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Thiruchirapalli district, located in the Central part of Tamil Nadu has a total population of 1504485 as per 1991 Census, more than 85 per cent of which resides in rural area. The district has been sub-divided into 8 taluks and 14 blocks for administrative convenience. Agriculture is the main occupation of the rural populace in the district and the net sown area accounts for about 42% of the total geographical area of the district. Paddy, jawar, choloam, groundnut, cotton and banana are the important crops grown in the district. Dug wells are the major sources of irrigation, followed by canals and tanks. The average annual rainfall over the district ranges from 730 to 900 mm, with the coefficient of variation ranging from 24 to 31%. Rains are received during both southwest (June to September) and Northeast (October to December) monsoon seasons. There is considerable spatial variability in the distribution of rainfall, which shows a gradual increase from west to south, east and north. Thiruchirapalli district falls under Cauvery river basin. All the rivers except Cauvery are ephemeral in nature. Prominent geomorphic units identified in the district from Satellite Imagery include alluvial plains, shallow and deep buried pediments, pediments, valley fills and structural hills. The district has one reservoir, 135 canals and 1767 tanks, which serve as surface water irrigation sources. Geologically, the district is underlain by formations ranging in age from Archaean to Recent. Crystalline rocks comprising Charnockites, gneisses and associated rocks occupy a major part of the district. Upper Gondwana and Cretaceous formations are found exposed in parts of Pullambadi block. Recent alluvial deposits are restricted to major drainage courses and foothill zones. The important aquifer systems in the district are constituted by weathered and fractured crystalline rocks. Ground water occurs under phreatic conditions in the weathered residuum and under semi-confined to confined conditions in deeper fracture zones. Recent alluvial deposits and semi-consolidated formations are found to form localised, discontinuous aquifers with low to moderate field potentials. Depth to water levels in observation wells of CGWB tapping the phreatic zone ranged from 1.38 to 17.60 m bgl during May 1999 and from 1.51 to 13.13 m bgl during January 2000. The shallowest water levels in pre and post monsoon period were observed in Andanallur and Musiri blocks. The elevation of water table in phreatic aquifers during January 2000 ranged from 64.67 to 230.88m above Mean Sea Level. The gradient of ground water table, in general, is toward Cauvery River. The fluctuation of water levels during January 2002 in comparison with water levels during May 2001 ranged from a decline of 0.58 m to a rise of 14.95 m. Rise in water levels were observed in about 92% of wells analysed, indicating recharge sufficient to compensate withdrawal in a major part of the district. Comparison of water levels with the average water levels for the previous decade indicate a rise in 90% and 81% wells analysed during pre and post monsoon periods respectively. The depth to piezometric surface in the district ranged from 0.60 to 20.85 m bgl during May 2001 and from 0.20 to 7.85 m bgl during January 2002. Elevation of piezometric surface during January 2000 ranged from 52.50 to 260 m above Mean Sea Level. The fluctuation in piezometric levels between May 2001 and January 2002 ranged from a decline of 2.0 to a rise of 16.45 m. Computation of the ground water resources available in the district based on 1984 methodology show that the total replenishable ground water resources available in the district is of the order of 91910 ha.m/yr. The net utilisable ground water resources available in the district for irrigation have been computed as 70311 ha.m/yr. The net ground water draft through various abstraction structures is of the order of 38463 ha.m/yr. The balance ground water available, computed as the difference between ground water resources available for irrigation and net ground water draft has been computed as 39661 ha.m. The level of groiund water development projected to year 5 and assuming an annual increase of 0.6% in ground water draft, ranges fr4om 26.22% in Thiruveambur block to 86.09% in Uppiliapuram block. Based on the level of ground water development, Uppiliapuram block has been categorised as “DARK” (85–100%), whereas in Thuraiyur and Tattyyangarpet blocks, the level of ground water development is between 65 and 85% and are hence categorised as “GREY”. The remaining blocks, where the level of ground water development is below 65%, has been categorised as “WHITE”. The status of irrigation poitential from groundwater varies considerably from block to block. Considerable potential is available for development in all the blocks of the district except Uppiliapuram, to be developed through suitable ground water abstraction structures. The balance irrigation potential available in the district can be fully developed by constructing 45135 ground water abstraction structures, each having an average command of 1.2 ha. The ground water in the phreatic zone, in general, is potable. Considerable spatial variation exists in the chemistry of ground water in these aquifers. A majority of the samples collected from phreatic zone may cause high to very high salinity hazard and medium to high alkali hazard when used for irrigation. Proper soil management strategies are to be adopted in these areas while using these waters for irrigation. Major ground water–related issues in the district include the depletion of ground water levels in localised patches due to over-exploitation. In order to ensure sustainable development of variable resources, further development of ground water has to be carried out in a judicious manner. Artificial recharge through rainwater harvesting also needs to be popularised in the district as a cost-effective method for arresting the decline of ground water levels and water quality deterioration. |