During the drilling of an exploration well in the 1960s, an underground blowout occurred near Sleen, The Netherlands. During approximately 25 months, near-continuous leakage of large amounts of natural gas was released into the subsurface. After the blowout, the local drinking water production company installed a network of groundwater monitoring wells to monitor for possible adverse effects on groundwater quality at the blowout site. Today, more than 50 years after the blowout, the groundwater is still impaired. Data has been correlated with previously published data by Schout et al. (2018) covering description of geology and well depths. During two fieldtrips (November 2019 & October 2020) water samples were collected from several wells covering: - Bulk gas compositions (methane, ethane, propane, oxygen, nitrogen, CO2, Argon). Bulk isotope compositions of methane (δ13C & δH), carbon dioxide (δ13C) and nitrogen (δ15N). - Methane clumped isotope compositions (ΔCD & ΔDD). - Inorganic parameters (hydrocarbons, anions, cations, DOC, alkalinity, nitrate and ammonium). 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.