Uncharted waters: Probing aquifers to head off war

 

The water's edge (Image: George Steinmetz/Corbis)

Nearly all our fresh water comes from obscure underground deposits – now satellites and radioactive isotopes are telling us how much we have to go round

DEEP beneath the eastern Sahara, the Nubian Sandstone aquifer was in trouble. By the early 2000s, the aquifer - one of the largest and oldest groundwater deposits in the world, which supplies Libya, Egypt, Chad and Sudan - was emptying fast. Egypt was tapping the aquifer to feed its growing desert cities far from the Nile. Libya, whose only other water source is the salty Mediterranean, was drawing water off by way of an underground network of pipes and aqueducts known as the Great Man-Made River, which Libyans describe as the eighth wonder of the world.

Soon the Sahara's oases began to dry up, causing water shortages for nomadic groups and wildlife. But no one could agree on who was to blame. The ancient aquifer system was just too complicated: it was impossible to pinpoint who was taking too much water, or even estimate when it would run out.

Because none of the countries trusted the others to provide an unbiased analysis, they couldn't agree on what steps, if any, to take to protect the aquifer. Mistrust and a lack of cooperation threatened to spiral into something worse.

This conflict exposed an ordinary truth that had somehow been forgotten: most of the world's drinking water is hidden underground, and we don't have a clue what's happening to it. But as global populations grow and climate change kicks in, one thing is certain: we can no longer count on the water to be where we expect to find it. Our groundwater is dissipating into the ocean, being consumed at record rates and being irreversibly contaminated; even as claims to what remains become increasingly contentious. It won't be long before shortages cause widespread droughts and the first water war begins.

How can we stop this? The first step is knowing where the water is. Conventional maps are no longer enough when you're dealing with an invisible, moving target. But there is hope. Impressive new physics and engineering tools are beginning to yield the first clear pictures of the world's hidden water. These have already revealed some unexpected good news, but their real promise is in the possibility of a world map of a resource more precious than oil.

See graphic: "Mapping the world's unseen water"

Although it comprises 97 per cent of the world's accessible fresh water - the UN Environment Programme's latest estimate - we have never really bothered to get a clear picture of the water beneath our feet. Most hydrologists prefer to study the water on the Earth's surface. "Certainly groundwater has suffered from an 'out of sight, out of mind' problem," says Peter Gleick, a hydroclimatologist who runs the Pacific Institute, an independent think-tank based in Oakland, California.

Most people would be surprised to hear that. After all, it's been easy enough to exploit the water hidden in aquifers. These underground stores are vast; the 40,000 cubic kilometres of water in the Guarani aquifer in South America, for example, far exceeds what's in all five of North America's Great Lakes. But this water isn't held in a vast underground lake. Instead, it moves, often slowly, through complex layers of permeable rock, sand and other geology. And unlike a lake, how useful it is depends not only on how much water it contains, but on how quickly it is refilled by rainwater or snowmelt.

Parched landscapes

Plenty of maps show where the world's aquifers are located, but they make no mention of how much water they contain, how fast the water levels are changing, or even whether the water is safe to drink. As the world's population increases, so too does demand, but that isn't the only cause for concern. Climate change is gradually redistributing the world's water. As the Earth warms, precipitation is shifting from the mid-latitudes to the low and high latitudes. Wet areas are becoming wetter and dry areas drier, which may account for the record-breaking droughts in east Africa and Texas last year. Less rainfall in these mid-latitudes means less new water to refill the aquifers that are being depleted the fastest, and that means more aquifers will become like the Nubian - a precarious source in a parched landscape.

To figure out how much water we can sustainably take from such a fragile system, we need to know two things: how old the water is, and how quickly it is being replenished.

Until recently, the only way to get that kind of data was with costly and time-consuming borehole studies. These involve digging lots of narrow wells in order to monitor the speed and direction of the water flow, and then using that data to construct a model of the aquifer.

In the case of the Nubian aquifer, however, such a study was out of the question. For one thing, it would have been too expensive, and many of the drilling sites would be in remote desert. More than anything, however, such intensive studies require a political appetite to set them in motion, and since previous borehole studies had met with scepticism, a dense network of boreholes across four countries would be hard to justify.

The solution came from an unexpected corner: the International Atomic Energy Agency (IAEA). Alongside its work with nuclear energy and weapons, the organisation also uses isotopes for water analysis. Pradeep Aggarwal, who runs the IAEA's isotope hydrology division, had been using isotopes to test surface water for decades, and he saw an opportunity to help the Nubian aquifer. In 2006, Aggarwal and Zheng-Tian Lu, a physicist at Argonne National Laboratory in Illinois, launched a major project with The UN Development Programme and other agencies to create a comprehensive map of the aquifer. Not only is isotope testing much cheaper, because it only requires taking a few water samples from existing wells, but this handful of samples can reveal the state of the whole aquifer. "Measuring isotopes in one location can tell you what is happening tens, and even hundreds of kilometres away," says Aggarwal.

First, they needed to work out how old the aquifer was, and for that, the team turned to carbon-14. Just like ancient artefacts, water can be dated using this radioactive isotope. Above ground, water absorbs some of the gases in the atmosphere, including carbon dioxide, causing it to take on the atmosphere's signature cocktail of isotopes. When the water later disappears into an aquifer, it takes this unique signature with it. As time passes, the carbon-14 undergoes radioactive decay. The amount that remains in a sample of water can reveal the water's age.

It turned out that there was practically no carbon-14 left in their samples. That meant the water was extremely old.

The finding squared with previous carbon-14 studies, which had suggested the Nubian aquifer was 40,000 years old. But Aggarwal realised this figure was close to the isotope's 50,000-year dating limit, raising doubts about its accuracy. So he turned to krypton-81, a rare isotope that researchers have only recently learned how to trap and count, which can accurately determine the age of water back to 2 million years. Lu discovered that indeed, the carbon-14 studies had been way off: a good portion of the water in the Nubian aquifer is closer to a million years old (Geophysical Research Letters, vol 31, p L05503).

But simply knowing the water's age wasn't enough. To get a complete picture of the aquifer, Aggarwal needed to understand whether any part of it was being refilled with new water.

The team did this by looking at two isotopes of the atoms that make up the water molecule - deuterium and oxygen-18. Every drop of water has a telltale ratio of these two isotopes, which offers clues to the climate at the water's source. Both of these heavy isotopes decreases in cooler climates. Therefore, a sample with a lower concentration of oxygen-18 and deuterium would suggest that the last time new water was being deposited, the climate around the Nubian aquifer was cooler.

Sure enough, the deuterium and oxygen-18 in the samples confirmed that no new water had been deposited there during modern times. The aquifer contained only "fossil water" that had been trapped underground long ago. In other words, this aquifer was not being refilled and would one day run dry.

This was bad news. However, thanks to the isotope studies, Aggarwal was also able to calculate how much total water remained in the aquifer. It may be old, but it turned out that enough fossil water remains in the aquifer to last at least several centuries. Not only that, but the water flows so slowly that one country's pumping doesn't immediately affect another. "So Chad doesn't have to worry about Libya stealing its water," says Aggarwal.

In the wake of the IAEA's conclusions, the countries that rely on the Nubian aquifer finally agreed that they need to work together to protect it, which broke decade-long tensions. "We were able to build a model that all four countries accepted as a reliable simulation," Aggarwal says. The IAEA has nearly completed a world atlas that extends the work they did on the Nubian Sandstone aquifer, and Aggarwal ultimately hopes to map the world's groundwater according to age and recharge rate (see map).

But none of that will help the local oases, which are still drying up - along with a lake in Libya. That's because political tensions are not the only problem in the global water picture.

Even when an aquifer supplies only a single country, it's not always clear who is using the water or for what purpose. "We've been overpumping groundwater and we've known about it for decades," says Gleick. "But policies still can't get a handle on it." A complete account of groundwater use would require nations and industries to measure their indirect as well as real water use, which few do.

