29. Existence of Stagnant Fresh Groundwater and Diffusion-Limited Chloride Migration in a Sub-Sea Formation at Yatsushiro Bay, Japan

We attempted to evaluate long-term behaviour of saline groundwater by analyzing chloride concentration and chlorine isotopic ratios of porewaters obtained from the sub-sea formation at Yatsushiro Bay, southwest Japan. Chloride concentrations and stable chlorine isotopic ratios were measured on 13 porewater samples. Porewaters with chloride concentrations higher than 16400 mg/L are found at depths shallower than 1.5 metres below sea floor (m b.s.f.). Chloride concentrations decrease downwards gradually, and become lower than 250 mg/L below 7.7 m b.s.f. The stable chlorine isotopic ratios show a minimum value of –1.27% at 5.5m b.s.f. and those from other depths show minor fluctuation, from –0.45% to 0.11%. From these results, diffusion is considered to be the dominant process for the transport of chloride at the location studied.