Non-invasive hydro-geophysical imaging of landfill leachate source-to-plume dynamics using electromagnetic induction methods, geochemistry and hydrological modelling

Landfill leachate constitutes a major environmental threat due to its capacity to degrade soil and groundwater quality, particularly in situations where heterogeneous waste deposits and complex interactions between waste and natural subsurface hinder the characterization of source zones and plume evolution. This study aims to develop an integrated hydro-geophysical and hydro-chemical framework to investigate leachate generation, migration, and dispersion within a complex landfill system in Central Italy. The approach links surface hydrology, subsurface electrical properties, groundwater flow dynamics, and chemical indicators to reduce interpretative uncertainty and improve the conceptual understanding of source-to-plume generation mechanisms. Surface drainage and infiltration patterns were analyzed using a high-resolution (0.5 m) LiDAR-derived digital elevation model. Shallow groundwater flow directions were reconstructed from piezometric measurements at 19 monitoring points, while groundwater samples were analyzed for key leachate tracers, with particular emphasis on sulfate and boron. Subsurface characterization was performed through a multi-frequency frequency-domain electromagnetic (FDEM) survey acquired at 3, 6, and 15 kHz, providing depth-dependent apparent electrical conductivity information down to approximately 4 m from the ground surface. Results reveal a strong spatial coherence between runoff concentration zones, subsurface conductive anomalies, and groundwater flow pathways. Areas prone to water stagnation and enhanced infiltration correspond to pronounced conductive anomalies within the unsaturated zone, which progressively deepen and widen with increasing investigation depth, indicating vertical percolation and lateral spreading of leachate beneath the waste mounds. Hydro-chemical data independently corroborate this interpretation, delineating a contaminant plume emerging from these conductive secondary source zones and migrating downgradient along the valley axis under advection and dispersion processes. Overall, the proposed integrated framework provides a robust source-to-plume conceptual model and represents an effective, transferable tool for landfill monitoring, groundwater risk assessment, and remediation planning.