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mrz
09-05-03, 08:02
Seawater 'salt pump' threatens drinking water
19:00 08 May 03
NewScientist.com news service

Coastal freshwater wells could be sucking more pollution from the ocean than previously thought, according to a laboratory experiment which shows that salt in seawater pumps pollutants into neighbouring freshwater.

That could spell trouble for coastal communities that rely on well water to drink, as they do in some parts of Spain and North Africa.

Brian Berkowitz and colleagues at the Weizmann Institute of Science in Rehovot, Israel, split a fish tank into two compartments with a barrier of sand to study how pollution passes through aquifers - areas of porous rock that hold freshwater.

When organic pollutants like benzene, toluene, and trichloroethylene were dumped into freshwater on one side of the tank, with clean freshwater on the other side, the pollutants slowly leaked through the sand.

But when the pollutants were added to salt water, they shot through the sand much more quickly. Thatıs partly because pollutants are less soluble in salt water than they are in fresh, says Berkowitz. This effect, known as "salting out", is well known to chemists dealing with organic solvents, but seems to have been ignored by people studying aquifers. "No one thought of this before," Berkowitz says.


Salting out

The team says it cannot be certain that the same thing is happening to the aquifers that supply coastal communities. Everything from different kinds of pollutants and salt, to the effects of tides and groundwater flow, might dramatically affect their results, they say.

But Berkowitz says people should start looking more closely at the problem. "This might be a serious but localised phenomenon," he says. People in southern Spain, for example, near where the Prestige spilled its oil some months ago, might find their well-water badly hit by pollution.

Perhaps most worrying is the fact that salt water can hold more pollution when shaken up by wind or waves. Instead of floating on the surface, contaminants are more likely to become suspended in the water in such conditions. When Berkowitz shook his salty samples, they absorbed up to 20 times as much pollution, so that more was available to be pushed through into the freshwater.

Journal reference: Science (vol 300, p 950)


Nicola Jones

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http://www.newscientist.com/news/news.jsp?id=ns99993712