Scenario-Based Modelling of Contaminant Transport in the Karst Vadose Zone of Avetrana (Southern Italy): An Applied Case Study Using the Equivalent Porous Medium Approach

Abstract

Groundwater is among the most vital freshwater reserves worldwide, accounting for roughly 25–40% of global drinking-water supply. Where karst aquifers occur, dissolution-enhanced fracture networks make these systems especially prone to contamination. This study presents a scenario-based three-dimensional numerical case study of contaminant transport within the vadose zone of a karst aquifer system adjacent to the municipality of Avetrana, Apulia Region, southern Italy, where a planned stormwater collection channel introduces a potential pathway for pollutant infiltration. The Equivalent Porous Medium (EPM) approach was adopted, allowing groundwater flow and solute transport to be simulated using the FEFLOW finite-element platform. Four simulation phases were executed: steady-state flow, steady-state mass transport with and without an injection well, and transient mass transport over five years. A sensitivity analysis was carried out on the equivalent porosity of the limestone, considering values of 50%, 70%, and 90%. As porosity increases, the contamination plume expands, while the geological structure of the vadose zone serves as a natural barrier, slowing down pollutant propagation. Under transient conditions, the peak concentration at the deepest monitoring slice dropped from 297 mg/l (90% porosity) to 32 mg/l (50% porosity), supporting the protective effect provided by the lithological succession. Mass transport velocity analysis revealed decreasing velocities with depth, with a clear contrast at the calcarenite–limestone interface. Rather than a methodological innovation, the study offers a transferable framework for assessing groundwater vulnerability in karst environments subject to managed aquifer recharge or stormwater infiltration, together with site-specific evidence of the protective role of the vadose zone. All quantitative outcomes are scenario-dependent, spatially averaged estimates intended for pre-construction screening. Limestone equivalent porosity is the dominant control on plume extent and velocity. Simulated five-year loading attenuates 97–70% at depth, decreasing as porosity rises. Injection wells partly bypass the vadose-zone protection given to surface sources.