The combined vulnerability: a novel hybrid index for the aquifer vulnerability assessment

The aim of the present work is to extend the existing overlay and index method for aquifer vulnerability assessment to groundwater transport. This novel procedure falls into the category of “hybrid” methods since it combines the overlay and index methods, that considers only vertical transport through the vadose zone, with the horizontal transport through groundwater. Based on a simple probabilistic analysis, we can use any overlay and index method for the assessment of the probability that the contaminant reaches the groundwater then such probability is propagated within the aquifer using the piezometric surface as proxy of the groundwater flow. For this purpose, geomorphological methods, usually available in Geographic Information Systems (GIS) software, were used. This study leads to the definition of a new index called combined vulnerability index υ which considers the transport of the contaminant from soil surface that takes care of both transport in the vadose zone and the aquifer. The procedure is applied to a groundwater catchment area in the Campania region (Southern Italy) with the use of QGIS software. The method is independent from the index and overlay method used and the hydraulic characteristics of the aquifer. Furthermore, this procedure is simple as it requires a few data and it can be used to analyze large areas, demonstrating its effectiveness in assessing the intrinsic vulnerability of groundwater.