HYDROGEOCHEMISTRY IN THE NARROW ZONE OF CLIMATIC VARIATION IN THE KABINI RIVER BASIN, SOUTHERN INDIA

摘要

Major ion - chemistry of groundwater is spatially influenced by mineral dissolution, rainfall input, climatic variability and water level fluctuations in different geological formations. This paper presents an analysis of basin ion chemistry in a narrow zone along a river basin, as a function of 'Climatic Variation' in terms of 'Rainfall'. The influence of landuse patterns in this climate zone is also compared. Geochemical indices are used to analyse the behaviour of (a) Forested - Humid (b) Forested-Semi arid and (c) Agricultural - Semi arid zones. In the Kabini river basin, Moole Hole is in the Forested-Humid (1200 - 1500 mm/year rainfall) zone while a part of Gundal basin lies under both Forested and Agricultural Semi-arid (500 -800 mm/year rainfall) zones. A total of 21 wells sampled for a period of 4 years are analysed for different chemical parameters. Using Na vs CI and (Ca + Mg) vs CI plots, dissolution of carbonates is identified as the dominant geological process. Ion exchange and reverse ion exchange processes cycle between Cl & S0_4 and Ca & Na. Excess Na in Gundal is contributed by Silicate weathering while Evaporation contributes for high CI in Moole Hole. (Ca+Mg)/HC0_3 ratio (~0.5) affirms that Ca and Ma solely originated from weathering of pyroxenes and amphibole minerals. "3-D block" plots are used for observing the effect of rainfall and climatic gradient in this narrow zone. Moole Hole is more calcareous due to high Ca and low Na concentrations (Na~65mg/L; Ca/Cl up to 6). On the other hand, semi-arid zone which receives lesser rainfall is more silicaceous having high Na/CI ratios (up to 8). Agricultural zone in Gundal basin exerts a control over Na~+, Cr and S0_4~(2-) chemistry. In summary, Forested-Moole Hole (unimodal rainfall) has smaller range of molar ratios (clustered together) while Forested - Gundal (bimodal rainfall) has a wide range of molar ratios though both the areas lie in forest zone. In contrast, both Forested and Agricultural zones of Gundal basin behave similarly. The commonality in the three zones is that Forest - Bimodal - Gundal, which is located in the transition zone, behaves both like Forest-Unimodal and Agriculture - Bimodal zones. Thus, the variation in amount of rainfall and type of rainfall (climatic influence) along this gradient produced distinct chemical weathering products and could geochemically separate them.