The dataset contains details of field collection of groundwater samples with use of different water intake devices and the measurement results of gaseous compounds (methane) obtained during analytical method validation performed in order to develop a methodology of groundwater sampling for analysis of dissolved gases. The dataset is not intended to be used for any site characterisation. Sampling sites were chosen based on high probability of occurrence of measureable methane content in groundwater. Furthermore, the data will be used for formal procedure to obtain the methodology accreditation from the Polish Centre for Accreditation (PCA). The dataset was created within SECURe project (Subsurface Evaluation of CCS and Unconventional Risks) - https://www.securegeoenergy.eu/. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 764531