Assessment and mapping of groundwater vulnerability to pollution: Current status and challenges

Increasing groundwater contamination across the globe triggered the concept of "aquifer vulnerability", which has been extensively used worldwide during past three to four decades by researchers and policy makers for protecting groundwater from pollution. However, only a few recent studies have focused on the performance evaluation of two or more vulnerability assessment methods. Some of these studies have resulted in contrasting findings. Given this fact and considering growing threat of groundwater contamination due to increasing human activities across the globe, it is necessary to critically review existing methods, understand current research trends, and identify major challenges associated with the assessment of aquifer vulnerability. Hence, the aim of this study is to present a comprehensive review of the methods and approaches used for the evaluation of aquifer vulnerability for 'resource' and 'source' protection. First, the concept and types of aquifer vulnerability along with the definitions evolved over the years are presented, and then the methods for assessing aquifer vulnerability are suitably classified and briefly discussed. Second, the concept of vulnerability assessment for 'source' protection is highlighted, and the evolution of groundwater vulnerability evaluation methods is presented with an enlightening block diagram. Third, current research trends and critiques on past studies are discussed. Fourth, the major challenges of vulnerability assessment are highlighted and a way forward is suggested. It is concluded that the progress of vulnerability evaluation methods has not kept pace with the advancement of knowledge and tools/techniques. There is an urgent need for developing a scientifically robust and somewhat versatile methodology for the evaluation of 'intrinsic' and 'specific' groundwater vulnerability for 'resource' and 'source' protection under varying hydrogeologic and hydro-climatic conditions. It is emphasized that more studies should be devoted to vulnerability assessment for 'source' protection using 'Source-Pathway-Receptor/Target' approach. Also, spatial decision support systems should be developed using modern tools/techniques including artificial intelligence to improve decision-making process for protecting vital groundwater resources.