HYDROGEOLOGICAL STUDIES OF THE LOWER PARTS OF GHATAPRABHA BASIN, BELGAUM AND BIJAPUR DISTRICTS OF KARNATAKA, INDIA

Abstract

The lower part of the Ghataprabha (tributary of river Krishna) basin in Belgaum and Bijapur districts of Karnataka, India has been investigated for its hydrogeological aspects. Geologically, the area comprises of sedimentary rocks of Kaladgi Group of Precambrian age (1400 million years) rest ing unconformably over the phyllites, schists and granitic gneisses of the Dharwar Supergroup (2000 million years) which are in turn overlain by the Deccan Traps of Late Cretaceous age. Rocks of the Kaladgi Group form a basin which trends in WNW-ESE direction. The main rock types are sandstones, quartzites, shales, limestones and dolomites. Quartzites form ridges whereas limestones and shales occupy low lands. These rocks are folded into a large syncline the axis of which runs in a WNW-ESE direction. The carbonate rocks in the central part of the basin along the synclinal axis are intensely disturbed, karstified and cavernous and form potential aqui fers. The Deccan Traps overlying these sediments have two contrasting hydrogeological features - the weathered, vesicular fractured basalt forms better'aquifer as compared to the fine grained, poorly weathered, massive basalt. Five small drainage sub-basins representative of the area have been studied for their geomorphological parameters. Among the various morphometric parameters drainage density appears to be the most important one from the point of view of groundwater occurrence. The drainage density varies between 2 • ' - 1.63 and 2.1 km/ktn in limestone terrain and 0.80 and 1.92 km/km2 in basalts. ii The main water bearing formations in the area are limestones, shales, and weathered basalts. The limestones are permeable mainly because of their solution cavities and other karstic features. The occurrence of groundwater in shales is chiefly controlled by their calcareous material along cleavage planes, joints and weathered mantle. Weathered, vesicular and relatively fractured basalts form a good aquifer as compared to hard, compact, poorly weathered basalt. Stru ctural and geomorphological study using Landsat image has revealed that wells drilled on the synclinal axis and on the lineaments in the calcareous pediment zone have given better yields. Wells drilled on the resistant ridges along the basin boundary have given very poor yields. In the study area, large-diameter shallow wells are common which are used for domestic and irrigation purposes. The cross-sectional area of these wells varies from 4 to 145 m2 and the depth ranges from 5.4 to 35 m below ground level. The deepest wells are in shale. Monthly water levels monitored in 27 large-diameter observation wells for the period from 1973 to 1985 have been used to find out the relationship between rainfall and groundwater levels and to prepare groundwater level fluctuation maps*, ft *is seen that rainfall is one of the important factors that influences groundwater levels. In limestone area hydrographs of pre-monsoon water levels indicate continuous declin ing trend in recent years which may be due to overdraft iii conditions during the lean periods of rainfall. Groundwater fluctuation maps for normal rainfall years show different trends from those of drought years. During the years of normal rainfall, areas of higher fluctuation are seen in the south eastern lime stone terrain and north and north western basalt terrain. Excess ive exploitation of groundwater in the recent lean rainfall years has led to continual lowering of water levels in the limestone terrain as observed in the water level fluctuation maps Pumping test data both from exploratory bore holes and large-diameter wells have been analysed. Theis type curve method, Jacob's drawdown method, Theis recovery method and Chachadi method (1984) have been used to determine the aquifer parameters from the pumping test data on exploratory bore 2 holes. Transmissivity values range from 170 to 616 m /day in limestone, 146 m2/day in dolomite and 82 m2/day in basalt. Pumping test data from large-diameter wells have been analysed by the methods of Papadopulos and Cooper, Boulton and Streltsova, Rushton and Singh, Theis recovery and drawdown recovery method of Mishra and Chachadi (1985). It is found that transmissivity values obtained using the methods of Papadopulos and Cooper and Mishra and Chachadi are in comp arable range. Transmissivity values range from 53 to 109 m2/day in fine grained massive basalt, 106 to 289 m2/day in weathered vesicular basalt, 105 to 270 m2/day in fractured karstified limestone, 38 and 61 m2/day in shale, 121 m/day in sandstone and 163 m2/day in laterite. IV Frequency analysis of specific capacity index i.e. productivity values indicates that wells in limestones are more productive than those in basalts. Using the water balance concept, specific yield of the aquifers and groundwater recharge from rainfall have been estimated. The specific yield values determined for limestone are !•/., 4.3^ and 5.5^, and for basalt are 1.6/and 1.4*. The percentage rainfall recharge values in limestone are 6.2, 10.3 and 28 and in basalt are 12.0 and 9.8. Groundwaters from various rock types of the study area have been analysed to assess their quality for their suitab ility for drinking and irrigation purposes. The classification of waters according to Piper's diagram shows that most of the water from basalt and limestone are characterised by the dominance of calcium, magnesium and bicarbonate. A few samples falling in area 7 indicate primary salinity, i.e. chemical pro perties are dominated by alkalies and strong acids which may be due to water logged conditions. Groundwaters from shales show the dominance of sodium, potassium, sulphate and chloride. Not much variations are noticed in water quality during the span of thirteen years and also with depth. Groundwaters from limestones and basalts have been differentiated on the basis of their chemical characters using multivariate discriminant function analysis. Groundwaters, in general, are found to be quite suitable for drinking and irrigation purposes.