<p>Groundwater in coastal regions is increasingly threatened as a result of seawater intrusion caused by over-extraction, rapid urbanization, and climatic variability. This study assesses the vulnerability of aquifers in Chengalpattu District, Tamil Nadu, using the Groundwater Occurrence-G, Aquifer Hydraulic Conductivity- A, Groundwater Level above Mean Sea Level- L, Distance from Shore- D, Impact of Existing Seawater Intrusion- I, and Thickness of Aquifer- T [GALDIT] index - a composite model based on six hydrogeologicalparameters. Additionally a total of twenty-five groundwater samples were collected during the post-monsoon period of February 2025, and essential water quality parameters such as hydrogen ion concentration (pH), electrical conductivity (EC), total dissolved solids (TDS), salinity, Calcium (Ca²⁺), Magnesium (Mg²⁺), Sodium (Na⁺), Potassium (K⁺), Chloride (Cl⁻), Carbonate (CO₃²⁻), Bicarbonate (HCO₃⁻), and Sulphate (SO₄²⁻) were analysed. Hydrogeochemical evaluation expose the dominance of salinity and ion exchange process. Thematic layers corresponding to each GALDIT parameter were processed to generate a spatial vulnerability map. The results revealed that the eastern coastal and estuarine regions, notably Kelambakkam, Nemmeli and Mahabalipuram, exhibited high vulnerability due to shallow aquifers, elevated Cl⁻/ (CO₃²⁻+HCO₃⁻) ratios, and proximity to the seawater intrusion. Inland zones such as Madurantakam and Padalam also displayed elevated vulnerability, potentially linked to ion exchange processes, inland salinity, or excessive groundwater extraction. The outcomes of this study underscore that more than 30% of samples fall within highly vulnerable zone needs sensitive utilization and conservation of groundwater in coastal region.</p>