The behavior of ground water in the Indian sub-continent is highly complicated due to the occurrence of diversified geological formations with considerable lithological and chronological variations, complex tectonic framework, climatological dissimilarities and various hydrochemical conditions. Studies carried out over the years have revealed that aquifer groups in alluvial / soft rocks even transcend the surface basin boundaries. Broadly two groups of rock formations have been identified depending on characteristically different hydraulics of ground water, viz. Porous formations and Fissured formations.
1.1 Porous Formations :
Porous formations have been further subdivided into Unconsolidated and Semi – consolidated formations.
1.1.1 Unconsolidated Formations
The areas covered by alluvial sediments of river basins, coastal and deltaic tracts constitute the unconsolidated formations. These are by far the most significant ground water reservoirs for large scale and extensive development. The hydrogeological environment and ground water regime conditions in the Indo-Ganga-Brahmaputra basin indicate the existence of potential aquifers having enormous fresh ground water resources. Bestowed with high incidence of rainfall and covered by a thick pile of porous sediments, these ground water reservoirs get replenished every year and are being used heavily. In these areas, in addition to the annual replenishable ground water resources available in the zone of Water Level Fluctuation ( dynamic ground water resource), there exists a huge ground water reserve in the deeper passive recharge zone below the zone of fluctuation as well as in the deeper confined aquifers which is nearly unexplored. Although the mode of development of ground water is primarily through dug wells, dug cum borewell and cavity wells, thousands of tube wells have been constructed during last few decades.
1.1.2 Semi-Consolidated Formations
The semi-consolidated formations normally occur in narrow valleys or structurally faulted basins. The Gondwanas, Lathis, Tipams, Cuddalore sandstones and their equivalents are the most extensive productive aquifers. Under favourable situations, these formations give rise to free flowing wells. In select tracts of northeastern India, these water-bearing formations are quite productive. The Upper Gondwanas, which are generally arenaceous, constitute prolific aquifers.
1.2 Fissured Formations (Consolidated Formations)
The consolidated formations occupy almost two-third of the country. The consolidated formations, except vesicular volcanic rocks, have negligible primary porosity. From the hydrogeological point of view, fissured rocks are broadly classified into four types viz. Igneous and metamorphic rocks excluding volcanic and carbonate rocks, Volcanic rocks, Consolidated sedimentary rocks and Carbonate rocks.
1.2.1 Igneous and Metamorphic Rocks Excluding Volcanic and Carbonate Rocks
The most common rock types are granites, gneisses, charnockites, khondalites, quartzites, schists and associated phyllites, slates, etc. These rocks possess negligible primary porosity but develops secondary porosity and permeability due to fracturing and weathering. Ground water yield also depends on rock type and possibly on the grade of metamorphism.
1.2.2 Volcanic Rocks
The predominant types of the volcanic rocks are the basaltic lava flows of Deccan Plateau. The contrasting water bearing properties of different flow units controls ground water occurrence in Deccan Traps. The Deccan Traps have usually poor to moderate permeabilities depending on the presence of primary and secondary porespaces.
1.2.3 Consolidated Sedimentary Rocks excluding Carbonate rocks
Consolidated sedimentary rocks occur in Cuddapahs, Vindhyans and their equivalents. The formations consist of conglomerates, sandstones, shales, slates and quartzites. The presence of bedding planes, joints, contact zones and fractures control the ground water occurrence, movement and yield potential.
1.2.4 Carbonate Rocks
Limestones in the Cuddapah, Vindhyan and Bijawar group of rocks are the important carbonate rocks other than the marbles and dolomites. 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.
1.3 Hydrogeological Units and their Potentials in India:
The distribution of hydrogeological units in the country is given in Table I.
Table I. Distribution of Hydrogeological Units in the Country and their Potential