It is accumulation of water the surface of ground, caused by the portion of rainfall (about 60 percent) which percolates through soil pores or rock crevices, flow gravity till it reaches an impervious stratum, when it moves lateral direction to some point of escape appearing as springs well infiltration galleries.             The surface of the ground water is called ground water table is more generally as the water table.  The general characteristics of water table are: (a)   It usually follows the profile of the ground surface, sloping the same direction with little variations in slope (b)   The groundwater level is not static.  It fluctuates rising with the increase in percolation during wet season and falling with the repletion of storage during dry season. (c)    Where the groundwater meets streams or natural water sources, the level of ground and surface waters will coincide. (d)   The slope of the water table depends upon the quantity of ground water moving through the ground and resistance to flow.  Percolation results in an increase of the ground water storage, raising hereby the water level, lesser at the outlet than at some distance way from it.  The slope of the water table, consequently, is increased resulting in an increased rate of flow.  Conversely, depletion of the water storage during dry season lowers the water table, flattens the slope, decreasing the rate of flow, and in some cases, when the water level goes below the level of the stream bed, into which it normally discharges, the stream flow may stop. The Ground waters may be generally classified as (i) Springs (ii) Infiltration galleries (iii) Infiltration wells (iv) Shallow wells & Deep wells  1.4.1 Springs.  Springs may be regarded as outcrops of ground water, which often appear as small water holes at the foot of fall or long riverbanks.  Springs are generally of two types (1)   Gravity Spring (2)   Artesian Spring The gravity spring may result either from the out-cropping of the impervious stratum underneath the water bearing formation or from the overflow of water table by the continuous rise in the water table into the sides of the valley.  The yield of the gravity spring varies with the position of water table of rainfall and is therefore uncertain.             The artesian spring is one resulting from the water-bearing stratum being under pressure, underlain and overlain impervious strata.  Water flows to the surface through the weak spots in the upper impervious strata, either some fault or crevice the rock.  The yield of the artesian spring is more uniform and almost constant through the year. 1.4.2 Infiltration Galleries.  These are wells constructed in a horizontal position along the riverbanks or other seepage areas from where waster can be tapped across the line of flow. 1.4.3 Infiltration Wells.  These are in fact shallow wells constructed along the banks of rivers with the object of tapping water from them.  The water infiltrates through the bottom of these – wells and as it has to pass through sand beds on the way, it gets clarified.  The water thus collected into various infiltration wells flows by gravity into a sump well hence it is pumped for supply.  1.4.4 wells   A well is a vertical cylindrical opinion, which extends from the surface of the ground down into the water bearing formation.  The water bearing formation is termed as the aquifer. Classification. Wells may be classified into 3 general ways as described below Wells according to the Aquifer tapped.  This is general classification and broadly includes two types – (1) Shallow wells (2) Deep wells.             A shallow well is one, which is constructed by tapping the upper most water-bearing stratum.  If, however, deeper and more extensive aquifers have to be tapped in order to get larger and more reliable supplies of water then it is called a deep well.  A deep well is usually constructed by tapping a water-bearing stratum underneath an impervious stratum.             As compared to the deep well, a shallow well has the following drawbacks: (a)        Yield of the well i.e., the quantity of water available from the well per unit time is uncertain due to large variations in the ground water level throughout the year.  Against this, the yield of a deep well is large and uniform since there are no quick fluctuations in the water level. (b)        Quality of water is generally poor, since only the uppermost underground stratum is tapped, the well water may be open to the risk of contamination due to surface wash or close existence of septic tanks, privies etc., On the other hand, water from the deep well would be more reliable since much of the pollution would have been removed by the longer travel of water and the straining action provided by the intervening porous strata. Wells according to the condition of flow.  Accordingly, wells may be classified as (a) gravity wells and (b) pressure wells.  This classification is less general.       A gravity well is one is which the surface of the water in the aquifer outside of and surrounding the well is at atmospheric pressure.  The water flows under gravity into the well and rises to the height of saturated material surrounding it.  In a pressure well also called as an artesian well, the aquifer is confined or sandwiched between two impervious strata.  One above and the other below, so that the water flows under pressure into the well, the water in the aquifer being at a pressure greater than the atmospheric.  In some cases, the pressure may be so great that the water rises from the ground surface and flows from the well.  This is termed as a flowing well.             Wells according to the type of construction.  Accordingly, wells may be classified as (1) Dug well or percolation well (2) Driven well (3) Tube wells.

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