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Many people have the impression that underground water occupies vast caverns, flowing from one cavern to another along underground rivers. This is a common misconception: underground caverns are fairly rare, but huge quantities of water exist underground, within rocks. This is because many rocks contain pores, spaces that come in all shapes and sizes.
In sediments, and consequently sedimentary rocks, there are often pores between grains which can be filled with water. There may also be spaces between rock beds or along joints, fractures or fissures which can also contain water. However, before we look at pores in more detail we will examine how water gets into the rock.
Precipitation that reaches the ground either runs off at the surface, or sinks into it. Infiltration is the movement of water through the ground surface into the soil and on downwards. The rate at which infiltration can take place depends, among other things, on the permeability of the soil or rock.
Permeability is a measure of the ease with which water can move through a substance - the greater the permeability, the easier the infiltration. The total amount of infiltration also depends on the time available for water to seep into the ground. Heavy rainfall usually results in rapid runoff, and relatively little infiltration into the ground.
There are two distinct zones containing water beneath the ground surface. The unsaturated zone has mainly air-filled pores, with water held by surface tension in a film around the soil or rock particles.
Water moves downwards by gravity through this zone, into the saturated zone beneath, in which all the pores are filled with water. The boundary surface between the unsaturated zone and the saturated zone is the water table, which is the level of water in a well (strictly, in a well that just penetrates to the water table).
Water below the water table, in the saturated zone, is groundwater. Just above the water table is a zone called the capillary fringe, in which water has not yet reached the water table, because it has been held up by capillary retention.
In this process water tends to cling to the walls of narrow openings. The width of the capillary fringe depends on the size of the pore spaces and the number of interconnected pores. It is generally greater for small pore spaces than for larger ones.
The thickness of the unsaturated zone depends mainly on the climate (particularly the precipitation), but also on the topography.
In arid and mountainous regions this zone may be hundreds of metres thick, whereas in areas of high rainfall it may be only a few metres thick. Beneath swamps, lakes or rivers, the saturated zone reaches to the surface.
The thickness of the capillary zone will depend on the soil or rock texture, from a few millimetres in gravel to several metres in chalk or clay, and sometimes this zone may reach the surface.
The saturated zone extends downwards as far as the permeability of the rock will allow - a few tens of metres in some places, a kilometre or more in others. Water movement in the saturated zone is predominantly sideways (unlike in the unsaturated zone, where it is mainly downwards).
Dense vegetation, steeply sloping land, roads and buildings, and frozen subsoil, all have an effect on the total amount of infiltration.
Dense vegetation increases interception, which reduces infiltration, although the effect will be offset to some extent by the dense vegetation reducing the rate of runoff and thus increasing the time for, and therefore the amount of, infiltration.
Water rapidly runs off steeply sloping land surfaces, so there is little time for significant infiltration to occur.
Tarmac, concrete and roofing surfaces are relatively impermeable, so that roads and buildings promote overland flow and reduce infiltration.
Frozen subsoil is relatively impermeable and will reduce infiltration.
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