Groundwater

Groundwater is fresh water stored deep underground in spaces between rocks and soil grains. It is the largest store of fresh water on Earth that is not frozen in ice, more than 100 times the amount of water in all the world's rivers and lakes put together. About 2 billion people get their drinking water from groundwater, and farms around the world rely on it to grow crops. But because groundwater is invisible, it is easy to take for granted, and many parts of the world are now pumping it out far faster than nature can replace it.

• % of Earth's fresh waterApprox. 30%Most of the rest is frozen
• People depending on itApprox. 2 billionFor drinking and farming
• Largest aquiferGreat Artesian BasinBeneath 22% of Australia
• US Ogallala aquiferLost approx. 1/3Since pumping began in 1940s
• Time to refill an aquifer100s to 1000s of yearsMuch slower than pumping it out
• Springs areGroundwater coming back upThrough cracks in the rock

What is groundwater?

Some of the rain that falls on the ground does not just run off or evaporate; it slowly seeps down through the soil and into the rock below. Bits of rock are not as solid as they look: even hard rocks like sandstone or limestone are full of tiny spaces (called pores) between the grains. Other rocks have cracks, joints and underground channels. Together these spaces can hold huge amounts of water, which becomes groundwater.

The water table and aquifers

If you dug a hole almost anywhere, you would eventually hit water. The level at which the soil and rock below are completely saturated is called the water table. Below the water table, all the pores and cracks are full of water. Above it, they may be partly damp but contain air too.

An underground layer of rock or soil that holds and releases lots of water is called an aquifer. The best aquifers are made of porous rocks like sandstone or limestone, or loose materials like gravel. The largest aquifer in the world is the Great Artesian Basin, which lies under approximately 22% of Australia and is a vital water source for sheep and cattle stations across the outback.

Where groundwater comes back up

Groundwater is not stuck forever. It moves slowly through aquifers (sometimes only a few centimetres per day) and eventually comes back to the surface in several ways.

• Springs: places where the water table meets the ground surface, often at the edge of a hill or where one layer of rock meets another. Most rivers are fed partly by springs.
• Rivers and lakes: groundwater seeps in from the sides and bottom, keeping rivers flowing even during dry weather (called the baseflow).
• The ocean: where aquifers reach the coast, fresh groundwater seeps out below the sea surface.
• Wells: humans drill holes through the rock to reach the water table and pump groundwater up.

How long groundwater takes to form

The water in shallow aquifers may be only a few years or decades old. Deeper aquifers can hold water that fell as rain thousands or even millions of years ago. Some "fossil" aquifers in deserts (like parts of the Sahara) contain water from the last ice age, when those regions were green and rainy. Once that water is pumped out, it cannot be replaced on any human timescale.

Fact The Ogallala Aquifer beneath the central United States is one of the largest in the world and a vital water source for farming the wheat, maize and beef that feed America (and much of the world). It contains water that has been collecting for millions of years. Yet since pumping began in the 1940s, approximately one third of the aquifer's total water has been used up. Some parts have dropped by 30 metres. At current rates, large sections may run dry within decades.

Why over-pumping matters

When humans pump groundwater out faster than nature can replace it, several things go wrong.

• The water table drops: wells have to be drilled deeper, costing more.
• Springs and rivers dry up: when the groundwater that feeds them is depleted.
• The ground sinks (called subsidence): when water is removed from rock pores, the rock can compact under its own weight. Parts of Mexico City have sunk by over 9 metres in the last century.
• Salt water intrudes: in coastal areas, pumping out fresh groundwater can suck salty seawater into the aquifer, ruining it for drinking.
• Land becomes useless for farming: when wells finally run dry, the irrigated fields above can revert to desert.

Did you know? The world's longest underground river is in Mexico, in a system of flooded limestone caves called Sistema Sac Actun in the Yucatan. So far, divers have mapped over 375 km of interconnected underwater passages. The whole system is hidden inside the bedrock and filled with crystal-clear fresh groundwater. The Mayans considered the cave entrances (called cenotes) to be portals to the underworld.

Deeper dive: how artesian wells produce water without pumping

One of the most interesting features of groundwater is the artesian well: a well where water flows up to the surface (and sometimes shoots into the air) without any pump. The water is pushed up by natural pressure inside the aquifer.

Here is how it works. Some aquifers are sandwiched between two layers of impermeable rock (often clay or shale) that water cannot flow through. The aquifer is filled with water from a higher area (a "recharge zone", often in distant mountains), and that water pushes downhill through the aquifer. When you drill a well through the upper impermeable layer, the trapped water shoots up because it is under pressure from the recharge zone behind it.

The word "artesian" comes from the Artois region of northern France, where this kind of well was famously used in the Middle Ages. The most spectacular example today is the Great Artesian Basin of Australia, where boreholes can produce flowing water from depths of over 2 km. Without artesian wells, much of the Australian outback would be too dry for any farming at all.

Sadly, many of the world's artesian basins are being pumped faster than their recharge zones can refill them. Pressure is dropping. In some areas, wells that used to flow freely now need pumps, and in others they have stopped producing entirely. The slow process of recharging would take thousands of years to undo the damage. Like fossil aquifers, artesian water is essentially a one-shot resource.

For more on the water cycle, see what is the water cycle and run-off.