COASTAL wells could be sucking up more pollution from the ocean than we thought, according to a lab experiment showing that salt water can actively pump pollutants into neighbouring fresh water. That could spell trouble for coastal communities that rely on wells for drinking water.
Fluid-flow expert Brian Berkowitz and his colleagues at the Weizmann Institute of Science in Rehovot, Israel, used a simple model to investigate how pollutants pass from the sea to coastal aquifers 鈥� natural reservoirs of fresh water held in porous rock. They divided a tank into two compartments, separated by a barrier of sand to mimic the rock, and dumped organic pollutants such as benzene, toluene or trichloroethylene into the water on one side.
With fresh water on both sides of the tank, the pollutants leaked though the barrier relatively slowly. But when the polluted water was salty and the water on the other side was fresh, the pollutants passed through the sand much more quickly (Science, vol 300, p 950).
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That is partly because pollutants are less soluble in salt water than they are in fresh, says Berkowitz. This effect, known as 鈥渟alting out鈥�, is well known to organic chemists, but seems to have been ignored by people studying aquifers. 鈥淣o one thought of this before,鈥� says Berkowitz.
His experiments also show that seawater probably holds more organic pollutants than people have assumed. While only a small amount of these substances dissolves in salt water, a lot more gets mixed in as tiny suspended droplets when the sea is shaken up by wind or waves. When Berkowitz shook his salty samples, they absorbed between 2 and 20 times as much pollution as before.
The team cautions that things are much more complicated outside the lab: the flow of water through the ground might dramatically affect what happens, for example. But Berkowitz thinks it would be wise to look at the problem more closely.
In Israel, for example, most of the country鈥檚 drinking water comes from a coastal aquifer that is heavily polluted with nitrates. Many researchers think this is due to farmers pouring too much fertiliser on their fields or irrigating with sewage. But Berkowitz says polluted water seeping in from the sea might be contributing to the problem, though he has not tested this idea.
And while Israel鈥檚 water goes to central purification stations before being distributed, the same is not true everywhere. In northern Spain, for example, near where the Prestige spilt its oil last year, some people drink well water. 鈥淭his might be a serious but localised phenomenon,鈥� says Berkowitz.