An aquifer is an underground rock layer that stores water underground (groundwater) and from which the water can be extracted via a well or borehole.
People often think that underground water is stored in underground rivers that flow beneath us. While these do exist in some areas of limestone, in most of the country the rock type does not permit this and the water on the ground is held in cracks and fissures – or even between the grains of a sandstone. Although you would think that you can't get much water in between these grains, when you have a few million tons of rock in the ground you can get an awful of water stored there.
One of the key features is that the stored water must be able to flow, albeit slowly. Generally speaking most water in aquifers lies in the zone of a couple of hundred metres depth but the water can actually penetrate much further, down to 2 km. Mind you, at this depth the water tends to have a lot of minerals dissolved into it.
The chalk areas around London and Hampshire are very important source of aquifers. Chalk contains tiny fissures that hold water and it is not uncommon for boreholes in this area to yield 10,000,000 litres of water a day.
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