Characterization of groundwater chemistry under the influence of lithologic and anthropogenic factors along a climatic gradient in Upper Cauvery basin, South India

Hydrogeological and climatic effect on chemical behavior of groundwater along a climatic gradient is studied along a river basin. ‘Semi-arid’ (500–800 mm of mean annual rainfall), ‘sub-humid’ (800–1,200 mm/year) and ‘humid’ (1,200–1,500 mm/year) are the climatic zones chosen along the granito-gneissic plains of Kabini basin in South India for the present analysis. Data on groundwater chemistry is initially checked for its quality using NICB ratio (<±5 %), EC versus TZ+ (~0.85 correlation), EC versus TDS and EC versus TH analysis. Groundwater in the three climatic zones is ‘hard’ to ‘very hard’ in terms of Ca–Mg hardness. Polluted wells are identified (>40 % of pollution) and eliminated for the characterization. Piper’s diagram with mean concentrations indicates the evolution of CaNaHCO₃ (semi-arid) from CaHCO₃ (humid zone) along the climatic gradient. Carbonates dominate other anions and strong acids exceeded weak acids in the region. Mule Hole SEW, an experimental watershed in sub-humid zone, is characterized initially using hydrogeochemistry and is observed to be a replica of entire sub-humid zone (with 25 wells). Extension of the studies for the entire basin (120 wells) showed a chemical gradient along the climatic gradient with sub-humid zone bridging semi-arid and humid zones. Ca/Na molar ratio varies by more than 100 times from semi-arid to humid zones. Semi-arid zone is more silicaceous than sub-humid while humid zone is more carbonaceous (Ca/Cl ~14). Along the climatic gradient, groundwater is undersaturated (humid), saturated (sub-humid) and slightly supersaturated (semi-arid) with calcite and dolomite. Concentration–depth profiles are in support of the geological stratification i.e., ~18 m of saprolite and ~25 m of fracture rock with parent gneiss beneath. All the wells are classified into four groups based on groundwater fluctuations and further into ‘deep’ and ‘shallow’ based on the depth to groundwater. Higher the fluctuations, larger is its impact on groundwater chemistry. Actual seasonal patterns are identified using ‘recharge–discharge’ concept based on rainfall intensity instead of traditional monsoon–non-monsoon concept. Non-pumped wells have low Na/Cl and Ca/Cl ratios in recharge period than in discharge period (Dilution). Few other wells, which are subjected to pumping, still exhibit dilution chemistry though water level fluctuations are high due to annual recharge. Other wells which do not receive sufficient rainfall and are constantly pumped showed high concentrations in recharge period rather than in discharge period (Anti-dilution). In summary, recharge–discharge concept demarcates the pumped wells from natural deep wells thus, characterizing the basin.