OCCURRENCE AND MOVEMENT OF GROUNDWATER

Once water is introduced to the earth as rainfall or snow-melt it exists in the ground in several different environments. Broadly these environmental zones can be classified into two groups viz. a zone of 'Vadose Water' ( or 'unsaturated zone', where all the soil / rock interstices are partly filled with water) and a zone of 'Phreatic Water' ( or 'zone of saturation', where all the soil / rock interstices are fully filled and saturated with water). These two zones are separated by the groundwater table. See fig. 3.

In Vadose Zone, three types of water exist. In vertical downward sequence , they are Soil water, Intermediate Vadose and Capillary water. Soil water is the component available to plants and hence subjected most to transpiration and evaporation losses. The depth of Soil water component varies from 0.9 m to 9.1 m.. The Intermediate Vadose component lies in between Soil Water and Capillary water. At the bottom of Intermediate Vadose component, Capillary water exists which is formed due to capillary rise in the medium of Vadose zone coming in contact with water table. The thickness of the Vadose zone and its component mostly depend upon the soil type and geography of the area. Water, below the water table is generally called groundwater. The bottom of groundwater zone is nearly impossible to delineate because it grades almost imperceptibly into a region where openings in the rock, are more and more isolated. Groundwater in unsaturated zone moves downward under the influence of gravity, whereas, in the saturated zone it moves in a direction determined by the surrounding hydraulic gradient.. The groundwater zone may be imagined as a huge natural reservoir or system of reservoirs in soil or rocks whose capacity is the total volume of pores or openings that are filled with water.

In terms of the holding (porosity) and transmitting (permeability) capacity of groundwater the host medium or geologic unit can be uniquely identified as,

· AQUIFER -which can hold and also transmit significant quantities of water under ordinary hydraulic conditions;

· AQUITARD -which can hold and also transmit water but their permeability is not sufficient to allow the completion of    production well

· AQUICLUDE -which can hold but can not transmit

· AQUIFUGE - which can neither hold nor transmit water.