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Aquifers in India
An aquifer is a subsurface layer of permeable formation that can store as well as yield groundwater economically to the well tapping it.
In India, broadly two groups of rock formations, viz. porous formations and fissured formations, have been identified as aquifers, depending on their characteristically different hydraulics of ground water (CGWB, 2010, Ground Water Scenario of India 2009-2010). Porous formations have primary porosity from the time of deposition, while fissured formations have secondary porosity developed through to various geological and tectonic processes.
Porous formations comprise unconsolidated and semi – consolidated formations:
Unconsolidated formations consist of alluvial sediments of river basins, coastal and deltaic tracts. These are by far the most significant ground water reservoirs for large scale development in the high rainfall and recharge areas. They are not prolific in desert conditions with less rainfall recharge. The mode of development of ground water is primarily through dug wells, dug cum bore wells, tube wells, and cavity wells. Thousands of tube wells have been constructed during the last few decades. With regard to groundwater potential, there are aquifers having enormous fresh ground water reserve down to 600m depth in the Indo-Ganga-Brahmaputra basin. This ground water reservoir gets replenished every year and is being used heavily. In coastal areas there are reasonably extensive aquifers but these are at a high risk of saline water intrusion.
Semi-consolidated formations normally occur in narrow valleys or structurally faulted basins. The Gondwanas, Lathis, Tipams, Rajahmundry, Cuddalore Sandstones and their equivalents are the most extensive productive aquifers in this category. Under favorable situations, these formations give rise to free flowing wells. In selected tracts of northeastern India, these water-bearing formations are quite productive.
The fissured formations occupy almost two-thirds of the country. These are consolidated formations. Except for vesicular volcanic rocks, they have negligible primary porosity. From the hydrogeological point of view, fissured rocks are broadly classified into four types:
Igneous and metamorphic rocks, excluding volcanic and carbonate rocks: The most common rock types under this category are granites, gneisses, charnockites, khondalites, quartzites, schists, etc. These rocks possess negligible primary porosity but attain porosity and permeability due to fracturing and weathering. Ground water yield also depends on the rock type and grade of metamorphism.
Volcanic rocks: The predominant types of volcanic rocks are the basaltic lava flows of Deccan Trap Plateau. The highly variable water bearing properties of the different flow units control ground water occurrence in Deccan Traps. The Deccan Traps have usually poor to moderate permeability depending on the presence of primary and secondary fractures.
Consolidated sedimentary rocks: The formations consist of conglomerates, sandstones, slates, phyllites, and shales. The presence of bedding planes, joints, contact zones and fractures controls the ground water occurrence, movement and yield potential.
Carbonate rocks: In carbonate rocks, the circulation of water creates solution cavities thereby increasing the permeability of the aquifers. The solution activity leads to widely contrasting permeabilities within short distances.
For consolidated and semi-consolidated formations, the availability of groundwater depends on secondary porosity developed due to weathering, fracturing etc. In these formations, there is scope for groundwater availability at shallow depths (20-40 m) in the weathered zones as well as in fractures at depths down to 400 m. The weathered zone forms the most prolific aquifers.
The map below shows the distribution of consolidated and unconsolidated/ semi-consolidated formations in India with the groundwater yielding potentiality of the aquifers.
Source: CGWB, 2002