Surface geochemical survey in the vicinity of decommissioned coal mine shafts

Methane observed in soil atmosphere above active and abandoned bituminous coal mines has a thermogenic (coalbed) and/or microbial (produced by anaerobic bacteria or resulted from in situ coal biodegradation) origin or it may be a mixture of gases generated from various sources. Identification of the origin of methane may support or refute the thesis on the uncontrolled migration of mine gases in rock formations followed by their emission to the atmosphere at coal extraction sites. Surface geochemical surveys were carried out in the vicinity of the decommissioned mine shafts at the Murcki-Staszic Coal Mine, located in the Upper Silesian Coal Basin (southern Poland). Over a 3-month period, measurements were carried out cyclically at 2 locations. Totally, 43 fixed geochemical probes were installed in the area of the Murcki II "Stanisław" Shaft, in which measurements were carried out in six sessions. Next 29 probes were installed in the area of the Staszic III Shaft, where measurements were taken twice. Each measurement included concentrations of 5 components (methane, non-methane volatile organic compounds, total hydrocarbons, carbon dioxide and oxygen), which provided a total number of 1405 individual analytical determinations. For measurements, portable EcoProbe 5 instrument was used. The stable carbon isotope composition of methane and carbon dioxide was analyzed in selected 10 samples. The isotopic composition of hydrogen in methane was also measured in one sample. The comparative material consisted of eight samples of soil gases and two samples of mine gases, which were used as reference 'soil' gases. The isotopic analyses were conducted using a mass spectrometry method. Based on the results of our analyses, it is stated that both the Murcki II "Stanisław" and the Staszic III shafts were properly decommissioned. The successful sealing of mine shafts prevented mine gases from migration through the shaft into the atmosphere. However, at a single geochemical probe located more than 100 m from the former shaft center, methane concentration in soil gases reached over 1 vol%. This confirms the general observation that mine gas migrates from coal seams and/or gobs through crushed rocks into the atmosphere. The stable carbon isotope analysis of methane and carbon dioxide as well as stable hydrogen isotope analysis of methane suggest that methane in the soil atmosphere is of thermogenic origin with minor contribution of microbial component.