A multiple-isotope (δ³⁷Cl, ¹⁴C, ³H) approach to reveal the coastal hydrogeological system and its temporal changes in western Kyushu, Japan

Multiple isotopes were used to reveal the coastal hydrogeological system and its temporal changes, caused by transgression and deposition of marine clay, in a small catchment and its offshore extension in Kyushu, Japan. Chloride concentrations in extracted pore waters from an offshore borehole were higher than 16,400 mg/L at depths shallower than 1.5 meters below sea floor (mbsf), and decreased downwards gradually to become lower than 250 mg/L below 7.7 mbsf. Stable chlorine isotopic ratios were –1.50‰ at 6.2 mbsf and –1.27‰ at 5.6 mbsf, while those from other depths showed minor fluctuation. A one-dimensional diffusion-sedimentation model was successfully applied to explain both chloride and stable chlorine isotopic ratios. Apparent residence times of groundwater below the inter-tidal zone is on the order of 100 years while that below sea bottom is about 2,000 years, indicating that groundwater below the inter-tidal zone constitutes a part of the present-day active groundwater flow system while that below sea bottom moves extremely slowly. The coincidence of the apparent residence time of groundwater below sea-bottom and the age of the start of both deposition of marine clay and diffusion processes strongly suggests that the deposition of marine clay has controlled the hydrogeological system and resulted in the reduction of the extent of groundwater discharge.