Abstract The origin of the combustible gases in groundwater from glacial-outwash and fractured-bedrock aquifers was investigated in northern Tioga County, Pennsylvania. Thermogenic methane (CH 4) and ethane (C 2H 6) and microbial CH 4 were found. Microbial CH 4 is from natural in situ processes in the shale bedrock and occurs chiefly in the bedrock aquifer. The δ 13C values of CH 4 and C 2H 6 for the majority of thermogenic gases from water wells either matched or were between values for the samples of non-native storage-field gas from injection wells and the samples of gas from storage-field observation wells. Traces of C 2H 6 with microbial CH 4 and a range of C and H isotopic compositions of CH 4 indicate gases of different origins are mixing in sub-surface pathways; gas mixtures are present in groundwater. Pathways for gas migration and a specific source of the gases were not identified. Processes responsible for the presence of microbial gases in groundwater could be elucidated with further geochemical study.