The subsurface region below the water table where all pore spaces and fractures are completely filled with water. It is the zone from which wells draw groundwater.
The saturated zone, also called the phreatic zone, is the region below the Earth's surface where all voids in the soil, sediment, or rock are completely filled with water under positive hydrostatic pressure. Its upper boundary is the water table in unconfined aquifers or the base of the confining layer in confined aquifers. In the saturated zone, pore water pressure is greater than atmospheric pressure, and water can flow laterally toward discharge points such as streams, springs, and wells. The depth to the saturated zone varies from zero in wetlands and near water bodies to hundreds of meters in arid regions. The saturated zone is the primary target for groundwater extraction and is where most groundwater contamination assessment and remediation efforts focus. Water in the saturated zone moves according to Darcy's law, with flow directions determined by the hydraulic gradient. The boundary between the saturated zone and the overlying vadose (unsaturated) zone is not always sharp, as a capillary fringe of saturated material exists above the water table where water is held by surface tension but at less than atmospheric pressure.