 |
 |
|
|
Thiruvannamalai district, having a total geographical area of 6312.05 sq.km, and a population of 20,42,979 as per 1991 census is an agrarian district with the net sown area constituting about 41% of the total geographical area. The district’s population is predominantly rural with about 88% of the total population residing in villages. Paddy. Maize, jawar, ragi, groundnut, sugarcane and pulses are the major crops grown. Dug wells are the major source of irrigation in the district, accounting for about 60% of the total area irrigated. The district receives rainfall from both southwest monsoon (43%) and northeast monsoon (42%) in almost equal measures. Thiruvannamalai district forms part of Palar and Ponnaiyar river basins and is drained by rivers Palar, Cheyyar and Ponnaiyar. Various geomorphic units such as structural hills, buried pediments, shallow pediments, Palaeo channels, bazada zones, flood plains and valley fills have been identified in the district. There are 5 important surface water irrigation schemes and about 1800 minor irrigation tanks in the district, of which about 600 have ayacuts of 40 ha or more. The district is underlain by geological formations ranging in age from Archaean to Recent. Charnockites, gneisses and granites traversed by quartz veins and pegmatites underlain the major part of the district. Upper Gondwana formations are exposed in the northeastern part of the district, whereas Recent alluvial deposits are seen along major drainage courses. Numerous lineaments have been identified from LANDSAT imagery, the most prominent ones of which are those oriented in NE-SW. The thickness of weathering ranges from less than a meter to about 10 m. Central Ground Water Board has constructed two exploratory wells in sedimentary rocks and 24 exploratory wells in crystalline rocks as part of its ground water exploration programme. In the Gondwana sediments, the granular zones in the depth range of 79.71 to 94.18 m.bgl yielded about 32 lps of water. The depth of wells drilled in crystalline rocks ranged from 34.60m to 200 mbgl. Potential fracture zones were encountered in the depth range of 5.60 m to 177.80 mbgl. The yield of wells ranged from less than 1 lps to 13.88 lps. Interpretation of the data collected from vertical electrical soundings in the district has indicated three or four geo-electric layer substrata. The important aquifers in the district are constituted by (i) Fissured, fractured and weathered crystalline rocks and (ii) porous formations comprising Recent alluvial deposits and Gondwana sediments. Ground water occurs under phreatic to semi-confined conditions in these aquifers. Charnockites in the district have been found to have low water-yielding capacity in comparison with granites and granitic gneisses. Productive fractures have been encountered down to a maximum depth of 176.0 mbgl in the crystalline rocks. Ground water occurrence in Gondwana sediments is generally restricted to shallow depths due to their compact nature and lack of joints. Quaternary alluvial deposits comprising alluvium and colluvium constitute prolific aquifers of limited thickness along the banks of major drainage courses. Pre-monsoon water levels (May 1998) are in the range of 5-10 mbgl whereas post monsoon (January 1999) levels are in the range of 2-5 m in the major part of the district. Analysis of water level fluctuation between May 1998 & January 1999 indicated rise in water levels throughout the district, mostly in the range of 2-4m. Comparison of water levels of May1998 with long-term water levels of May for the period 1989-1997 indicated a rise in water levels in about 60% of wells analysed. The ground water in the district, in general, is potable and suitable for irrigation and industrial applications except for localized areas. However, ground water in the shallow zone in the major part of the district is likely to cause high to very high salinity hazard and medium to high alkali hazard when used for irrigation and hence proper soil management strategies are adopted while using the ground water for irrigation. Preliminary evaluation resources on GEC 1984 norms has indicated that the district still has balance in ground water resources which is of the order of 74,228 ha.m/yr and considerable potential remains to be developed in Thellar, Polur, Pernamallur, Chetput, Kalasapakkam and Tiruvannamalsi blocks. Based on the level of ground water development at year 5, Chengam block has been categorized as “Dark”, whereas Vembakkam, Cheyyar, Anakkavur, Vandavasi, arani, Turinjapuram, Kilpennathur and Thandram ablocks have been categorised as “Grey”. Owing to the comparatively low level of ground water development in most of the blocks, shallow water levels and lack of major industries causing ground water pollution, the district is not highly vulnerable to perils of over-exploitation, water logging or ground water pollution. |