Spatial variability of groundwater quality in four mega-cities of India and impact of land use types on groundwater quality

The deterioration of groundwater quality is a global concern, particularly in densely populated urban areas. This study examines spatial variability in groundwater quality across four major Indian cities - Mumbai, Delhi, Chennai and Kolkata - using water quality index (WQI) and remote sensing techniques. Groundwater data from the 2018 report of the Central Groundwater Board was analyzed using 12 parameters, while Normalized Difference Vegetation Index (NDVI), Normalized Difference Water Index (NDWI) and Normalized Difference Built-up Index (NDBI) indices were calculated from Landsat imageries to assess land use impacts. Findings indicate significant variability: Delhi shows 44.62% unsuitable samples, Chennai 12.12%, Kolkata 0% and Mumbai 2%. Urbanization (NDBI) negatively affects WQI, while green spaces (NDVI) improve it. NDWI impacts vary, being negative in Mumbai but positive in Kolkata, Chennai and Delhi. Welch's analysis of variance (ANOVA) and post hoc analysis confirms significant WQI differences across cities. These results highlight the urgent need for sustainable urban infrastructure, increased green spaces and pollutant-free waterbodies to improve groundwater quality in urban India.