The UNEP is urging action to better manage the world's groundwaters and warns that cities, industries + agriculture may soon struggle to meet their needs...

This is because ground waters are being exhausted by over-extraction and becoming tainted by salt contamination.

Places as diverse as Arizona, Bangladesh, Bangkok, Cairo, Calcutta, London, Mexico City + Jakarta are vulnerable to excessive extraction depleting their water supplies, while many semi-arid regions are at risk of salt contamination...

Salt contamination occurs in hot countries where excessive land clearance and/or irrigation have taken place... once the native trees are cut down, the grasses that farmers tend to grow as crops, such as wheat, often fail to release water into the atmopsphere as fast as it is being pulled up to the surface by the sun... this leads to surface waterlogging (which drowns plants) - irrigation speeds up the waterlogging step - eventually the water table is raised to such an extent that it becomes impossible to drain the land and prevent the water as well as the salts it carries (which have been washed deep underground by millions of years of rain) from reaching the surface... over many decades the water-borne salts precipitate out at the surface (due to evaporation) and become highly concentrated... eventually, even salt-tolerant plants stop being able to grow + the land turns into desert....

In Australia, previously productive farmland is being lost to salt on a massive scale and cities such as Adelaide + Perth are having their drinking water tainted... this problem is even more severe in poor countries which cannot afford to pay for desalination, have large populations + lack the resources (or knowledge) necessary to revegetate their landscapes...

You can find out more about this hidden, but critical problem here.