Groundwater may turn salty
The world's food chain relies on groundwater, but it may be becoming unusable
by Bas den Hond
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 freshwater are stored in volumes of buried, permeable rocks called aquifers. Often the basis 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 can not 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 salinization. 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 $23billion in crops annually. They conclude that the first stage of salinization 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 LaMancha 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 infeasible, 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.