Deep groundwater flow in the Kamikita Plain, Japan, inferred from geochemical tracers including 36Cl

Long-term changes in deep groundwater flow in a coastal sedimentary plain affected by sea-level changes were investigated using a paleohydrological approach with 36Cl. The 36Cl/Cl ratios of the saline component in the Na-Cl type deep saline/brackish waters extensively distributed inland were found to be comparable to the secular equilibrium value of the aquifer. On the other hand, younger saline/brackish waters with low 36Cl/Cl are distributed in coastal areas, particularly in the deep aquifer. Deep freshwaters of the Na-Cl to Na-HCO3 type found at slightly shallower depth than −1000 m a.s.l. have δD values ∼10 ‰ lower than recent precipitation, suggesting that they were recharged during a colder period than present. The presence of low-δD deep freshwaters inland and the distribution of relatively young saline/brackish waters in coastal areas both indicate that older saline water was flushed out by meteoric water during the glacial period, but to a limited extent and area.