The hard rocks constitute 73% of the total geographical area of the State, which mostly cover the western and central parts. These rocks contain mainly fracture which act as repository for groundwater. The fracture porosity is generally not uniform either laterally or with depth. It is fairly known that the hard rock aquifers are heterogeneous, as indicated by the variations in lithology, structure and texture within short distances. This phenomenon renders it difficult to generalise any uniform hydrogeological regime in hard rocks, and as such every individual area needs to be examined in depth independently, depending upon the hydrogeological set-up of each area. The occurrence and movement of ground water mainly depends on the degree of weathering, topography and interconnection of fracture zones.
The important hard rock aquifers noticed in the State are the weathered gneiss of Coimbatore, Dharmapuri, Salem and Ramanathapuram district, weathered and jointed Charnockite and Khondalite of Madurai, Theni, Tirunelveli, Vellore, Coimbatore and Kanniyakumari district, weathered and jointed ultramafic rocks of Nilgiri district and the cavernous limestone of Tirunelveli district. The hard rock aquifers occurring in south and south-eastern part of Tiruvallur and Kancheepuram districts, western-most part of Thanjavur district, northern and north-western part of Tiruchirapalli district and western part of Pudukkottai district are not highly productive.
Ground water occurs in the hard rocks normally under water table conditions in the weathered mantle and under semi-confined to confined conditions in the fractured rocks. The folded quartzite bands and solution cavities in the limestone of the Tirunelveli district also form potential aquifers. The available hydrogeological data on hard rock aquifers suggest that the depth of weathering varies from 5 to 30m.bgl and the depth of open wells varies between 6 and 50m while depth of borewells in general varies from 30 to 100m. Potential hard rock aquifers are located in Dharmapuri, Coimbatore, Madurai, Theni and Ramanathapuram districts. The yields of open wells range from 50 to 140m3/day in Dharmapuri district and from 10 to 256m3/day in Madurai and Theni district. The yields of borewells very from 13 to 363m3/day in Madurai and Theni districts and from 10 to 256m3/day in Ramanathapuram district. The yields of open wells range from 50 to 140m3/day in Dharmapuri district and from 10 to 256m3/day in Madurai and Theni districts. The yields of borewells vary from 13 to 363m3/day in Madurai and Theni districts and from 50 to 575m3/day in Ramanathapuram district. Similar aquifer zones are also expected in the fractured and jointed rocks of the remaining districts. In the water balance studies carried out by the CGWB in collaboration with Swedish International Development Agency (SIDA) in parts of Coimbatore district, the depth of borewells ranged from 25 to 305m and their yields range from 1.4 to 42m3/hour. Transmissivity of the weathered and partly fractured aquifers varied from 3.5 to 167.8m2/day whereas in limestones it has range of 86 to 114m2/day. Transmissivity of the gneiss in Noyil varied from 0.2 to 496.9m2/day and in Ponnani basin it varied from 2.06 to 61.75m2/day.
The unconsolidated formations are considered more prolific in comparison to hard crystalline formations. These includes formation ranging in age from U.Gondwana to Recent sediments, comprising Shales, Sandstones, Limestones, Laterites and Alluvium.
The Jurassic Formations, represented by the Upper Gondwanas, are encountered in Tiruvallur, Kancheepuram, Vellore, Tiruchchirappalli and Ramanathapuram districts. Due to the low transmissivity and the compact nature of these formations, they do not contribute much to the ground water resources of the State. The Tertiary formations are the most prolific aquifers and ground water in these formations is under great pressure and flowing wells have been constructed notably in Cuddalore and Villupuram districts, in the Cauvery delta of Thanjavur district, eastern part of Pudukkottai district and in the north-eastern part of Ramanathapuram district.
The Quaternary sediments in the State are represented by the laterite and older alluvium of Pleistocene age and the Recent alluvium, 'Teris' and coastal sands. Cauvery alluvium occupying a major part of the Thanjavur district includes potential alluvial aquifers. Other important alluvial aquifers occur in Tiruvallur, Kancheepuram, Vellore, Pudukkottai and Ramanathapuram districts. Ground water occurs both under water table and confined conditions in the alluvium.
Filterpoint and cavity wells in the alluvium have recorded yields in the range of 15 to 60m3/hour. Important alluvial aquifers occur between depth ranges of 7 and 68m b.g.l. The maximum discharge recorded from the tubewells constructed in alluvium is about 82m3/hour. The transmissivity of the aquifers is found up to 2000 m2/day. The thickness of these aquifers varies from place to place. The "Teri" deposits of Tirunelveli district have a thickness ranging from 5 to more than 30m. In Cuddalore, Villupuram and Ramanathapuram district, tube wells were constructed tapping all the possible granular zones of the soft rocks. The average transmissivity of the aquifer in this region is 550 m2/day with storage co-efficient of 2 x 10-4.
SUMMARY OF AQUIFER DETAILS