The top or upper surface of the saturated zone of water that occurs underground, except where it is confined by an overlying impermeable layer. This surface separates the zone of aeration—the layer of soil or rock that is unsaturated most of the time—from the zone of saturation below. The water table is the level at which water rises in a well that is situated in a groundwater body but not confined by a layer of impervious material. In wetlands, the water table can be at the soil surface or a few centimeters below the soil surface, but in drylands it can be several hundred meters below the soil surface. Perched water tables occur where there is a shallow impervious layer (such as a clay layer) that prevents water from percolating downward to the more persistent, regional groundwater zone. As a result, water tables can occur at different depths within a region. Generally, if water tables are close to the soil surface, groundwater is susceptible to pollution from human activities.
