Hydro‐Chemical Dynamics and Irrigation Suitability of Kapatakkha River Water Under Seasonal Tidal Influence in Southwestern Bangladesh

ABSTRACT This study investigates how tidal and seasonal fluctuations impact the Kapatakkha River's irrigation suitability, addressing the oversight of tidal‐scale dynamics in traditional water quality assessments. The problem centres on a suitability paradox in southwestern Bangladesh. The objectives were to profile spatiotemporal hydro‐chemical dynamics and evaluate overall suitability using an integrated Water Quality Index. Methods involved analysing major ions (Na + , K + , Ca 2+ , Mg 2+ , Cl − , SO₄ 2− , HCO₃ − and PO₄ 3− ) and field parameters (pH, EC, TDS and DO) from eight stations during pre‐ and postmonsoon cycles, utilising principal component analysis (PCA), hierarchical cluster analysis (HCA) and spatial interpolation. The findings indicate that premonsoon river water chemistry is governed by evaporative concentration and mineral dissolution, whereas postmonsoon river water exhibits stronger hydro‐chemical connectivity among EC, TDS, Na + , Cl − , SAR and Na%. Mean EC increased from 3521 to 4939 μS/cm, chloride from 1513 to 5328 mg/L and SAR from 16.9 to 37.2 between pre‐ and postmonsoon conditions. Only 38% of sampling locations showed excellent water quality, and 42% fell into the rejection category. Critical findings reveal a divided river system. The study highlights the importance of incorporating tidal‐stage variability into irrigation water‐quality assessments and supports the development of adaptive, tide‐aware water management strategies for coastal river systems.