Physically-based groundwater vulnerability assessment for groundwater protection and land-use management

Numerous groundwater vulnerability and risk mapping techniques have been developed taking into consideration a variable number of factors. Most common techniques produce maps of indexes based on a relatively empirical combination of weighted factors reflecting the protective effect of underground formations overlying the groundwater resource. The limitation of such methods is related to their use of a qualitative definition of groundwater vulnerability, as opposed to a definition based on a quantitative description of contaminant migration. A physically-based point of view and definition of the vulnerability is proposed and based on three factors describing a pollution event, which are the contaminant transfer time from the hazard location to the 'target', the contamination duration at the 'target' and the level of contaminant concentration reached at the 'target'. This concept allows a clear distinction between conventional aspects and physically-based results in the building of a final vulnerability indicator. This methodology has the further advantage to consider the possible impact of runoff conditions occurring at the land surface and possibly leading to lateral contamination of groundwater through downstream preferential infiltration features, such as karstic features. Practically, this method needs to describe and simulate the pollutant migration in the unsaturated zone and possibly in the saturated zone in order to assess the breakthrough curve at the 'target'. Preliminary application is illustrated on a case-study located in a Néblon limestone basin in Belgium, one of the main groundwater resources for the city of Liège in the Meuse basin. Perspectives are proposed towards a generalisation of the vulnerability concept for risk assessment within a pressure - state - impact framework.