The rate at which water infiltrates into the ground depends on the permeability of the rocks and the state of the ground surface. Below the ground surface there is an unsaturated zone which has air in the pore spaces, and a saturated zone which has all the pores filled with water. The water table is the boundary between the unsaturated zone and the saturated zone, and is the level at which water stands in wells. Water below the water table is called groundwater. The water table follows the topography of the ground surface but with more gentle gradients.
Groundwater will flow in response to differences in elevation and pressure. Darcy's law relates the rate of the groundwater movement (Q) to the hydraulic conductivity (K), the cross-sectional area (A) and to the hydraulic gradient or slope of the water table (h/l):The hydraulic conductivity depends on the permeability of the rock and on the properties of the water. Water generally flows in the direction of the hydraulic gradient and slope of the water table.
A cone of depression is formed in the water level around a well from which water is being pumped. The difference in height between the water table before pumping and the water level in the well during pumping is called the drawdown.
There is usually saline groundwater under the land at a coast, with a wedge of denser saline groundwater under the fresh groundwater. The depth to the saline groundwater depends on the height of the water table above sea level and on the densities of the fresh and saline water.
The porosity of a rock is the proportion of its volume that consists of pores:Porosity is a measure of how much water a rock can store. The permeability of a rock is a measure of the properties of the rock which determine how easily water can flow through it. The porosity and permeability are generally greater in unconsolidated sedimentary rocks, particularly sands and gravels, than in consolidated sedimentary, igneous or metamorphic rocks. Both porosity and permeability can be increased by processes that occur after the formation of the rock, such as solution or fracturing. These are called secondary porosity and secondary permeability.
An aquifer is a body of rock that can store water, and through which water can flow. For a rock to be an aquifer it must be sufficiently porous and it must be permeable. Igneous and metamorphic rocks seldom make good aquifers unless they have both secondary porosity and secondary permeability.
The proportion of water that can be recovered from a saturated aquifer is known as the specific yield. This is less than the total amount of water stored in the aquifer (represented by the porosity) because some of the water is retained by surface tension around the individual grains (specific retention). Specific yield, like porosity, is expressed as a percentage of the total volume of the rock. The highest porosities are found in fine-grained sediments, but the greatest specific yields are in medium-grained sediments. The exploitable storage of a saturated aquifer is the volume of water it will give up when pumped or allowed to drain.
Aquifers can be unconfined or confined. Unconfined aquifers crop out at the ground surface; water normally has to be pumped to the surface from the water table in these aquifers. Confined aquifers are separated from the ground surface by an impermeable layer. Water in confined aquifers is called artesian water, and wells that penetrate confined aquifers are called artesian wells. The water in an artesian well may be under sufficient pressure to reach the surface of the ground without pumping (a flowing artesian well).
The potentiometric surface is an imaginary surface joining the heights to which water will rise. For an unconfined aquifer, the potentiometric surface is the water table.
The safe yield of an aquifer is the maximum rate of extraction of water that does not produce a long-term decline in the average water table level or have any other adverse effect, such as a significant reduction in the flow to springs and rivers. Exceeding the safe yield (i.e. 'mining' groundwater) would necessitate pumping from progressively greater depths to obtain water, and might lead to a reduced flow to springs and rivers, and a deterioration in water quality.