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
http://www.newscientist.com/