Water Table

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In common usage, the term water table expresses the surface dividing the unsaturated and saturated groundwater zones. More accurately, the water table lies within the saturated zone and separates the capillary fringe from the underlying phreatic zone. The phreatic zone is the area in which water will freely flow from pores in the geologic material. Within the capillary fringe, however, water is drawn upward from the phreatic zone by capillary action within the pores of the material. Smaller pores produce greater capillary force and cause the water to rise higher, resulting in a thicker capillary fringe. The pores in the capillary fringe are fully saturated, as are those in the phreatic zone. However, the capillary action causes the water in the pores to have a pressure that is lower than atmospheric pressure. Water is not able to flow out of these voids.

A more precise definition of the water table is the surface within the saturated zone along which the hydrostatic pressure is equal to the atmospheric pressure. Water below this surface has a pressure that is greater than atmospheric pressure while water in voids above is at a pressure that is less than atmospheric pressure.

The shape of the water table is controlled by a number of factors including the water-transmitting characteristics of the geologic medium, and the amount and location of groundwater recharge and discharge. The water table often reflects the surface topography of the area with a moderated relief. Because mountainous areas have greater precipitation, water infiltrating these areas recharges the water table and forms a mound. Groundwater discharges at streams, lakes, and wells cause the water table to dip toward these points. Between points of recharge and discharge, the water table tilts from areas of high potential (recharge) to areas of low potential (discharge).

The water table moves with changes in the hydrologic system. In periods of drought or heavy withdrawals, the water table will fall. During periods of precipitation, the water table moves upward.

A true water table exists only in unconfined aquifers, those where water can percolate directly through the overlying medium to the phreatic zone and the water table is free to move up and down. In confined aquifers, overlying rock or sediments with lower permeability prevent the water table from moving upward beyond the lower limit of the confining bed. The water in the confined aquifer is under pressure and the level to which it would rise, in the absence of the confining bed, is known as the potentiometric surface.

In some instances, a perched water table can form within the unsaturated zone, well above the regional water table. This occurs when a layer of low-permeability material, such as a clay lens, intercepts percolating water causing it to pool above the layer. A localized phreatic zone forms with the perched water table as its upper limit.

Hydrogeology; Porosity and Permeability; Springs