<p>Identification and delineation of the aquifer vulnerable zones are essential for effectively managing groundwater contamination, especially in river basins with significant anthropogenic activities. Therefore, the present study aims to identify the aquifer vulnerable zones in the Sai River Basin (SRB) using the geographic information system (GIS)- based modified DRASTIC model. The modified DRASTIC model incorporates seven parameters: Depth to the water table, net Recharge, Aquifer media, Soil media, Topography, Impact of the vadose zone, and Land use-land cover (LULC). The hydraulic Conductivity parameter was replaced with LULC in the modified DRASTIC model. The outcome of the modified model was validated by the sensitivity analysis and nitrate (NO₃⁻) concentration of the groundwater of the study area. The computed modified DRASTIC index results show that about 92% of the SRB area falls in a high aquifer vulnerability zone. However, the area (2.5%) of the SRB, which is close to the Lucknow city region, belongs to a very high aquifer vulnerability zone. The rest of the area of the SRB belongs to the moderate aquifer vulnerability zone. The statistical analysis of the modified DRASTIC parameters and single sensitivity analysis suggest that the depth to water table, impact of the vadose zone, and LULC contribute significantly to the aquifer vulnerability index, followed by aquifer media and net recharge. The moderate to high concentration of nitrate in the groundwater of the SRB falls in the high to very high aquifer vulnerable zone of the SRB, except in a few places, and suggests the aquifers of the SRB could be more susceptible to contamination. The GIS-based maps and the outcome of the present study could be helpful to policymakers for developing effective management plans for appropriate sewage treatment, managing leakage from toilet septic tanks, minimising fertiliser overuse, and groundwater monitoring, especially in areas with very highly and highly vulnerable aquifers.</p>