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