Assessment of Groundwater Vulnerability by Modified DRASTIC Model and Analytic Hierarchy Process over Urban and Suburban Landscape of an Indian Megacity

Abstract Groundwater is one of the major sources of freshwater on earth, so its quality and quantity are important. However, domestic water supply is a serious issue in megacities due to the enormous population growth and urban sprawl. This study analyses groundwater vulnerability in an Indian megacity Kolkata, where ⁓52% of the population are dependent on groundwater. Over the last thirty years, impervious surfaces over the studied area have increased about three times which resulted in the decrease of groundwater recharge. Based on the data analyses of the last two decades, it is observed that groundwater table has been depleting substantially towards the east and west from river Ganga that flows through the study area. To reduce groundwater contamination and recover groundwater table, an assessment of groundwater vulnerability is essential. The present study focuses on the modified DRASTIC model for groundwater vulnerability assessment. An additional parameter, land use land cover (LULC), was added to form the DRASTIC-L model. The ratings of subcategories of eight parameters are allocated based on the local environmental condition. In addition, the analytic hierarchy process (AHP) is used to assign a weight to each parameter in order to reduce human subjectivity in vulnerability assessments. To validate the accuracy of the results, correlation between the groundwater vulnerability index and groundwater quality parameter data is performed. The correlation between DRASTIC scores and nitrate (R = 0.36) and phosphate (R = 0.35) levels improved to R = 0.56 and R = 0.51 respectively after using DRASTIC-L AHP. The overall results reveal that the groundwater aquifer in the study area is facing contamination due to LULC changes and shallow depth of groundwater. We suggest that modified DRASTIC AHP model can be used to deliver reliable results making it a valuable tool for initial assessments for groundwater vulnerability for stakeholders and policymakers.