Underground water is held in spaces or voids within a rock. Some rocks - especially sedimentary rocks - are porous. They hold water and other fluids in the pore spaces between the grains of the rock. These millions of minute spaces, each one holding its own minute drop of fluid, build up to significant underground reservoirs of water, oil and gas. The proportion of pore space within the rock is called porosity. If the grains are well-rounded and arranged with open packing, the porosity reaches a maximum value of almost 50% - this kind of sediment would be able to hold a large quantity of water. Porosity decreases if the grains are angular or if the rock is well-cemented.

In other cases, the fluids are held in joints, bedding planes and other fissures - this is fissure porosity.

If the voids are inter-conected, the ground water can flow through the rock, which is said to be permeable. Permeability allows the porous rock to give up its water as it flows towards springs and boreholes.

However, not all porous rocks are permeable; clay and pumice are two common examples of porous but impermeable rocks. Ground water can only be abstracted via a bore-hole or well if the rock is permeable.

A rock from which significant amounts of water can be extracted is called an aquifer ("water-bearing"). Most aquifers are clastic sedimentary rocks. Crystalline rocks can make good aquifers if they are well-fractured.

The upper level of saturation of the rock (the upper surface of the groundwater) is the water table. The water table: tends to follow the topography; changes with the seasonal rainfall; produces a spring where it meets the ground surface.

The difference in height between two points on the water table is a measure of the hydraulic gradient. This provides the difference in hydrostatic pressure needed to make the water flow, in the direction of the gradient.

An aquifer which outcrops at the surface is called 'unconfined'. This is the area where the aquifer is replenished by rain. It also the area which can more easily be polluted by water-borne chemicals. An aquifer which dips below a cover of impermeable beds is called 'confined'. A confined aquifer can only be replenished from the unconfined section. Some confined aquifers are artesian - see "Water Supply".

Today, we rely on bore-holes to abstract water from aquifers. Previously, springs were used. Springs occur where the water table meets the ground surface, either because the water-table is high, or because underlying impermeable strata direct the water to the surface.

A big advantage of underground water is its high quality, being filtered as it passes through the small pore spaces of the aquifer. However, the water may pick up soluble mineral ions from the aquifer. These are listed on the label of your bottled water, and can add up to quite a significant amount, especially in limestone areas where 'hard water' can present a problem if it furs up your kettle and hot water pipes - the chemistry is similar to the formation of stalactites.

Porosity, permeability and underground water are also important in the accumulation of oil and gas reserves, in waste disposal and slope stability - see the other Study Topics.

The topic of underground water continues in the Study Topic "Water Supply".

Geopix Study Topic "Underground Water"

• explains : porosity, permeability, and includes experiments to 'try at home';
• explains : water table, hydraulic gradient, hydrostatic pressure;
• describes the various types of springs;
• describes the various types of aquifers;
• describes the origin and quality of ground water, including a section on hard water;
• includes a glossary of terms.

To order the "Applied Geology" Study Guide on CD, or the "Water Supply" Study Topic by download,
please visit the Geopix on-line store

The "Water Supply" Study Topic includes the following pages: