Not enough water in the ground

Only a small amount of the groundwater used by humans can be replenished in the next 50 years, which means that one of the most important resources on planet Earth will become scarce, according to a new study from an international team of researchers from Canada, the US and Germany. John Dyer reports from Boston.

Image credit: The Earth's groundwater resources are at risk of becoming scarce within the next 50 years. (Image credit: Vicki Francis/Department for International Development)

Image credit: The Earth’s groundwater resources are at risk of becoming scarce within the next 50 years. (Image credit: Vicki Francis/Department for International Development)

The Earth replenishes its groundwater far more slowly than scientists previously believed. According to a new study from three universities, rain, snow and other precipitation renew only 6 per cent of the world’s groundwater in a span of 50 years.

In other words, the vast majority of the water people pump from the ground won’t return within half a century.

Groundwater resources used too fast

“This has never been known before,” said Tom Gleeson, a civil engineer at Canada’s University of Victoria and lead author of the study that recently appeared in the academic journal Natural Geoscience. “We already know that water levels in lots of aquifers are dropping. We’re using our groundwater resources too fast—faster than they’re being renewed.”

That could become a dire problem as the world’s population grows in countries that are too poor to build water treatment or desalination plants for drinking water. Around two billion people in the world depend on groundwater that’s drawn directly from the ground by wells and other mechanisms.

“Groundwater is a super-important resource,” Gleeson told the Canadian Broadcasting Corporation. “It’s used by more than a third of the world’s population every day for their drinking water and it’s used by agriculture and industry.”

23 million cubic kilometres of water

The study determined that there is around 23 million cubic kilometres of water underground. If all that water were pumped up the surface, it would cover the earth in a layer that would be more than 180 metres deep. But only around 350,000 million cubic kilometres had been added to that reservoir in the last 50 years.

Scientists last estimated the amount of water in the world in the 1970s. Gleeson and his colleagues from the University of Texas in Austin and the University of Göttingen in Germany collected data from a million watersheds for their research using satellite imagery and computer models of 40,000 rock tests—far more information than ever compiled before.

Recent groundwater in Amazon Basin

They tracked the age of groundwater by testing for tritium, a radioactive hydrogen isotope that dates back to nuclear testing in the 1960s.

“Our maps and estimates show where the groundwater is quickly being renewed and where it is old and stagnant and non-renewable,” Gleeson said.

The most recently deposited groundwater sat in the Amazon Basin, Congo, Indonesia, and in North and Central America along the Rocky Mountains and Andes to the tip of South America. The Sahara Desert and other arid regions contain the least amount of groundwater.

Modern groundwater is more vulnerable

In 2012 Gleeson mapped out regions where people were pumping more water from the ground than was being replenished. Those regions included northern India and Pakistan, northern China, Iran, Mexico Saudi Arabia and California, Kansas and Texas in the United States.

“Since we now know how much groundwater is being depleted and how much there is, we will be able to estimate how long until we run out,” he said.

He noted that the oldest water — commonly accessed for industrial purposes — was often briny and not suitable for drinking without costly treatment. So drinking water supplies were actually lower than the total amount of water in the ground.

“Knowing the volume of modern groundwater is important because it is a more renewable resource than older stagnant groundwater,” said study co-author Elco Luijendijk from Göttingen University’s Geoscience Centre. “However, modern groundwater is also more vulnerable to climate change and contamination by human activities.”

Wake-up call

In the same issue of Nature Geoscience, Rutgers University Hydrologist Ying Fan, who is not connected to Gleeson’s work, wrote an accompanying article about his findings. Fan believed the study was a wake-up call to world leaders who need to regulate water so that future generations have enough to drink.

“This global view of groundwater will, hopefully, raise awareness that our youngest groundwater resources — those that are the most sensitive to anthropogenic (man-made) and natural environmental changes — are finite,” she wrote.

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