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Demarcation of High Mineralization Zones Identified with Respect to Aquifer Type using GIS in and around Vellar Basin, Tamil Nadu, India
3. Methodology
The following methodology was adopted in the present study,
- The base map of the study area was prepared using existing Map.
- Using GIS technique, the thematic maps have been prepared.
- Digitizing, Editing & labeling the various thematic maps, Integration and preparation of derived maps using GIS software (Arc/Info)
4 Study Areas
The area selected for such study to demonstrate the newer concepts of higher mineralization pockets is located near the east cost of Tamil nadu in and around velar basin.
The Present study area Velar basin, of Tamil Nadu state, India, in between Longitude 79º15’ to 79º50’ E and Latitude 11º05’ to 11º55’. Much importance is associated with this area because of its proximity to southeastern coast of India. Being a tropical area, rainfall distribution is uneven with an annual average rainfall of about 1200mm, received from both southwest and northeast monsoons, and has 37ºC and 20ºC as annual average maximum and minimum temperature respectively. The rivers that flow in the study area are Vellar and Ponniyar rivers.
5. Geology of the area
The study area is underlain by formations of Archean to recent age. Crystalline rocks are found on the Western side and Sedimentary formations of Cretaceous, Tertiary and Quaternary ages on the eastern side. The geological map of the area is shown in The Oldest formation is crystalline rocks of Archean age, which comprises charnockites, granite gneisses and pink granites. The Archean and Sedimentary formations are separated by a lineament along NNE – SSW direction. The sedimentary formation include Cretaceous and Tertiary deposits consisting of clay, marls, siltstones, alluviate layers of sand and clay (fossiliferous) and the Quaternary alluvium which covers the narrow stretch of coastal area consists of sand, coastal alluvium and backwater lagoon deposits. The Tertiary (Miocene and Pliocene) formations cover a major portion of the study area.
6 Base Data for the year 1990 (DR. Sukija Report)
It has been mentioned that Vellar Ground water basin is quite complex. The basis of selection of the sampling sites has revolved around the main objectives of the study. As one of the main objectives has been to establish the area extent of principal recharge area for the confined aquifer and groundwater flow rate. We have collected a large number of samples from areas, where it is hypothesized that the well might lie in the recharge area. Another important consideration has been given to the discharge area and flow in the upper and lower confined aquifers. Unfortunately, we have very few samples, which may represent the lower confined aquifer. A large number of samples were collected from the I and II mine areas for the observation of spatial-temporal variation. In order to study the aquifer interconnections, samples were collected from different wells tapping different aquifers. A few shallow well samples were also collected in order to compare the groundwater movement with that of deeper aquifers. For regional groundwater flow study, we have utilized the isotope and chemical data for the deeper aquifers, as that has been the main concern of our study. We also have a few collections from some wells, which have shown relatively high chloride waters.
Since, the groundwater samples were collected from different aquifers wherever possible, they are respectively labeled with their tapped aquifers and accordingly classified. Based on such classification the radiocarbon results are discussed as that of
- Phreatic aquifer group
- Semi confined aquifer group
- Upper confined aquifer group
- Lower confined aquifer group.
In many cases, such a distinct classification is difficult to make, because this classification assumes regional existence of such aquifers, which in reality is strictly not valid. We have opted for such a classification because of observed similar radiocarbon ages and chemical concentration patterns in different group of aquifers.
In this context, a uniform scale is very important. It has been decided to generate thematic maps on same scale. Accordingly, the same scales were used for preparation of base map. The basemap details for various themes should be uniform. Adequate ground features, as available on the topomaps were shown on the basemap for facilitation of quick ground references. There are 142 wells analyzed in different aquifer system in Vellar basin (1990). (Shown in fig)
7 Well Locations
Well Locations data were collected from existing NGRI, report. After that the latitude and longitude were marked (Point features) using arc/info software. Nearly 145 wells and have been maintained. The GIS Data base of 142 wells and hydro chemical data editing and labeling and generated using arc/info and arc view software’s.
In the present study, various GIS aquifer databases were generated for the following parameters.
7.1 Total Dissolved Soilds
Total Dissolved Solids mainly various kinds of minerals present in water. In natural water, dissolved solids are mainly composed of Carbonates, Bicarbonates, chlorides, sulphates, phosphates and Nitrates of ca, mg, Na, K, Fe, and Mn.
Concentration of Dissolved solid is an important parameter in drinking water. They given a particular taste to the water at higher concentration and reduce its possibility. Contour maps were prepared for Total Dissolved Solid for Vellar region.
7.2 Chlorides
Chloride, in the form of Chloride ion is one of the Major inorganic anions in water. Chlorides are present in all potable water supplies and in sewage, as a metallic salt. When sodium is present in drinking water, chloride concentrations in excess of 250 mg/l give a salty taste. If the chloride is present as calcium or magnesium salt, the taste detection level may be as high as 1000 mg/l chlorides.
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