The Geological Survey of India (1972) classified the country into various geological formations to their suitability as reservoirs of groundwater. The geo-hydrological maps of India show three broad classifications, namely:
(i) Hard rock area,
(ii) Alluvial terrain, and
(iii) Consolidated sediments.
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Over 70 per cent of the country’s geographical area of 3.29 million square kilometres is covered with hard rock formations. Groundwater is relatively scarce in such areas.
The alluvial surface of Indo-Gangetic plains forms the richest water bearing formations in the world. Consolidated sediment formations occur in some parts of Rajasthan and Gujarat in the Deccan Plateau where groundwater potential is limited. Alluvial formations in Peninsular India occur in narrow belts along the coast with varying quantity. Cauvery Delta forms the best aquifer in this zone.
However, it is noteworthy that recent investigations have revealed that hard rock of peninsular India also holds more groundwater than it was assumed earlier.