Factor Analysis of Seasonal Hydrochemical Data of the Varthur Catchment Area, Bangalore Urban District, Karnataka

Seasonal groundwater-quality variability across the Varthur Catchment, Bangalore Urban District, was assessed using R-mode factor analysis applied independently to pre- and post-monsoon 2023 hydro chemical data from 58 bore wells. The correlation matrix showed strong positive associations among EC, TDS, TH, Ca, Mg, Na, Cl and SO₄ in both seasons, indicating that mineral dissolution and rock-water interaction govern groundwater mineralization. Four and three factors were retained for the pre- and post-monsoon datasets, explaining 77.22% and 72.70% of total variance, respectively. In both seasons, Factor I loaded heavily on Ca, Mg, Cl, SO₄, EC, TH and TDS and is interpreted as a hardness-mineralization factor reflecting geogenic rock-water interaction. Factor II reflected alkali enrichment and ion exchange, dominated by K pre-monsoon and by pH post-monsoon, while a distinct carbonate/bicarbonate factor emerged in both seasons. A separate nitrate-dominated factor (loading 0.972) was retained pre-monsoon, indicating concentrated anthropogenic contamination during the dry season; post-monsoon nitrate loadings were weak and diffuse, consistent with dilution by monsoon recharge. Spatial mapping of factor scores showed that hardness-mineralization zones coincide with deeper water tables and dense urban development in the south-central and northern catchment, whereas nitrate hotspots persist near dense settlement and residual agriculture in the north. Overall, Varthur groundwater chemistry is governed by lithogenic weathering and carbonate equilibrium, with a superimposed, seasonally modulated anthropogenic nitrate signal.