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The occurrence and distribution of groundwater are controlled primarily by the geology of the area and the quantum of recharge received by the existing aquifer formations. Thus the alluvial tracts of river valleys and the coastal plains containing alternative deposits of sand and clay of varying thickness are rich in groundwater. Regions underlain by hard rocks such as a large part of south India, are poor groundwater occurrence. In fact, from a compactness point of view, regions underlain by hard rock formations like basalt and granite are not expected to yield any significant quantity of groundwater. But nature has its way.

In many areas, the top portion of hard rocks develops extensive fractures and secondary porosity to a considerable depth due to weathering. The highly weathered granular hard rock derivative known locally as murrum functions as an excellent shallow water table aquifer. A deep sheeted intensive fracture system in granite, contact zones between two similar or dissimilar complex rock formations, sets of joints in sandstone, and solution channels in limestone are also known to contain and transmit a sufficient quantity of water under confined and semi-confined conditions.

Despite having such thick layers of sand formations, one might wonder why a place like Rajasthan is still poor in groundwater occurrence. The reason is that the recharge received from rainfall by these thick sand formations is never adequate to build up a water table. The entire infiltration gets disseminated and lost within the unsaturated zone before reaching an impervious layer at depth for building up a water table. However, shallow dug wells in sand formations underlain by hard rock or a hardpan at shallow depth yield a moderate quantity of groundwater depending upon the recharge received. Rajasthan and Gujarat also have some typical deep confined aquifers comprising sandstone, but with time the yield of these aquifers has reduced considerably due to over-extraction and lack of recharge.

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