<p>Groundwater salinization threatens water security and ecosystem integrity in the Bengal Delta, where intensive agriculture, expanding aquaculture, and inadequate sanitation infrastructure compound climate-driven coastal vulnerability. This study investigated spatial and temporal groundwater quality variations in Koyra Upazila, southwestern Bangladesh, analyzing 51 samples across pre-monsoon and post-monsoon using integrated hydrochemical, geochemical, statistical, and geospatial approaches. Results reveal severe salinization with predominant Na-Cl type groundwater (&gt; 50% of samples), indicating seawater mixing (as a primary control) through reverse cation exchange as the secondary salinity driver. Hydrochemical evolution from Ca-HCO₃ (freshwater signature) to Na-Cl (saltwater signature) dominance demonstrates progressive marine water displacement of terrestrial freshwater systems. Higher NO₃⁻ and SO₄²⁻ concentrations reflect anthropogenic contamination from agricultural fertilizers and sewage effluents. Spatial analysis reveals the highest salinity (11.15 ppt), EC (18500.00µS/cm), and TDS (9300.00&#xa0;mg/L) concentrations in the northeastern zone, with progressively lower values toward the southwestern zone. Water quality assessment classifies groundwater as very poor for drinking, though sodium adsorption ratio analysis indicates &gt; 50% of samples remain suitable for irrigation. These findings provide essential baseline data for integrated coastal zone management in Bangladesh and offer transferable insights for vulnerable deltaic regions globally (Mekong Nile, Mississippi deltas) facing similar pressures. Recommended management strategies include groundwater extraction regulation, diversified water supply development, ecosystem-based adaptation integrating Sundarbans mangrove conservation, and community-based governance frameworks critical for sustaining livelihoods and ecosystems in climate threatened coastal landscapes worldwide.</p><p><b>Trial registration number</b>: Not applicable.</p>