Groundwater vulnerability of megacity under changing climate and land use scenarios

Rapid urbanization and climate change pose a serious threat to the groundwater resource of urban regions in terms of contamination and excessive groundwater abstraction. To understand and protect groundwater resources from anthropogenic activities, an index-based vulnerability assessment model which integrates future climatic variables and land use (LU) is needed. This study attempts to identify the impact of climate and land use change on future groundwater vulnerability of rapidly urbanizing South Asian city, Chennai Metropolitan Area (CMA), India using DRASTIC-LU model under RCP4.5 and RCP8.5 scenarios for near (2020-2035) and far future period (2035 - 2050). The dynamic variables of the DRASTIC-LU model such as depth to groundwater - D, net recharge - R, and land use - LU were projected using regional climatic models and land change modeler. Three regional climatic models (RCMs) namely ICHEC-EC-EARTH, MIROC-MIROC5, and CNRM-CERFACS-CNRM-CM5 were compared and selected using Taylor’s diagram. The ICHEC-EC-EARTH model was found to perform better than the other two RCMs for this region and was used to project future climatic variables under RCP4.5 and RCP8.5 scenarios for two future periods (4 scenarios). Terrset land change modeler (LCM) was used to project future land use for the years 2030 and 2050. Vulnerability condition of the base period (2018) was assessed using nitrate concentration and vulnerability indices, the developed DRASTIC-LU model produces an accuracy of (AUC = 0.796). The projected groundwater vulnerability depicts an increase in vulnerability area from the base period as 21.41%, 30.09%, 20.98%, and 22.01% for scenario-1,2,3 and 4 respectively. Variation in precipitation pattern contributes to change in future net recharge and groundwater level and increased built-up region, i.e, change in land use attributes to increase in future groundwater vulnerability. Based on future vulnerability analysis, it is identified that the CMA groundwater system is in critical condition of high vulnerability for near and far future periods.