Modeling VOC Transport in a Large-Scale Thick Vadose Zone

Groundwater contamination by volatile organic compounds (VOCs) presents a pressing environmental challenge, particularly in complex subsurface systems. As persistent pollutants, VOCs degrade water quality, threaten ecosystems, and pose serious health risks, necessitating effective containment and remediation strategies. In this study, we employ transport modeling to investigate industrial contaminant spreading in the Jerusalem Mountains, Israel, characterized by a thick, stratified, karstic, faulted, and folded vadose zone, including two-story perched aquifers. Despite limited monitoring data and a 50-year delay in detecting VOC leakage, our FEFLOW model successfully reconstructed key contaminant transport processes, including convection, dispersion, retardation, volatilization, and attenuation. We integrated a transport model with a calibrated flow model to trace the evolution of the VOC plume over 70 years, revealing critical dynamics within the vadose zone. Preferential flow occurs horizontally via perched aquifers and vertically through faults, enhanced by karstic networks. The vast extent of contamination, with the plume extending into the deep regional aquifer several decades after the presumed onset of pollutant dispersion, underscores the vadose zone's dual function as both a buffer and a facilitator of pollutants. While rapid flow paths enabled the aquitards to be traversed, effectively spreading contaminants into the regional water source, most of the unsaturated zone successfully contained the VOCs, mitigating their migration toward residential neighborhoods and critical groundwater resources. Our modeling predicts that, without remedial actions, the VOC plume will persist in the vadose zone, leaching slowly over time, with negligible attenuation in the coming decades. By uncovering these dynamics, our research not only aids local remediation efforts but also provides a framework for addressing similar challenges in complex hydrogeological settings worldwide.