Geological unit containing fresh Groundwater that can be extracted and used. It must have sufficient porosity to hold water and sufficient permeability to allow easy flow and release of water. Porosity is the space between grains of rock and in cracks and fissures. Aquifers occur on many scales, ranging from small and local units to 100s of square kilometres. Thickness ranges from a few metres to 100s of metres. A water table (or unconfined) aquifer lies just below the ground surface, and is vulnerable to pollution. A confined aquifer lies below an impermeable rock layer (such as clay or shale) which helps protect it from surface pollution.
Associated terms:
Aquitard: a layer of rock of low permeability but sufficient to allow important flows of groundwater to pass through it vertically between aquifers above and below. Permeability is too low to withdraw water directly from it.
Aquiclude: a layer of rock of very low permeability that does not allow water to pass through it, thus separating groundwater flow between upper and lower aquifers. The underlying aquifer is a confined aquifer.
Karst aquifer: In most aquifers, water is contained in minor pore spaces between grains of rock or in tiny fissures and moves very slowly (<1 meter/day). However, a karst aquifer contains open caves and water channels created through a geological history of dissolution. This is where there is where groundwater flow is closest to the concept of underground rivers and lakes. Water (and pollution) can move very rapidly just as in surface rivers.
