Groundwater

Groundwater resources of the world Source: BGR & UNESCO

Groundwater flows in the upper part of the Earth's crust and thus represents the underground and invisible part of the water cycle. It is formed mainly by the infiltration of precipitation into the ground, but a smaller proportion is produced by the seepage of surface water. Unlike surface water, groundwater does not flow along channels or sinks or, as is often mistakenly thought, along "water veins", but occurs everywhere in the saturated zone below the water table. There it generally flows at low velocity through porous media or through fractures and karst zones in hard rock environments, following the gradient to discharge streams or rivers.

Depending on the hydraulic conductivity of the media, an aquifer or an aquitard can be defined. Aquitards are mainly composed of fine and poorly conducting material such as silt and clay. There are three types of aquifers: porous, fractured and karstic. Porous aquifers are composed of sand or gravel and are the most productive aquifers. Fractured aquifers are hard rock aquifers where groundwater flows along fractures and fissures. They generally have a lower storage capacity than porous media. If the fractures become channels or cavities as a result of underground leaching processes, the aquifers are said to be karstified. This type of aquifer is typical of carbonate rocks such as limestone and dolomite, but sulphate and saline rocks can also karstify. Karst aquifers are very sensitive to pollution because of the general absence of protective soils, the high flow velocities and the short residence time of groundwater in the aquifer.

The DIN 4049 defines groundwater as the underground water that fills the openings in the earth's crust. Its movement is determined exclusively or almost exclusively by gravity and the frictional forces generated by its movement.

Groundwater is not only a natural resource but also an economic commodity. As such, it fulfils important economic functions: it is the most widely used raw material and an important source of drinking water. It is also an essential factor of production for industry and agriculture.