Water flows downward through soil and bedrock because of the force of gravity. It continues in that direction until a depth of about 5 kilometers (3 miles) is reached, where porosity and permeability cease. The pore space above this level begins to fill progressively upward with groundwater.
The saturated zone. The rock and soil in which all the open spaces are filled with water is called the saturated (or saturation) zone. As the top of the saturated zone rises toward the surface, it reaches a level of equilibrium with the overlying unsaturated zone.
The unsaturated zone. The unsaturated zone (or zone of aeration) is the rock and sediment in which pore spaces contain mostly air and some water and therefore are not saturated. The unsaturated zone typically starts at the surface and extends downward to the saturated zone. The contact between the saturated and unsaturated zones is called the water table (Figure 1).
Figure 1
The Water Table
There is “room” for air in the unsaturated zone because the water is held to the sides of the soil particles through the force of surface tension. Surface tension also causes water to rise up into the unfilled pore spaces from below through a process called capillary action. The lower part of the unsaturated zone that draws water upward from the water table is called the capillary fringe, which is usually only a few feet thick.
Perched water tables. A perched water table (Figure 1) is an accumulation of groundwater that is above the water table in the unsaturated zone. The groundwater is usually trapped above an impermeable soil layer, such as clay, and actually forms a lens of saturated material in the unsaturated zone. A perched water table is generally insufficient to supply domestic groundwater needs, and often runs dry after being drilled. If the perched water table intersects a sloping surface, it may be manifested by springs or seeps along the line of intersection.
Migration of groundwater. The movement of groundwater is generally slow and ranges from 1 inch to a 1,000 feet per day. In addition to moving vertically downward for hundreds of feet, it also flows laterally, roughly parallel to the slope of the surface of the water table.
The slope of the water table is generally proportional to the slope of the overlying land surface: the steeper the topography, the steeper the slope of the water table. The steeper the slope of the water table, the faster the groundwater flows. The groundwater also moves more quickly in those sedimentary or rock formations that have a higher permeability relative to other formations.