Physically-based groundwater vulnerability assessment : from concepts to objective mathematical formulations and applications

Many different approaches have been developed for groundwater vulnerability assessment and mapping. Most are based on the selection of a number of factors that control the sensitivity of groundwater resources to contamination. These factors are subsequently weighted and rated into a global, yet most often empirical, index of groundwater vulnerability. Unfortunately, such such approaches and concepts present strong lacks and drawbacks related to the fuzzy and ambiguous definitions on which these concepts rely. As a consequence, they are hardly used for groundwater quality management and protection because they cannot be straightforwardly translated into efficient decision making procedures. The problem becomes acute when hydrogeologists wish to take into consideration the complex fate of contaminants and to develop efficient modelling tools for the evaluation of groundwater vulnerability. Most limitations relate to the use of qualitative and subjective definitions of groundwater vulnerability, as opposed to objective definitions based on physical, mathematically formulations reflecting the fate of contaminants in the underground or the sensitivity of groundwater to pollution. In this context, the objective of this talk is to discuss in details the physics behind the concepts of groundwater vulnerability and to describe different physically-based approaches that have been developed for a quantitative assessment of groundwater vulnerability. One of the approaches is based on factors describing a pollution event, such as contaminant travel time to groundwater, contamination duration, level concentration or relative mass of contaminant reaching the groundwater table. The other approach is based on the definition of sensitivity coefficients of groundwater state variables to stress factors. Both approaches can be adapted to integrate most processes governing the fate of contaminants to and into groundwater and incorporated into advanced groundwater flow and transport models. Examples of applications on synthetic and real cases are used to illustrate the presented concepts.