Evidence of methane and carbon dioxide migration to the near surface zone in the area of the abandoned coal mines in Wałbrzych District (Lower Silesian Coal Basin, SW Poland) based on periodical changes of molecular and isotopic compositions

摘要: Abstract The objective of this paper is to determine the reasons, origin and character periodical changes methane carbon dioxide concentrations in near-surface zone as well emission these gases atmosphere after closing bituminous coal anthracite mines former Walbrzych Coal District (SW Poland). measurements were performed during period including active impact “water piston effect” on migration a coal-bearing stabilization water table Pennsylvanian aquifer. In years 1997–2005, 20 sampling sessions completed which 3210 soil gas samples collected at depth about 1.2 m along 4 measurement lines located Gorce Sobiecin depressions. Maximum measured reached 32.4 15.7 vol%, respectively. Isotopic study shows that coal-bed gases, recent microbial methane, multigenetic from degassing ground-waters Upper aquifer occur within District. Within Serpukhovian Bashkirian-Moscovian Žacleř formations three genetic types occur: Variscan thermogenic (methane, higher gaseous hydrocarbons dioxide), endogenic (abiogenic) “late” (Late Cretaceous) dioxide. Changes mean recorded studies demonstrated influence effect”, drives coalbed towards earth surface. both depressions, migrates with various intensity. Generally, lower 2004–2005, compared 1999–2000, resulted groundwater whereas confirms shallow groundwaters. Anomalous related numerous faults fracture systems cutting steeply dipping Mississippian formations, outcrops rocks. However, zone, multiproportional mixing observed. Periodical stable isotope composition particular are caused by time-related change location complicated structure Persistently anomalous near surface still observed several-years-long table, seem indicate continuous supply through pervious dislocations. Such an interpretation confirmed extreme value detected site cessation piston” effect. It was presumably secondary intensification flux deep sources, triggered neotectonic movements area.