Integrated Risk Assessment Framework for Abandoned Mine Methane (AMM) Emissions in Urban Environments: Methodological Development and Application to the Lupeni Case Study (Romania)

Abandoned mine methane (AMM) emissions represent a significant public safety and environmental hazard in post-industrial urban areas. Uncontrolled subsurface gas migration may lead to explosive accumulations in confined spaces, human exposure, infrastructure damage, and additional greenhouse gas emissions. This study develops an integrated risk assessment framework for AMM in urban environments, combining quantitative analysis of field monitoring data with semi-quantitative probability–consequence risk matrices and multi-factor evaluation. Methane concentrations were measured at 41 monitoring points during three campaigns (August–September 2024). A total of 42 influencing factors were identified and classified into seven categories (geological, mining, hydrogeological, meteorological, anthropogenic, biological, and special phenomena). Exceedance probabilities of critical thresholds were estimated with 95% confidence intervals. Consequence weights were derived using the Analytic Hierarchy Process (AHP) based on a five-expert panel (CR = 0.043). The framework was applied to the urban area of Lupeni, Romania, where methane concentrations of up to 54% vol. were measured during borehole screening measurements (subsurface probe points). Elevated concentrations were detected four days after commissioning of a new gas pipeline. Gas chromatographic analysis excluded pipeline leakage and confirmed a mining-related source. Results indicate a localized critical risk (R = 25 on a 1–25 scale) in hotspot P2, with a 95% confidence interval for the probability of exceeding the 3% vol. alert threshold of [0.885–1.00], justifying immediate mitigation through controlled drainage. Post-intervention monitoring showed a reduction to instrumentally undetectable levels by February 2025. The study demonstrates that administrative mine closure does not eliminate residual methane risk and proposes a complete decision-support algorithm (URBAN-MINE-RISK) for similar urban settings. The applicability of structural reliability methods (e.g., FORM) is discussed as a future research direction. The methodology is transferable to other post-mining regions in Central and Eastern Europe.