Isotope and chemical alteration of coalbed gases during migration from underground coal mine into the atmosphere

Methane release into mine workings and the atmosphere poses a serious environmental and safety risks. This study examines the composition and stable isotope geochemistry of coalbed methane in a hard coal mine of the Upper Silesian Coal Basin (Poland), with emphasis on gas migration pathways from the coal seam through mine workings and ventilation shaft to the atmosphere. Methane concentrations decrease progressively with distance from the active excavation zone toward the exhaust shaft. In addition to CH 4 and CO 2 , the coalbed gas contained notable amounts of nitrogen heterocyclic compounds, sulfur-bearing species, and chlorinated gases, depending on the sampling location and temporal window. Geochemical results reveal a strong link between gas migration and isotopic variability in the underground environment. Zones of methane accumulation, commonly associated with bends in return airways, showed enrichment in 13 C, consistent with diffusive fractionation. Variations in CH 4 and corresponding isotope effects during coal transport indicate multiple gas origins within the coal seam. Isotope signatures point to predominantly thermogenic CH 4 at the active mining level, whereas 13 C-depleted methane at the exploited and flooded levels suggests secondary microbial generation. The microbial contribution is likely promoted by infiltrating meteoric waters supplying nutrients for methanogenic archaea, as supported by δ 2 H, δ 18 O signatures of mine water, and measurable tritium activity. The absence of enriched δ 13 C values in CO 2 and dissolved inorganic carbon (DIC) further implies mixed thermogenic-microbial gas origin in abandoned mine workings. These findings highlight the combined influence of mining operations, diffusion-desorption processes, microbial activity, and gas mixing on the molecular and isotope composition of coalbed gases transported toward the exhaust shaft. Moreover, isotopic data from downwind emission plumes demonstrate the value of integrating underground measurements with atmospheric observations to trace and quantify methane emissions from coal mining operations. • Thermogenic CH 4 dominates at the active mining level, mixed gas origin in exploited level. • Mixing, desorption, and diffusion processes alter CH 4 isotope composition in a mine. • Variations of gas content with isotope shift occurred during coal transportation. • Tracers vary most greatly for N-, S-heterocycles, sulfur gases, chlorinated compounds. • Mine ventilation-driven CH 4 emission displayed significant temporal heterogeneity.