The origin of GHG's emission from self-heating coal waste dump: Atmogeochemical interactions and environmental implications.

The aim of this work was to analyse the origin of the exhaust gases containing large amount of carbon dioxide (CO 2) and methane (CH 4) from self-heating coal waste dump and its potential health and environmental impact. Air samples were collected from the coal waste dump and its vicinity in Nowa Ruda − Słupiec (NR-S), Southwestern Poland, during ground survey campaigns in spring (March 30, 2019) and winter (December 1, 2019). In order to detect the origin of the exhaust plume emission, stable carbon isotopic composition (δ13C) of CH 4 and CO 2 was measured using a cavity ring-down spectrometer (CRDS). We investigated the gases collected from the hotspots at the surface of the coal-mine dump, from the air above the dump and finally from surroundings of the dump. This allowed to estimate the stage and magnitude of CH 4 oxidation inside the coal dump. Many gas seeps from the places, where the dump is actively burning were identified during the ground surveys. CH 4 mole fraction in those seeps (hot-spots) reached up to 10,000 ppm. Methane found in the most of the air samples collected from the dump was strongly enriched in 13C, with an average of −27.4 ± 5.8‰, which is heavier than methane coming out from formerly excavated coal seams what indicate coal pyrolysis as the predominant source of methane seeps. The covarying with methane, CO 2 mole fraction from active burn dump sites reached up to 73,000 ppm with an average δ 13 C value of −27.3 ± 8.4‰. Therefore, CO 2 resembles isotopically it's predominant precursor, the pyrolytic methane, via incomplete combustion in the shallow oxygenated parts of the hot spots. Substantial amounts of methane escape from the self-ignited spots on the NR-S coal waste dump, however, the mobile surveys did not reveal the enhanced methane mole fraction at the ground level in distance between 1 and 3 km from the dump. Even at distances of 150 m from the hot spots impact was limited, implying convective transport of these localized hot pollutant plumes. The results highlight changes in the fire intensity, prevailing winds, dump architecture, and surface roughness in the surrounding regions are the most important factors of air pollutant emission from complex coal mine dump system. • The noticeable increase of number of thermally active spots on coal waste dump was observed • High temporal and spatial heterogeneity of GHGs was caused by intensifying self-heating • Enriched δ13C clearly indicated generating thermogenic CH 4 inside the intensely self-heating coal waste dump • Carbon dioxide predominantly originated from methane oxidation. [ABSTRACT FROM AUTHOR]