Hydrogeomorphological studies in the Trichirappalli environs,Tamil Nadu, India using Remote Sensing technology
Paleochannel:
Paleochannels or old river courses, buried by subsequent sedimentation, are generally potential reservoirs for groundwater and hence useful for groundwater exploration. In addition to that, the paleochannel identification useful in flood mitigation. In the identification of paleochannel contrasting dark tone in a characteristics winding fashion in association with cropping pattern and linearly oriented vegetation.
Several paleochannel are found to be deltaic region of the Cauvery river delta filled with sands. Soils in this region more moisture content as compared to the surrounding areas due to the groundwater flow zones. The paleochannels are observed mainly in the south eastern and eastern parts of the study area especially deltaic region of the Cauvery River. This areas falls under the low-lying topography. The origin of the paleochannels may be due to the frequent changes in the river of Cauvery.
Groundwater prospects in buried channel are excellent because of hydrophilic nature of alluvial and colluvium deposits, retention of foot waters, location of contiguous to the river mainly responsible for recharge.
Lateritic uplands/ sandstone:
Lateritic uplands/Sandstone are developed over early Pleistocene to Recent age groups. The thickness varying from 10-15m and its characterized by moderate to good infiltration from rainfall and considerable water table fluctuations. Groundwater prospects in this unit are moderate to good in this zone because of the aquifer horizons sand. A number of laterite uplands are found to be northeastern, eastern and southeastern parts of the study areas.
Lineaments:
A lineament is defined as a large-scale linear feature, which expresses itself in terms of topography, which is in itself, an expression of the underlying structural features. From the groundwater point of view such features may include, valleys controlled by faulting and jointing, hill ranges and ridges, displacements and abrupt truncation of rocks, straight streams and right angles off setting of stream courses etc., (Merh et.al., 1989 and Ramesh, 1990). A lineament is a regional scale linear or curvilinear feature, pattern or changes in pattern that can be identified in a data set and attributed to a geologic formation or structure (Qureshy and Hinze,1989). The linear features can be measured and treated quantitatively like measurements of other geological properties, but it is necessary to use formal statistics that reflect the circular nature of the directional data (Cheeney, 1983; Gaile and Burt, 1980 and Gambel et al 1953; Mardia 1972). Lineaments are linear fractures commonly associated with dislocation and deformations. They provide the pathways for groundwater movement and are hydrogeologically very important Sankar et.al 1996. Lineaments are important in rocks where secondary permeability and porosity dominate and inter-granular characteristics combine in secondary openings influencing weathering, soil water and groundwater movements. The fracture zone forms an interlaced network of high transmissivity and serve as groundwater conduits in massive rocks in inter-fracture areas. The lineament intersection areas are considered as good groundwater potential zones. The combination of fractures and topographically low grounds can also serve as the best aquifers horizons (Karanth, 1989; Subba Rao, 1992). A numerous lineaments were marked in this study area. The lineaments are trending in the directions of northeast to southwest, north to south. A few of them east-west direction also. The distribution of lineaments is maximum in the western part where as minimum are found to be eastern part. The length of the lineaments a few km to several kilometres ( Fig.3).
Finally, Trichirappalli environs has been classified into three zones on the basis of their groundwater potential zones such as low, moderate and good after integrating well inventory, yield characteristics along with geological, geomorphological and hydrogeological informations (Fig.4).
Summary and conclusion:
Visual interpretation of IRS 1A Satellite imageries provided information related to the geology, geomorphology, lineaments interpretation was helpful in knowing the nature and water potentiality of different geomorphic units. The composition of materials of landforms, and there from inferred rechargebility and other hydrogeomorphic characteristics aid in identifying the groundwater potential zones.
Geologically, the area is occupied both hard and sedimentary formations. The hard rocks include hornblende biotite gneisses, granitic gneisses, charnockites, anorthosites, quarzites, and pegmatites. In the sedimentary formations comprised of sandstone, shale, clay, limestone, shell limestone and kankar etc. The majority of the fractures patterns northeast to southwest, southeast to northeast and east to west directions.
Hydrogeomorphologically, the investigated areas occupied by denudational hills, Residual hills, structural hills, Pediments, Pediplains, flood plains, buried pediments shallow, medium, deep and deltaic plians. By studying the hydrogeomorphological conditions of the area, it is possible to deciper the groundwater potentiality. Most part of the area moderate to good, while rest of the area buried pediments medium, pediplains having moderate groundwater potential zones. In this area dug wells are recommended. In the shallow pediments, pediments areas dug-cum-bore wells are recommended because, the water level will lower during pre-monsoon period. Bore well are recommended the lineaments and intersections areas. In addition to that electrical resistivity survey is required for to locate the exact location of good bore wells/tube wells sites in the areas.
In the investigated area has undergone successive tectonic activities and is traversed by a number of lineaments whose analyses from IRS 1A satellite imageries interpretation provides useful information about tectonics and the structure of the area.