Recent developments in the Apsû method for processes-based groundwater vulnerability mapping

The Apsû method (Popescu et al. 2019) was developed to assess the intrinsic and specific vulnerability of groundwater in support of pollution risk studies. In its original version, Apsû allows mapping the vulnerability of the first (phreatic) aquifer encountered from the land surface. Vulnerability maps are obtained from a quantitative analysis of contaminant transport processes across the partially saturated zone. Vulnerability classes are based on physical criteria reflecting the sensitivity of groundwater to contamination events. They are the minimal travel time of pollutant from the surface to the saturated zone, the maximal concentration of pollutant breakthrough in the saturated zone, and/or, the relative quantity of pollutant that reaches the saturated zone. One of the specific features of Apsû is that it also incorporates a concept of land surface hazard that reflects the conditions of infiltration (direct hazard) and runoff (lateral hazard) that prevail in the groundwater recharge basin. In the direct vicinity of watercourses, the default assumption is that the direct hazard is maximum whether the river is infiltrating or draining. Apsû has recently been adapted to take a more consistent account of groundwater-river interactions and the potentially protective nature of alluvial aquifers connected to these rivers for aquifer systems located below the alluvial deposits. Also, the possible presence of protective horizons such as a layer of barely permeable clay and their effect on the vulnerability of deeper aquifers has also been considered. The improvements made to the Apsû method are presented and illustrated with cartographic results obtained for aquifers in Walloon Region of Belgium.