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Groundwater basins that provide water for much of the world鈥檚 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 basin is in an 鈥渙pen鈥� state, which means water is constantly flushed through it. But if the water level falls too low, the basin can become 鈥渃losed鈥�, which means water cannot leave the aquifers via rivers or underground flows.

Once a basin is closed, salt leaching into the groundwater will never 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.

Hydrologists Graham Fogg and Rich Pauloo at the University of California, Davis call 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.

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, who wasn鈥檛 involved with the study, agrees that the problem Pauloo and Fogg have identified is important and worrying for the world鈥檚 food supply in the long term.

鈥淚t 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鈥檛 be realistic,鈥� says Bierkens.

He 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. 鈥淚n most of the important regions, you鈥檒l reach that point,鈥� he says.

If refilling an aquifer isn鈥檛 feasible, farmers and cities will have to deal with the salty water as best they can, according to Bierkens. 鈥淵ou could desalinate the water,鈥� he says, though doing so costs money, which means the approach may not be economical for many small-scale farmers.

Pauloo agrees that desalinisation may be needed to deal with the problem. Yet the world should also start to see aquifers for what they are, he says: freshwater reservoirs that dwarf the storage capacity of human-made ones and could potentially replace them. This might have the added benefit of opening them up again. Compared with water flows above ground, 鈥渨e haven鈥檛 really developed the same kind of infrastructure, planning and management around groundwater storage鈥�, he says.