2021-01-16 New Scientist

18 | New Scientist | 16 January 2021

News

GROUNDWATER basins that

provide water for much of the

world’s food production are in

danger of becoming too salty

for plants and animals. This risk

will remain even if care is taken

not to deplete them further.

A groundwater basin is a large

geological structure in which vast

quantities of fresh water are stored

in volumes of buried, permeable

rocks called aquifers. Often

the basin is in an “open” state,

which means water is constantly

flushed through it. But if the

water level falls too low, the basin

can become “closed”, and water

cannot leave the aquifers via

rivers or underground flows.

Once a basin is closed, salt

leaching into the groundwater

won’t be flushed out of the

aquifer again, so it accumulates.

Irrigation may cause both

the closure of a basin and

worsen the resulting problems.

As groundwater is pumped up

for agriculture, part of it will

evaporate and leave behind

salt deposits. These are

eventually washed into

the aquifer again, making

it more saline from the top down.

A team led by Graham Fogg

and Rich Pauloo at the University

of California, Davis, calls this

process ABCSAL, which stands

for anthropogenic basin closure

and groundwater salinisation.

They have just conducted a

detailed study of the important

Tulare Lake basin in the southern

Central Valley of California, where

just over 12,000 irrigated square

kilometres of land produce more

than $23 billion in crops annually.

They conclude that the first

stage of salinisation is already

happening there (Journal

of^ Hydrology, doi.org/fp62).

Shallow groundwater may

deteriorate over decades, says

Fogg. The quality of deeper

reserves may only become

a problem after two or three

centuries. Yet he notes that

this could come sooner than

the current estimates of the

expected exhaustion of a basin.

Marc Bierkens at Utrecht

University in the Netherlands

agrees that the problem

the researchers have identified

is important and worrying

for the world’s food supply

in the long term.

“It means that stopping the

overexploitation of aquifers

is not enough. You must make

them open again, for a time

adding much more water than

is pumped out. In many cases,

that won’t be realistic,” he says.

Bierkens thinks that a

number of important aquifers,

such as the Indus basin in South

and East Asia, the Ogallala Aquifer

in the US and the La Mancha

aquifers in Spain, could very

well be affected. “In most of the

important regions, you’ll reach

that point,” he says.

If refilling an aquifer

isn’t feasible, farmers and

cities will have to deal with

the salty water as best they can,

according to Bierkens. “You could

desalinate the water,” he says,

though doing so costs money,

which means the approach

may not be economical for

many small-scale farmers. ❚

Bas den Hond

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Astrophysics

Jumping into a

wormhole could

make it collapse

WHEN a wormhole reaches the end

of its life, the “mouths” at either end

may expand and get closer to one

another before disconnecting and

carrying on as regular black holes.

The trigger for this death could be

energy that perturbs the wormhole,

including a person leaping in.

Wormholes are hypothetical

tunnels between two black holes

that connect distant regions of

space-time or even separate

universes. There is no observational

evidence that they exist – nor that

separate universes exist – but they

are theoretically possible under the

laws of physics as we know them.

Igor Novikov at the Niels Bohr

Institute in Copenhagen, Denmark,

and his team simulated what would

happen if a wormhole connecting

two universes was pushed out of

equilibrium, such as if it were hit

by a blast of energy. Previous work

has shown that this could unite the

universes, but Novikov and his

colleagues found that wasn’t the

case. According to their calculations,

the “throat” of the wormhole –

the tunnel connecting the two

universes – would become wider

and wider while shortening in length

at the speed of light (arxiv.org/

abs/2012.13788). Eventually, the

connection between the two black

holes would disintegrate, leaving

behind a pair of black holes, each

in the universe in which it started.

One way to add energy and push

a wormhole out of equilibrium is by

jumping into it. This would mean

that travelling through a wormhole

to a different universe is impossible

as it would sever the connection

between the two universes, leaving

the jumper falling into a black hole.

Don Marolf at the University

of California, Santa Barbara, says

this calculation relies on a type of

quantum field that is unlikely to

exist. He says these wormholes

“violate cherished principles in

addition to there being no evidence

for their existence”. So, a wormhole

falling apart in this way is “a

hypothetical and highly unphysical

event”, he says. ❚

Leah Crane

Environment

Groundwater may turn salty

The world’s food chain relies on groundwater, but it may be becoming unusable

“The tunnel connecting two

universes would become

wider and wider while

shortening in length”

The Tulare Lake basin

in California is already

beginning to turn salty