Pre-monsoon hydrochemistry and multivariate analysis of deep groundwater, Sagar Island, Sundarbans

Abstract Deep Quaternary aquifers in Holocene mega-deltas are often assumed to be protected by low-permeability clay aquitards, yet they remain vulnerable to climate-driven and anthropogenic stress. This study investigated the confined aquifer system of Sagar Island, India, during the 2025 pre-monsoon peak-stress season to clarify the hydrogeochemical processes governing solute evolution under present-day pressure conditions. Multivariate geostatistics (varimax-rotated Principal Component Analysis and Cluster Analysis) combined with thermodynamic modeling indicate a spatially bifurcated system, with a northern sector dominated by freshening signatures associated with meteoric recharge and a southern sector influenced by saline modification and mixing. Conservative estuarine overprinting is partly separated from non-conservative base-cation exchange, suggesting localized vertical leakage under pumping stress within a heterogeneous aquitard system. The results highlight that groundwater quality in the southern sector is more vulnerable to abstraction-driven salinity transfer, whereas the northern sector retains a stronger freshening signal. Overall, the study shows that sustainable management of coastal groundwater in stressed deltaic settings requires spatially targeted abstraction control and closer attention to aquitard integrity rather than reliance on basin-scale averages alone.