Geohydrological Studies of Mulki River Basin Karnataka, India

Abstract

Water being the vital natural resource of concern, the sustainable development of the same is of much importance at present. A river basin or a watershed is a clearly defined basic unit to understand the water resources considering groundwater and surface water as a single resource for integrated development in a given terrain. The complex relationship that exists among soils, geology, land forms, rainfall, vegetation, land use practices and the condition of water resources is better understood and managed in a river basin or watershed. Characterizing and deriving the sustainable developmental plan for watershed, demands accurate information pertaining to its land use, soil, geology, geomorphology, meteorology, hydrogeology, quality of water, etc., in spatial domain. The erstwhile Dakshina Kannada district being blessed with many watersheds and heavy rainfall needs special attention in this regard thanks to its fast development in the coastal part of India. Mulki, a typical midland draining river basin in this coastal part of Karnataka in India has been studied and analyzed in order to understand the various aspects of its groundwater resources for the sustainable development and management. To attain this, an integration of various field and laboratory data of current and historical interest have been carried out in a spatial domain with different thematic maps derived from Survey of India (SOI) Toposheets and imageries of Indian Remote Sensing (IRS) Satellites. They have been visually and digitally processed and analyzed using geoinformatic technologies and statistical analysis. The geomorphologic studies and morphometric analysis revealed that the Mulki river having a drainage area of about 350 sq. km. is a 6th order, very coarse textured, rectilinear, dendritic, midland draining basin of matured topography with high discharge capacity and meager groundwater potential. Mulki River Basin has elbow bends in its midland area other than an offset of the NE-SW trending land mark structural ridge and WNW-ESE trending lineaments indicating structural deformation into its major course. Morphometric analysis and geomorphologic evidences such as pediplain, structural ridge, lateritic plateaus (mesas) of varying geological origin and time, waterfall, high sinuosity, cross bedding, meandering course and elbow bends inAbstract x Geohydrological Studies of Mulki River Basin, Karnataka, India. Ph.D. Thesis-2013 Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal the middle stretches, river terraces, water pooling, etc., suggest neotectonic activity in this area which may be responsible for the groundwater storage and movement. The geology of the study area depicts rocks of Archaean to the Recent age, exposing fifteen lithological units representing Sargur group and Peninsular Gneissic complex of Archaean age, South Kanara Granite batholith, younger intrusive basic dykes and acid veins of Palaeoproterozoic age, laterites of Cenozoic age and coastal sands of Quaternary period and the Recent sediments of the alluvial deposits. But predominantly the rocks like Granite/Granitic gneisses with occasional laterite capping and unconsolidated river and marine sediments, occupy the area. Basic intrusives like dolerite and gabbro, and acidic intrusives like pegmatite and quartz veins are found at many places shaping the hillocks, ridges and mounds in the terrain. Many sets of joints are parallel to the river course in the ENE-WSW directions other than NE-SW, N-S, E-W and NW-SE which influence or guide the groundwater potential of the area. The NNW-SSE trending dolerite dykes are numerous and relatively abundant in granite. Foliations and joints in rocks dip either north or northeast directions favouring groundwater storage and movement in the upstream directions. Geology of the area and lineament studies confirm the earlier report of three active faults in the area which are responsible for the offset of the river and recent neotectonic activity in this area. There are some lineaments in the basin extending up to the Western Ghats which will be highly potential for groundwater explorations. NE-SW trending structural ridge of intrusive rock (mainly gabbroic) forms a major lineament in the eastern part of the basin. There are about seven genetic types of soils spread over the basin, broadly divided into four types depending on geohydrological condition as alluvial soils, loamy soils, gravelly clayey skeletal soil and lateritic soils of different encrustations. The soils and geological structures such as lineaments and joints in the otherwise hard and impervious rocks have an influence on the geohydrological conditions of this study area. Water quality, being an important criteria for deciding the suitability for drinking and irrigation purposes, about 21 important physico-chemical water quality parameters (of groundwater and surface water) have been analyzed and seven irrigation water quality parameters have been computed for 154 randomly selected water samples collected during pre-monsoon seasons of 2008 and 2009; and 95 samples collected during postmonsoon period of 2009 to understand the utility values of the same and its spatial variation. Statistical analysis of the above parameters and a comparison with acceptable drinking water quality standards revealed that most of the water samples collected during pre- and post-monsoon periods found to have quality parameters well within permissible limits. However, the spatial variation maps of vulnerable parameters and vulnerability map of water quality has been prepared, and it has been found that certain parameters like Fe, Ca, Chloride, pH, TDS, Total Hardness and Turbidity were beyond permissible limit of potable water during pre-monsoon period especially near to the coast. Salt water intrusion has taken place up to about 7 km. from the coast along the river course. The majority of the post-monsoon water samples are found to be acidic in nature influenced by the geology of the area. The drinking water qualities of surface water sources during pre-monsoon have been affected more compared to groundwater sources in the study area. There is a significant difference between the bore well water chemistry and open well water chemistry, as well as the pre-monsoon water quality and post-monsoon water quality. Bore well waters, except near the coast, are influenced by the silicate weathering of the igneous rocks, whereas the open well waters are influenced by the clay mineral reaction except those near the coast, influenced by the chloride dissolution from the salt water ingression in the study area. The total hydrochemistry in the study area is dominated by alkaline earth (64%) and strong acids (55%) with carbonate hardness (33%) (secondary alkalinity) and primary salinity (26%) influenced by the weathered granitic gneisses and leached laterite besides the influence of saline water. During pre-monsoon season Rock interaction Domain is having a dominating influence on the groundwater whereas in post-monsoon season Precipitation domain influences open well water. The difference in the hydrochemistry during pre-monsoon and postmonsoon periods indicates the influence of weathering, infiltration, mixing and leaching in the study area. A great variation is noticed in irrigation water suitability based on different characteristics and is found to be 95% (pre-monsoon) and 100% (post-monsoon) based on EC, 100% (pre-monsoon) and 98% (post-monsoon) based on SAR, 84% (pre-monsoon) and 33% (post-monsoon) based on % Na, and 60% (pre-monsoon) and 03% (post-monsoon) based on RSC. All groundwater samples fall under no problem category of sodium water type irrigation quality with low to medium salinity. Therefore, they can be used for irrigation on all types of soil with little danger of exchangeable sodium. An analysis of the Water Quality Indices (WQI) and its mapping in spatial and temporal domain in the study area found to be a faster and better tool in assessing and rating the suitability of groundwater for drinking water based on quality weightage. The very high WQI at the coastal front near the mouth of the river and its extension along the river course upstream up to a certain distance during pre-monsoon indicates the influence of saline water and its migration along tidal water in this area. The temporal variation in distribution pattern and density of WQI points to the significant role of precipitation and infiltration playing in the determination of water quality. As a part of systematic identification, quantification and management of regional hydrologic regime, geophysical investigations especially Vertical Electrical Soundings (VES) have been carried out at about 129 selected stations representing different terrains in the basin to understand the subsurface lithology, groundwater quality and aquifer characteristics at different depths. The data has been interpreted using resistivity cross sections and correlated with a few available drilled data/geology of the area in order to understand the aquifer characteristics and resource potential. Saline water ingression studies also have been carried out using these data along the coastal stretch. From the hydrometeorological analysis, the study area is found to be falling under tropical humid climatic zone where hot humid climate prevails throughout the year. The rainfall of the study area falling in the wet climatic zone is uneven and shows an overall decreasing trend for the last four decades. From the last four decade’s rainfall data analysis, the average annual rainfall is found to be about 4264.09 mm falling in the wet climatic zone giving rise to an average volume of about 1496.70x106 m3 storm water per annum in the basin. The rainfall spread for about five to six months in the study area shows spatial and temporal variation in its distribution with a minimum of 66 days at Mulki during 1973 to 161days at Karkala during 1978. The average daily rainfall also found to be varied from 12.3mm during 1973 to 53.2mm during 1997. A balance in distribution of rainfall found to be characteristic of this area where deficient years or dry years almost equaled the wet years in all three point stations during the 30 years observations period. Temporal and spatial variations in rainfall have been noticed in the study area, where 87% of rainfall is contributed from southwest monsoon spread over a period of four months and the decrease in trend is from Karkala to Mulki in a northeast to southwest direction. However, the moving average curve of rainfall denotes a nine year trend of continuous variation in its pattern and periodicity. The frequency probability analysis of magnitude and return period of rainfall in the basin demonstrate that rainfall above 3176.9mm have a chance of returning every year with 67% to 98% dependence. The computed water budget of the area shows an actual evapotranspiration of 30% of rainfall against the maximum potential evapotranspiration of 46.6% and a runoff about 771.16 x 106 cubic meters per annum from the study area. Groundwater assessment and management studies revealed that there is a surplus availability of storm water and groundwater balance in the 43 villages of the 17 Panchayats falling in the Mulki River basin, but not managed properly underutilizing its huge potential. Estimation of groundwater recharge and potential has been carried out using water table fluctuation data obtained from three observation wells other than 36 observation wells periodically monitored for a period of one year. The average annual decline in the water table of the study area is found to be about 6.85m during last one decade giving rise to a draft of 2,404.35x106 cubic meters per annum. Approximate water demand and utilization has been estimated based on Village Panchayat resource data. Even with a net annual recharge of 24,302 ha.m. of groundwater available for development in the study area, the annual net draft for utilisation from the aquifer is estimated to be 993.93 ha.m. only and a balance potential of 23,308 ha.m. is available for future utilization, but unevenly distributed in the area and lost as groundwater flow. Cultivated area of about 27% in the basin has been found in clustered strips irrigated by a good practice of rain fed traditional structures like tanks, ponds, etc. About 73 such structures ideally located with a concentration of one in 4.8 sq. km. spread over an area of 13.5 hectares with a maximum storage capacity of 0.23 MCM have been identified, delineated and mapped from the study area. Rehabilitating old Rainwater Harvesting Structures (RWHS) and increasing the density of tanks at the rate of at least one in one sq km. area can use more than 50 per cent of the runoff water considering the future needs of development. Case studies carried out to understand the viability of these resources proved these to be very effective. About 56 microwatersheds, significant in the sustainable development, spread over an area of about 46.14 sq. kms. comprising 13% of the Mulki River basin have been identified and delineated. Development of microwatersheds along with rehabilitation of abandoned and silted traditional rainwater harvesting structures found to be an efficient management practise to improve the water resource of this area for drinking and irrigational purposes. Geoinfomatic application found to be a very useful tool in the preparation of various thematic maps and integration of data for efficient planning and management of the water resources in the study area for sustainable development.