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The factors to be carefully studied before selecting a site for sinking a well are:
(i) Topography:
The valley regions are more favourable than the slopes and the top of hill rocks.
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(ii) Climate:
Maximum temperature, the annual rainfall region, intensity of sunlight and humidity are of considerable value. Semi-arid zones are more favourable than arid zones.
(iii) Vegetation:
(a) Phreatophytes:
Draw other water requirements directly from the zone of saturation.
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(b) Xerophytes:
Observing the moisture in the zone of aeration of store water in their thick fleshy leaves and stems.
Vegetation indicates large storage of ground water at shallow depths whereas bald hillocks with large number of xerophytes indicate the scarcity of ground water at shallow depths.
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(iv) Geology of the area:
(a) Areas comprising of thick soil or alluvium cover, highly weathered, fractured, jointed or sheared and porous rocks indicates good storage of ground water.
(b) Bald hillocks of massive igneous and metamorphic rocks or impermeable shales indicate paucity of ground water.
(v) Porosity, permeability and alteration of rocks:
Highly porous permeable and altered zones of dense rocks encourage storage of ground water.
(vi) Joints and faults in rocks:
The movement of water is through joints, fractures, fissures and cracks, which are interconnected.
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(a) Wells sunk into the highly jointed rocks or along the fault plane yielding copious supply of water.
(b) Faults in certain areas behave as barriers for the movement of ground water and create artesian conditions where flowing wells and springs are commonly noticed.
(vii) Folded strata:
(a) Synclines are favourable for storage of ground water in the previous layers and water is stored under pressure under artesian conditions.
(b) A well sunk in the anticlinal valley will be failure since it will be at the point of ‘Ground water divide’ and the water flow will be away from the crest of the anticline towards the synclinal basin.
(viii) Proximity of tank, rivers, etc.:
The wells sunk in these areas will yield water throughout the area.
(ix) Study of existing wells in the vicinity:
(a) The subsurface geology, rocks formations, depths, fractures, etc. can be observed in the existing wells in the neighbourhood.
(b) Observing the water levels can assess the depths of water table and the yield.
(c) Care should be taken to see that no dykes, veins or faults are situated in between the existing wells and the proposed wells.
In addition to the above factor, aerial photography’s and hydrogeological maps are helpful in making rapid reconnaissance of the area, where a large-scale well sinking programme is on hand.
Electrical resistivity method is found to be more helpful in the selection of well site. Subsurface exploration by test drilling and studying the various rock formations at different depths and their water bearing properties can be done by more sophisticated methods.
Spacing of wells:
If the wells are situated very closely, the supply water will be greatly affected, due to interference, when both the wells are pumped simultaneously.
This is roughly estimated to be around 200-300 m in alluvial areas and around 75-150 m in hard rock areas.