Modern and fossil seawater identification using ground- and pore-water dating from the western coast of the Miura Peninsula, Japan

Abstract

Deep groundwater flow in coastal areas has attracted considerable attention in the context of radioactive waste disposal and CO2 storage from transportation point of view. It is important to show the stability of groundwater flow for a long time, which relates to identify modern and fossil seawater in coastal area. Borehole investigation was conducted at a coastal area along the west side of the Miura Peninsula in Japan, where is a typical of the accretionary wedge environments. As part of this investigation, groundwater ages have been evaluated based on dating ground- and pore-water samples (obtained by groundwater pumping and rock core squeezing, respectively) from three boreholes of up to 500 m deep. At the borehole locations, the Miura Group, which consists of sandstone and siltstone, is observed to depths of ∼200 m and the Hayama Group, which primarily consists of mudstone, is observed below ∼200 m. In the Miura Group, 14C was abundant; high-Cl samples showed high 14C and low 36Cl/Cl, equivalent to modern seawater. In the underlying Hayama Group, where the low hydraulic conductivity of the formation means that the assessment was based almost solely on porewater samples, the Cl concentration below a depth of 300 m was close to that of seawater and 36Cl/Cl was close to in situ secular equilibrium (as estimated from rock properties). Furthermore, 4He was abundant in the Hayama Group, and an observed shift in δ37Cl is inferred to have been controlled simply by diffusion. The results indicate that the groundwater in the Miura Group is modified modern seawater, while the porewater in the Hayama Group is fossil seawater. Thus, groundwater dating, using a combination of 14C, 36Cl, and 4He, is a useful tool for the identification of modern and fossil seawater in coastal areas.