TECTONIC MODEL OF THE CHINNATADAGAM BASIN

The Chinnatadagam basin is a small natural drainage basin in the northwestern comer of the project area. The basin is surrounded by hill ranges on all sides except towards the east and it has a well defined watershed boundary. The drainage is from west to east.

The northern hill ranges are made up of chamockite, while the hill range on the southern side of the valley consists of granite. The central part of the basin is underlain by well foliated hornblende-biotite gneiss. Outwash material de- rived from the hill ranges has been deposited in the central part of the basin in the form of colluvial deposits, the thick- ness of which exceeds 80 m in the centre of the valley.

Outcrops of an umetamorphosed dolerite dike of about 30 m width, trending E-W, are found in the western part of the valley. This dike appears to be continuous in the east, below the colluvial cover. Seismic and resistivity surveys carried out in the basin indicate the existence of deep, E-W trending channels, cutting the basement rock in the central parts of the valley.

A prominent fracture zone with an ENE-WSW trend was located by interpretation of air-photos and satellite imagery in the southwestern part of the basin, close to the contact zone between granite and gneiss. At this locality the rocks are highly crushed with well developed slickensides indicating shearing. In the northern chamockite hill ranges and, to a less extent, in the granite hills in the south, well developed overthrust planes, dipping in moderate angles towards east and west were noticed.

Based on this field evidence, a tectonic model of the Chinnatadagam basin was constructed (Figure 4.4.6).

The basement rocks in the area have been subjected to tectonic compression in an E-W direction after they have attained solid state by regional metamorphism. The compression has produced an E-W trending set of parallel open

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Figure 4.4.6 Tectonic model of Chinnatadagam basin.

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tensile fractures, one of which is filled by the dolerite dike. The same compression has also produced the ENE-WSW shear fracture as well as the overthrusts.

Three of the exploratory wells drilled in the area are located in one of the E-W trending open tensile fracture zones.

All these wells, which tap mainly the fracture zones occurring below the colluvial cover, have given high yields in- dicating the openness of the fractures. When water was pumped from well no. 3 during drilling operations, the water level in the open well no. 1, at a distance of about 3 km, showed appreciable fluctuations, indicating an hydraulic con- nection along the open tensile fracture. This means that the open tensile fractures act as subterranean channels in the basement rocks, storing and transmitting large quantities of water.

4.4.6 GEOPHYSICS

The surface geophysical methods employed, included seismic. resistivity and magnetic methods, while the geophysical logging comprised electrical, radiation and caliper logging. The resistivity surveys were used on a larger scale than the seismic surveys. AC Aquameter and DC Terrameter were used in these surveys.

4.4.7 HYDROCHEMICAL INVESTIGATIONS

To determine the suitability of ground water for domestic, agricultural and industrial use, to locate areas of probable contamination, and to study the genetic character of ground water in the project area, water samples were collected twice a year from 260 observation wells, and from 190 wells in the detailed study basins. Water samples were also col- lected during drilling and pumping tests. In addition, surface runoff samples and samples of rain water and atmospheric dust were taken.

The results of the analysis of the pre- and post-monsoon samples show that the mineralisation of the water generally does not show wide variation. However, variation in the individual ions can be noticed. In general, ground water in the

CEIITRAL SRO”Y0 WATER BOAR0

SIDA ASSISTED GROUND WATER PROJECT

IN TAMIL NADU AND KERALA STATES

HYDROCHEMICAL MAP

INDEX

TYPE OF WATER QUALITY HAZARD

Figure 4.4.7 Hydrochemical map of project area.

Noyil and the Vattamalaikarai basins has a nitrate content in excess of the permissible limit of 50 ppm and also a higher

Some of the water samples have moderate hardness and moderate chloride content. Sulphate and nitrate concentrations are usually low. The water appears to be of bicarbonate and/or chloride type. in rain water, four strategic stations coinciding with the four meteorology stations were established in the area. Periodic collection of samples of atmospheric dust and rainwater was made for chemical analysis to determine the sodium and chloride content.

Chemical analysis of water samples from certain areas have shown a fluoride content above the permissible limits for drinking water (Figure 4.4.7). Laboratory studies have shown that the high fluoride content in some areas is genetically related to the occurrence of soluble fluoride present in the form of the calcareous precipitate called “kankar.” Such occurrence seems to be connected with slightly alkaline soils.

An experiment was carried out to study the effect on fluoride content of gypsum powder sprinkled around two will compensate the water pressure. Various materials such as clay, tarred felt or polythene sheets can be used besides brick and concrete depending on the local conditions and availability. The underground structure need not project above the surface but can be finished at a depth of 1 m below land surface.

1. A methodology was evolved for ground-water exploration in hard rock areas by integrating photogeological, geo- logical, hydrogeological and geophysical surveys. The selection of the exploratory-well sites was based on the metho- dology developed. Out of 53 exploratory wells drilled, 40 wells yield more than 10 m3/h and the specific capacity ranges from 2.83 - 1 086 1 pm/m drawdown.

2. Various parameters of the water balance were estimated using data from a network of observation stations.

3. An analysis of water level data from a network of observation stations has indicated that except for a small area in

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indicates that the ground-water reservoir gets fully saturated at the onset of the monsoon, allowing most of the preci- pitation to leave the area as surface runoff. Such areas have high potential for further ground-water development.

5. Fractured rock aquifers were located at depths of more than 150 to 200 m in hard rock areas. The geometry of the aquifers and the ground-water flow system in them were defined.

6. An experiment of arrest of ground-water runoff by constructing a sub-surface dike was carried out.

7, Areas with poor ground-water quality and hence of limited suitability for domestic and agricultural use were identi- fied. Experiments on the causes of high salinity and excess of fluoride were carried’ out and remedial measures for arresting deterioration of the chemical quality of the ground water were suggested.

REFERENCES

The complete list of references used in this case history is given in:

Baweja, B. K.; Raju, K. C. B. 1680. A case history of ground water potential of hard rock areas in Noyil, Amaravati and Ponnani Basins, South India. Report to the Central Ground Water Board, New Delhi.

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4.5 GROUND-WATER INVESTIGATIONS IN A GRANITE AREA OF SARDINIA, ITALY