Source Water Development - Public Water Systems - page 187

Fig. 8-14. Subsurface distribution of water
Such plants, called phreatophytes, grow without dependence upon the belt of soil
water.
Water that does pass through the belt of soil water enters the intermediate belt and
continues its movement downward by gravitational action. The intermediate belt holds
suspended water by molecular attraction and capillarity, the latter being the more
important of the two forces. The suspended water in this belt is dead storage because
it cannot be recovered for use. The thickness of the intermediate belt varies greatly,
and has a significant effect on the time it takes water to pass through this belt to
recharge the zone of saturation.
The capillary fringe lies immediately below the intermediate belt and above the
zone of saturation. It holds water above the zone of saturation by capillary force acting
against the force of gravity. The thickness and the amount of water held in the capillary
fringe depend on the grain size of the material.
The capillary fringe in silt and clay material is sometimes as much as 8 feet
(2.43 m) thick. In coarse sand or gravel, it may be a fraction of an inch.
Water in the zone of saturation is the only part of all subsurface water that is
properly referred to as groundwater. An exception to the above description of ground-
water is ancient seawater found entrapped in some sedimentary formations. Ground-
water of this origin is called connate water. Fresh water from precipitation percolating