Aquifers are underground layers of water-bearing permeable rock, gravel or sand, the upper limit of the saturated zone being the water table. Many aquifers are potentially renewable as water resources, and could produce water at a certain rate for ever, or at least until climate change stops the entry of new water.
This is more or less the rate that people once used them, before we discovered how to burn fossil sunlight, such as coal and oil, to extract fossil water directly from underground, using mechanical pumps. These can take water from an aquifer very much faster than it can be recharged naturally. Much of this water goes into bottles for shipping around the world (see: Bottling aquifers), most of the rest to irrigation, industries and cities.
The danger usually comes when large numbers of people use powered drills to send many boreholes down into a single aquifer, and powered pumps to suck water out of it. These pieces of equipment used to be rare and expensive, but since the 1980s tens of millions have been made and distributed around the world, with prices reduced by mass production, and the equipment often subsidized or given away by donor agencies or governments to encourage farmers.
The result has been a worldwide ‘groundwater revolution’, in which millions have become dependent on water drilled and pumped from underground.
Technology is only part of the story, though, since insatiable markets are also needed for over-exploitation to happen. This is where ‘virtual water’ comes in – the amount of water used to make things.
It takes about 2.8 tonnes of water to make a single cotton T-shirt. Well, on average, to make a kilo of wheat takes about a tonne of water, of sugar three tonnes, of milk up to four tonnes, of rice up to five tonnes, of coffee 20 tonnes, and beef 24 tonnes. All of this adds up to 1,000 km3 or a trillion tonnes of virtual water being traded every year. This demand is more than enough to drain many aquifers when it’s combined with open access and cheap technology.
Coup de grâce
Once an aquifer is being thoroughly over-exploited relative to its natural recharge rate, it can be finished off by reducing that rate. This can be caused by anything that interferes with the way that water soaks into the ground in critical places, those points where water finds its way most easily into the body of the aquifer.
This can happen when the water is blocked, such as by building waterproof houses, roads and car-parks in the area. Or when evaporation rates are increased, such as by planting thirsty crops like cotton, or thirsty trees like eucalypts. Or when the swamps and other wetlands that naturally form there are drained for farming. It can also happen when the flow of water off the critical place is speeded up, such as by straightening rivers, preventing floods, or by using impermeable culverts to drain the land.
These all prevent water from sinking into the ground, where some of it would normally help to recharge the aquifers beneath. The combination of over-pumping and blocking recharge can cause an aquifer to be completely ‘de-watered’, as the hydrologists put it.
As an aquifer is drained, the spaces once occupied by water are filled by dust, sand and rock fragments pressed down from above, and the structure of the aquifer collapses.
This has two consequences. One is that the ability of the water-bearing layer to hold water is greatly reduced, so the aquifer is irreversibly damaged and incapable of being recharged. The other is that the land surface will subside, exactly like earth over a grave, which sinks as the coffin and cadaver below collapse and decay.
A city built on the subsiding grave of an aquifer will sink and crack, and heavy rains will flood it.
Read on: The World to the Rescue?