Geochemistry of mineral water and gases of the Razdolnoe Spa (Primorye, Far East of Russia)

•Investigated Na–HCO3 water associated with coalbed methane.•7 isotopes (δ18O, δ2H, δ13CCH4, δDCH4, δ18OCO2, δ13CCO2 and 3H) were analyzed.•The chemical changes of water during methanogenesis are geochemically estimated.•Marine microbial origin of methane.•δ13СCO2 from the Razdolnoe Spa has carbonate reduction genesis.

New isotopic and chemical data on the sodium bicarbonate water and associated gases from the Razdolnoe Spa located in the coastal zone of Primorsky Kray of the Russian Far East, together with previous stable isotope data (δ18O, δD, δ13C), allow elucidation of the origin and evolution of the groundwater and gases from the spa. The water is characterized by low temperature (12 °C), TDS – 2.5–6.0 g/L, high contents of B (∼5 mg/L) and F (4.5 mg/L) and low contents of Cl and SO4. Water isotopic composition indicates its essentially meteoric origin which may comply with an older groundwater that was recharged under different (colder) climatic conditions. Major components of bubbling gases are CH4 (68 vol%), N2 (28%) and CO2 (4%). The obtained values δ13C and δD for CO2 and CH4 definitely indicate the marine microbial origin of methane. Thus the high methane content in the waters relates to the biochemical processes and presence of a dispersed organic matter in the host rocks. Based on the regional hydrogeology and the geological structure of the Razdolnoe Spa, Mesozoic fractured rocks containing Na–HCO3 mineral water and gases are reservoir rocks, a chemical composition of water and gases originates in different environmental conditions.