Chemical and isotopic characteristics of stagnant water isolated in a coastal area

Groundwater was investigated at the Kushiro Coal Mine after the mining area was extended by up to 8.5 km off the Pacific shore and 700 m below m.s.l. Three different types of water were found in the mine. The first is freshwater, which flows from the land and is present in the shallow mining area down to 150 m depth. The second is saline water with chloride ranging 5 000-22 000 mg/L, which was found in water drops from the tunnel ceilings throughout the entire undersea mining area and has the chemical properties of altered present-day seawater. The third is estimated to be fossil seawater; it is very saline water from boreholes, and has homogeneous chemical and isotopic properties. Based on the ratio of 36 Cl/Cl, the altered seawater had been in the Cretaceous formations and isolated from groundwater-seawater mixing for more than 2 million years, despite being located in a coastal area.