humans use groundwater for agriculture, but we don’t know how much of it is fossil groundwater. This is a matter of concern because fossil groundwater is non-renewable and won’t be recharged in human time scales,” says Yoshihide Wada, study participant and deputy director of the water programme at the International Institute for Applied Systems Analysis, Austria.
Jasechko and his colleagues estimate that more than half of the total groundwater in the uppermost one kilometre of the Earth’s crust is fossil groundwater. Their analysis suggests that these waters are more vulnerable to modern contamination than previously thought.
“This paper reminds us that groundwater resources are vulnerable,” says Alan MacDonald, professor, British Geological Survey. In a paper published in 2016, MacDonald and his colleagues reported widespread contamination of groundwater in the Indo-Gangetic Basin, which accounts for a quarter of global groundwater withdrawals.
However, Jasechko’s team did not carry out detailed analyses of fossil groundwater in Asia and Africa because of data paucity.
According to MacDonald, human activity can pollute groundwater particularly in the shallowest few hundred metres where it is relied upon for water supply and irrigation. “The combination of high abstraction and polluting land activities, such as heavy industry, poor sanitation or intensive agriculture, can draw contamination deep into certain aquifers,” he says.