GROUND WATER RESOURCES AND DEVELOPMENT POTENTIAL OF
 THENI DISTRICT, TAMIL NADU

Theni district, having a total geographical area of 2889.23 sq. km. and a population of 10,49,323 as per 1991 census is an agrarian district with the net area sown constituting about 41.51% of the total geographical area.  About 67% of the total population resides in rural areas of the district.  Paddy, Cholam, Cumbu, Ragi, Groundnut, Sugarcane and pulses are the major crops grown.  Dug wells and bore wells are the major sources of irrigation in the district accounting for about 75.8% of the total area irrigated.  The district receives rainfall from both southwest (30.3%) and northeast monsoons (46.8%).

Theni district is drained by river Vaigai and its tributaries.  There are three major irrigation projects in the district, viz., Periyar, Manjalar and Vaigai and about 206 minor irrigation tanks in the district, of which, about 55 have ayacuts of 40 ha or more.  Various geomorphic units such as structural hills, buried pediments, shallow pediments, Bazada Zone, Valley fills have been identified in the district.

The district is underlain by crystalline rocks of Archaean age, which are overlain by valley fill sediments and minor alluvium of Recent age along the drainage courses.  Calc-gneissic, Garnetiferous–Cordiorite–Sillimanite gneisses, quartzites, charnockite, granites and granite gneisses traversed by pegmatites and quartz veins.  Valley fill sediments comprise sand, clay, silt and calcareous mud.  Recent alluvium consists of sands, gravels, pebbles, kankar and clays.  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 metre to about 45 m in the district.

Central Ground Water Board constructed 12 exploratory bore wells and seven observation wells in the district as part of exploration programme. The depth of the wells drilled in crystalline rocks ranged from 53.34 to 202.00 m bgl.  The potential fracture zones were encountered in the depth range of 6.00 m to 137.00 m bgl.  The yield of the wells ranged from less than 1.00 lps to 18.97 lps.

The important aquifer systems in the district are classified into (i) fissured, fractured and weathered crystalline formation, (ii) valley fill sediments.  Ground water occurs under phreatic to semi-confined in these aquifers.  Charnockites in the area generally have low-yielding capacity when compared to granites and granitic gneisses.  Potential water yielding fractures exist down to a maximum depth of 150 m bgl, where as in Cumbum–Cudalur tract it is within 100 m bgl.  The yield of the open wells tapping the weathered crystalline rocks and the unconsolidated formations (valley fill sediments) generally ranges from 150 to 350 lpm for a draw down ranges from 2 to 4 m.

In major part of the district the pre-monsoon water levels are in the range of 5 to 10 m bgl.  Deeper water levels of more than 10 m bgl are noticed in the southwestern part of the district.  The shallow water levels of less than 5 m bgl are noticed in the northeastern part of the district.  Analysis of water level fluctuation between May 1998 and January 1999 indicated a net rise in water levels throughout the district mostly in the range of 2–4 m.  Comparison of water levels of May 1998 with mean water levels of the last decade for the respective periods (Mean May 1988 – May 1997) indicates a rise in water levels in about 75% of the wells analysed.  Similar analysis of post-monsoon water levels indicated a rising trend in about 85% of the wells analysed.

Preliminary evaluation of ground water resources based GEC 1984 norms has indicated that the balance ground water resources of the order of 2.100 x 10³ ha. m./yr and having very little scope for further development in Uttamapalayam, Cumbum, Bodi, Andipatti and Periyakulam blocks.  Based on the level of ground water development projected for years, Theni and Chinnamanur blocks have been categorised as “OVER-EXPLOITED” whereas Uttamapalayam, Cumbum, Bodi, Andipatti and Periyakulam blocks are categorised as “GREY”.

The ground water in the district, in general, is potable except for localised patches, where one or more constituents are in excess of permissible limits.  The quality of ground water in deeper aquifer is generally good and all the constituents are within the permissible limits except at Kottur and Cumbum, where the concentration of EC and chloride is high. This may be due to the shearing and magmatisation of the formation.