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Abstract
The city of Guwahati with a total municipal area of over 313 square km, has witnessed a rapid growth in population particularly during the last one and half decades. The population jumped from 1,23,783 in 1971 to 5,77,591 in 1991 and as per census 2001, the figure stands at 8,14,575. As a result, there is tremendous pressure on the natural resources like groundwater. Although the mighty river Brahmaputra flows through the northern periphery of the city, water supply from the river to the city inhabitants is grossly inadequate at present. In most of the cases people heavily depend on the dugwells and deepwells. However, it is found that due to the geological factors aquifers with adequate yield are not well developed and are mostly laterally discontinuous. The problem of depleted ground water table is acutely felt in the new growth centres like Beltala, Panjabari, Motoria etc. The high growth of built up areas and filling up of natural channelways have adversely affected the recharge of the ground water regime compounding the problem. In this backdrop, it is of vital importance to identify the groundwater potential zones of the city area taking into account of the geological and anthropogenic factors. Geologically, the city is characterized by mostly Precambrian granite gneiss and quartzites forming residual hills and occupying a major part of the landscape. Small elongated intermontane valleys with various thickness of sediment fill and alluvium form the rest of the areas. Study of satellite data supplemented by field check shows that a characteristic feature of the terrain is the presence of a number of palaeochannels which are perceived to be old channelways linked to the river Brahmaputra towards north. Using Analytic Hierarchy Principle (Saaty, 1980), a paired comparison matrix was prepared for the four criteria (geology, soil, slope and geomorphology) for Guwahati city. A set of four such matrices were also prepared for the classes in each of these four criteria. A programme was written in C++ to calculate the weights of the various criteria in each matrix using eigenvector method. The computed values show acceptable levels of consistencies. The consistency ratio (CR) for all the four thematic layers is found to be 0.00571 while for the individual criterion the CR values are: geomorphology (0.0039), Soil (0.00528), slope (0.0176 ) and geology (0) which are below the maximum acceptable level of 0.1. The result of this analysis was fed into SPANS 7.2 to determine the groundwater potential zones. The analysis shows that the palaeochannels and alluvium with high soil thickness are the potential zones for ground water in the city area.