Now Jay Famiglietti and his team at the University of California at Irvine have found a way around this: tracking real-time changes to the world's groundwater from space. To do this they rely on data from NASA's Gravity Recovery and Climate Experiment (GRACE), which measures variations in Earth's gravitational field. GRACE uses two satellites orbiting about 220 kilometres apart. When the first satellite flies over a spot where gravity becomes stronger - a mountain, say, or massive aquifer - it is temporarily drawn closer to the Earth and away from the trailing satellite. By measuring the changes in the distance between the two satellites, they can create a detailed map of the Earth's gravitational field.

Famiglietti's group has taken this idea one step further. They have been able to correlate the satellites' gravity data with significant changes to underground aquifers caused by wet and dry seasons, long-term droughts and water extraction for mining and agriculture.

A radical new map

The team raised eyebrows in 2011 when they published a controversial study showing that a major aquifer beneath California's Central Valley was being depleted much faster than anticipated, due to water-thirsty lettuce farms (Geophysical Research Letters, vol 38, p L03403). Last year, Famiglietti told New Scientist that if the unregulated nature of the farming continued, the aquifer would be depleted by 2100.

Like Aggarwal, Famiglietti is now putting together a global map based on the information he gathered from GRACE. His findings are sobering. We are depleting every one of the world's major mid-latitude aquifers. As in California, the main culprit is agriculture, which uses massive amounts of water.

Agriculture is not the only industry taking a toll. In Australia, Famiglietti and his team says their map may indicate serious depletion of the groundwater in areas where mining takes place. "Yes, we know that mining is intensive. But has anyone ever shown them a map like this? No," he says. Changes in rainfall are also playing a role, though his team has yet to worked out precisely how much. But the water is moving, that much is certain. "When you see this big-ass red spot that covers a fifth or a sixth of the continent, it raises your eyebrows," he says.

GRACE can only track changes in aquifers with areas greater than 150,000 square kilometres. But even so, multiplying GRACE's water output data with the isotope studies' age calculations gives us a rough estimate of the total amount of water in a given aquifer. Do this for all aquifers, Famiglietti says, and we can get the first good picture of how much fresh water is stored underground and where it's going. What's more, this information could allow us to predict water shortages, possibly with enough warning for countries to mitigate their effects or even stave them off altogether.

Such projects are still in their infancy. One is being carried out by Famiglietti's former graduate student, Matt Rodell, who analyses US water data at NASA. In December, Rodell's group showed that the record-breaking drought in Texas - the driest 12-month period on record since the 1890s - reduced groundwater levels in much of the state to their lowest in 60 years. Their prediction that recharging the aquifer would take months was enough to motivate the state to develop more effective water conservation methods.

In some cases, for example, it's possible to counter droughts by artificially recharging an aquifer with purified wastewater to keep it filled. California has done this for decades, and Egypt is experimenting with doing the same in a shallow, young part of the Nubian aquifer by feeding it with water from the Nile.

Develop a thorough enough understanding of the world's water, and we might even be in for a few pleasant surprises. Though finding large amounts of new fresh water is not on the cards, it is possible that such a map could unearth small supplies. On the parched Santa Elena peninsula in Ecuador, for example, residents had only three wells that gave water sporadically. But after an IAEA isotope investigation in 2009, residents dug four sustainable wells that now give water 24 hours a day. And in Bangladesh, where arsenic in the water was poisoning millions of people, isotope studies identified previously unknown aquifers whose waters were safe.

Maps alone won't refill the oases in the desert, but they are key to making sure everyone has enough. Ultimately, preventing water conflicts will be less about absolute amounts of water than about the fair distribution of what's there. The first step is knowing where it's going and who's taking it. Even amid all the bad news, there are bright spots: As more countries - from Canada to India to Australia - wake up to the problem and begin to track their hidden water, the tools to help them do so keep getting better.

Chelsea Wald is a freelance writer based in Vienna, Austria

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