Groundwater of Hard Rock Aquifers of India

Abstract

About two-thirds geographical area of India is occupied by hard rocks comprising granite, gneiss, schist, quartzite, charnockite, khondalite, banded gneissic complex, basalt and intrusive, etc. Out of the twenty-nine States in the country, in twenty-two States the spread of hard rock areas varies from about 0.3 million km(2) to more than 5000 km(2), and a number of mega-cities are located in the hard rocks. The occurrence of aquifers in these rocks is heterogeneous, in near-surface weathered zone, in underlying saprolite, in discontinuous fractured zones and along joints, veins and litho-contacts. The abstraction of these aquifers depends on their availability, storage, yielding capacity and water quality. To tap the deeper yielding fractured zone aquifer and obtain higher yield, the borewells are sunk to 200 m depth. The yield from the top weathered zone aquifer within 20-30 m depth ranges from 0.9 to 1.8 cubic metre per hour (m(3)/h). The cumulative yield from the weathered zone and underlying saprolite may range up to 9-10.8 m(3)/h. The yielding fractured zones are mostly encountered within 100-150 m depth and could occur deeper also, as observed in Karnataka State in southern part of India, but the frequency of occurrence is meagre. Generally, the weathered zone, saprolite and the yielding fractured zones up to 60-100 m depth are tapped by hand pumps and shallow borewells. The borewell yield less than 3.6 m(3)/h is termed 'low'. In India, the groundwater investigation in hard rocks is carried out using surface geophysical surveys generally comprising resistivity sounding, profiling and imaging supported by satellite imageries and lineament maps. Heliborne electromagnetic and magnetic surveys have also been conducted. In several parts, a declining trend in groundwater level is observed, for which artificial recharge methods are adopted. The higher concentrations of fluoride in groundwater are observed in several parts.