Global Inversion of ERT and IP Data using VFSA for Improved Detection and Uncertainty Assessment of Leachate Accumulation in Urban Landfills

Electrical Resistivity Tomography (ERT) and induced polarisation (IP) methods are widely employed in the characterisation of urban landfills due to their sensitivity to subsurface moisture and electrochemical properties of waste. Traditional local inversion techniques, typically based on smoothness-constrained Occam-type methods, can fail to resolve the high spatial variability of leachate accumulation areas. Additionally, these techniques do not provide an assessment of the uncertainty associated with the detection of accumulation areas, which is pivotal for informing quantitatively the landfill management. In this study, we apply a global inversion approach based on the Very Fast Simulated Annealing (VFSA) algorithm to ERT and IP datasets acquired on a municipal solid waste (MSW) landfill located in Central Italy. This site, characterised by a steep slope and high risk of lea-chate-induced instability, is currently monitored with time along multiple profiles. The global inversion process was implemented on a selected line where also piezometric level logged in wells are available. Posterior model ensembles were also analysed to derive uncertainty estimates, and petrophysical transformations were applied to extract water content and Cation Exchange Capacity (CEC) from the geoelectrical parameters. The results demonstrate the effectiveness of the VFSA method in detecting highly variable leachate zones, as confirmed by the good agreement with leachate levels logged in wells. The uncertainty assessment highlighting areas of higher and lower reliability of the geophysical model can further support the landfill monitoring, with implications for risk assessment and long-term management.