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Aquifer is made of two words ‘aqua’ and ‘ferre’ from Latin language. ‘Aqua’ means water and ‘ferre’ means ‘produce or bear’. Thus, aquifer is a geological composition which is porous and permeable. There is storage and transmission of water in it, as well as yielding of water to wells and springs in sufficient quantity.
Saturated part located below the surface of earth is called groundwater reservoir. From the geological point of view, it is homogeneous. Normally, a rock which is located in a certain condition, can be saturated if it possesses water qualities. Out of them, some rocks can become aquifers. Water reservoirs are considered as carrying the same meaning as aquifers. The permeable rock group or the layer through which water can enter underground and which can recharge wells or springs in sufficient quantity, is called ‘water bearing horizon’ or ‘water bearing layer’.
On such basis, any rock group or material which can supply water in sufficient quantity is called aquifer. Almost 90 per cent parts of all developed aquifers are unconsolidated rocks whose composition is from sand and gravel. Aquifers can be divided in four categories on the basis of water yielding capacity. First, ‘Water Course’, second, ‘Left Out’ or ‘Inside Valleys’, third, Plains, and fourth, Intermountain Valleys.
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The water route which forms the water channel, remains below the river bed. It is formed through alluvium of river. Wells located in most permeable rocks on river banks yield water in sufficient quantity because infiltration from the river maintains the quantity of groundwater. In internal valleys their formation streams remain absent.
Replenishing of giant reservoirs takes place in those areas where infiltration of rain water and reach of contemporary streams is up to lower levels of the earth. Volcanic rocks can also form permeable aquifers. Basalt lava flow is more permeable as compared to limestone.
Other permeable parts found in volcanic rocks are mainly permeable parts in flow breccias amidst lava layers, lava pipelines, fissures and joints are important. Big springs of the United States of America are mostly with basalt. Revolute has lesser permeability in comparison to basalt.
Types of Aquifers:
From the point of view of geological structure and expansion, aquifers are of different types. Sufficiently wide aquifers are also called underground water reservoirs. Water enters these reservoirs by natural or artificial recharging and again due to gravitational force it comes on the surface of the earth. It is drawn out through wells and it comes automatically through artesian wells.
From the geological angle, on the basis of existence or non-existence of water table, aquifers have been divided in following two categories:
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1. Unconfined Aquifers
2. Confined Aquifers
1. Unconfined Aquifers:
In such types of aquifers, groundwater level works like the upper layer of the zone of saturation. It is also called free or non-artesian groundwater. Wavy form and slope of groundwater is changeable, which depends on replenishing of groundwater, abandoned area and permeability.
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Fluctuation in the level of groundwater occurs according to changes in the stored groundwater in the space of the aquifer. On the basis of rise of water level in wells, a contour map of water level and sections can be prepared, with the help of which information about availability of water, and its distribution and transmission can be known.
One special state of unconfined aquifers is ‘perched aquifers’ which is also called the sorrowful state. In this situation, groundwater part is situated at some distance from the main aquifer. It has comparatively non-permeable layers and is separated by the aeration zone above the main groundwater unit. Such aquifers are mainly found in clay parts of sedimentation deposits, which are flat in shape. From wells located in perched aquifers, water can be available in small quantity and for short periods.
2. Confined Aquifers:
Confined aquifers are found at such places where pressure of groundwater is comparatively more than atmospheric pressure due to non-permeable layers. They are also called ‘artesian aquifers’. Such types of aquifers are found above the base of confined beds of water level in punctured wells.
Water enters confined aquifers from such areas where confined beds come up right up to the surface of the earth or disappear due to geological situations and aquifers become unconfined. The area supplying water to confined aquifers is called ‘recharge area’. In punctured wells of confined aquifers, fluctuation in water level is more due to changes m pressure than due to change in quantity of stored water.
Hence, there is hardly any change in storage water of confined aquifers and they mainly function as conduits for transmission of water from replenishing areas to natural or artificial final points. Pressure level of confined aquifer is imaginary, which is equal to the height of the hydrostatic level of the aquifer. Water level of a well boring confined aquifer shows its height at that point above piezometric surface. If pressure level is higher than the earth’s surface, it becomes a flow well.
Groundwater that percolates through the soil into the ground exists as free, confined and perched groundwater. The upper boundary of the zone of saturation determines whether the groundwater is free or confined. In fact it is this upper boundary of free groundwater which is known as the water table. C.F. Tolman defines this boundary as the “contact plane between free groundwater and the capillary fringe and this approximates to the level where the pore water pressure is equal to the atmospheric pressure”.
The water table definitely depends on the nature of the groundwater bearing material. In fine textured clay, there will be no clear cut ‘plane’ surface dividing the zone of saturation from the lowest layer of the capillary fringe. In the case of granite, the water table may vary considerably in height over short distances and also may be interrupted in places. While in the case of an open textured rock such as well-jointed limestone, the groundwater will move through the interstices to form a more horizontal surface.
A piezometric surface may be defined as the imaginary surface that coincides with the static level of the water in the aquifer. In the case of confined groundwater, the upper boundary of the water body is formed by an overlying confining stratum in which the spaces are so few and small that groundwater movement is extremely slow. The distinction between free and confined groundwater is made on the basis of hydraulic differences between the flow of groundwater under pressure of a confining layer and the flow of free unconfined groundwater.
Perched groundwater refers to a special case in which the confining layer is not continuous over a very large area and is situated at some height above the main confining zone. This confining zone is usually separated by open-textured strata in which the groundwater storage and movement can occur.