Groundwater

The water contained in a permeable fraction of the earth’s totally saturated crust, as a result of the infiltration of the water into the smallest cracks of the substratum and its accumulation above an impermeable layer. These bodies only form true underground rivers in karstic areas.

Groundwater corresponding to water that has infiltrated into the ground, moving around in the permeable rocks of the substratum, forms “reserves”. Different sorts of bodies can be distinguished using various criteria. These criteria may be:

• Geological:
  • Alluvial bodies (a porous strata close to the surface).
  • Bodies in fractured geological strata (limestone or volcanic)
  • Bodies in karstic strata (limestone)
  • Bodies in porous environments (sandstone, sand).
• Hydrodynamic:
  • Alluvial bodies, volumes of groundwater contained in alluvial areas, generally unconfined and connected to a waterway.
  • Unconfined bodies, volumes of groundwater with an unconfined surface, i.e. subject to atmospheric pressure. An unconfined body is included in an aquifer which incorporates an unsaturated zone above the saturated zone.
  • Confined bodies, a volume of groundwater isolated from the surface of the ground by an impermeable geological formation, subject to higher pressure than atmospheric pressure. Their piezometric surface is greater than the roof of the aquifer that contains them. Water, which is pressurised and moves very slowly around the body, is protected from potential surface pollution if there is no communication with the surface or other bodies (either naturally due to faults or caused by boreholes).
• Regulatory (France’s Water Law): interlinked bodies, linked to a waterway.

The same body may have an unconfined and a confined section.