A comprehensive review of groundwater vulnerability assessment using index-based, modelling, and coupling methods.

Groundwater has become increasingly vulnerable to quality degradation. An elaborate understanding of its flow, draft, recharge and pollutant transport processes needs to be developed to understand its risk to contamination. This paper has discussed different tools and methods that are used to map groundwater vulnerability around the world. To maintain the quality and impact of the study, rigorous search for relevant literature published in high impact scientific journals has been done, and the comprehensive information on groundwater vulnerability assessment methods being used, has been compiled. The GIS based overlay and index-based methods like DRASTIC, GALDIT, GOD, COP and PI takes into consideration various thematic layers, overlays them to calculate weighted index and identifies vulnerability classes. They have been criticised for the lack of numerical basis in their formulation. Therefore, over the years, many of the proposed indices have been modified to provide quantitative estimates of groundwater potential to degrade and deplete. However, where the data and software are not a constraint, the use of numerical based simulation models can be done for more elaborate and numerical based quantification of the vulnerability. These numerical models typically require extensive data and are exceedingly becoming more sophisticated with the introduction of new parameters. This study concludes that integrating the GIS with numerical models offers the advantage of data management and assists to spatially analyse the datasets. The difficulties that are associated with the differences between GIS and numerical model's data structures should be thoroughly understood, prior to coupling, to develop uniform conversion software.