SALIENT FEATURES OF GROUND WATER YEAR BOOK OF TAMIL NADU

GROUND WATER YEAR BOOK OF TAMIL NADU STATE AND PONDICHERRY AND KARAIKAL REGIONS OF UNION TERRITORY OF PONDICHERRY
(2005-2006)

ABSTRACT

The behaviour of ground water levels during the ground water year 2005-2006 in Tamil Nadu State and Pondicherry and Karaikal regions of U.T. of Pondicherry has been studied by monitoring a set of dug wells and purpose-built piezometers during the month of May 2005, August 2005, November 2005 and January 2006. As of January 2006, Central Ground Water Board, South Eastern Coastal Region, monitors 540 dug wells and 346 tube wells/piezometers to study the ground water scenario of Tamil Nadu State and Pondicherry and Karaikal regions of U.T. of Pondicherry.

The present compilation relates to the year 2005-2006. It provides information pertaining to water levels and also chemical quality of phreatic aquifer. Thematic maps depicting the ground water scenario along with geochemical quality are furnished and discussed in this report. In addition, the fluctuations in water level and piezometric surface between different time frames have been analysed and presented. Various thematic maps presented reflect the effect of rainfall received during the period of study and the long-term behaviour of water level according to recharge and draft conditions obtained in various agro-climatic zones.

In general, it was found that the water level in a major part of the state showed a decline in respect to either the previous year or the mean of last 10 years.

The data on seasonal rainfall during the year are furnished to correlate the effect of the vagaries of rainfall on water level.

Results of chemical analysis of water samples collected during May 2005 for understanding the spatial variation of EC, Chloride, Fluoride and Nitrate concentration in phreatic aquifer of Tamil Nadu Pondicherry and Karaikal regions of U.T. of Pondicherry are also discussed.

The salient features are as under.

77% of wells have recorded depth to water level within 10 m bgl during premonsoon period (May 2005), whereas during post-monsoon period (January 2006), about 94% of wells recorded water levels less than 10 m bgl (Plate-I&II).
Depth to water level over major part of the State covering the coastal plain and uplands was within 10 m bgl in nearly 77% of wells analysed during premonsoon (May 2005), whereas it was so in 68% of the wells analysed during the previous year. The increase in number of wells showing depth to water within 10m bgl is due to good summer rains.
57 percentage of wells have recorded rise in water levels during May 2005 and 43 percentage of wells recorded fall in water level during the same period in previous year.
The depth to water level over major part of the State covering the coastal plain and central midland, was within 10 m bgl, in nearly 93.9% of wells analysed during post monsoon (Jan 2005).
Majority of wells (85%) have recorded rise in water levels and 15% of wells have recorded fall in water levels during January 2006 in comparison to January 2005. A general rise in water levels during January 2005 in comparison to January 2004 is evident due to Normal to excess seasonal rainfall recorded over most of the districts.
53 percentage of wells have recorded rise in water levels during May 2005 in comparison with respective decadal mean while the comparison of decadal mean with respect to May 2004 have shown that 40 % of wells have recorded rise in water level.
47 percentage of wells have recorded fall in water levels during May 2005 in comparison with respective decadal mean while the comparison of decadal mean with respect to May 2004 have shown that 60 % of wells have recorded fall in water level.
88% of wells have recorded rise in water levels during January 2006 with respect to respective decadal means when the comparison of decadal mean with Jan 2005 recorded rise in 48 % of wells.
12% of wells have recorded fall in water levels during January 2006 with respect to respective decadal means when decadal mean with January 2005 recorded a fall in 52 % of wells.
Electrical conductivity of about 15% samples analyzed is fresh water quality (< 750 ms/cm at 25oC). In about 55% of the Ground Water Monitoring wells the EC varies between 751 -2250ms/cm at 25o C and 15% of wells are between 2251-3000 ms/cm at 25o C indicating that the ground water is slightly mineralized and about 15% of wells the EC is more than 3000 ms/cm at 25oC indicating that the ground water is highly mineralised (Plate-III).
In the case of Chloride, 92% of wells analysed show Chloride concentration within the prescribed limit, noticed in major part of the State. Rest of the samples (8%), in which it exceeds permissible limits, are found in the parts of Chennai, Cuddalore, Pudukottai, Ramanathapuram, Nagapattinam, Thiruvarur, Tuticorin and a small patches in districts of Tirunelveli, Dindigul,Namakkal, and Coimbatore.
The Fluoride content is less than 1.5 mg/l in about 89% of the sample analyzed and 11% of the sample shows more than 1.5 mg/l which are from the districts of Pudukottai, Sivagangai, Virudhunagar, Ramanathapuram, Salem, Namakkal, Erode, and Coimbatore.
The Nitrate content is less than 45mg/l in about 63% of the sample analyzed and 37% is above 45mg/l which are from the district of Salem, Namakkal, Erode, Coimbatore Kancheepuram, Thiruvannamalai, Villupuram, Dindigul, and small patches in the districts of Cuddalore, Kanyakumari and dindigul.