VERTICAL PROFILE OF PORE WATER CHEMISTRY TO 1000m DEPTH AT A COASTAL AREA
The drilling site is on an old sandbar, 5m above sea level, located 300m apart from the shoreline. The geology is characterized by thick layers of unconsolidated sandstone and half-consolidated siltstone since 3.0Ma.
Low chloride concentration of pore water (< 100 mg/L) suggests the existence of a primary aquifer in 75 - 345m bgs, which seems to be confined by shallower Holocene deposit. The lighter stable isotopes indicate that the water in the aquifer recharged during last glacial age. Below the 345m bgs, chloride concentration rises up to maximum 3100mg/L but decrease again toward 469 - 502m bgs (min. 420 mg/L). This suggests the existence of an impermeable layer and an underlying secondary aquifer. The stable isotopes in these layers are heavier than those of primary aquifer, and therefore the groundwater age might be older than last glacial age. The chloride concentration increases linearly between 502-782m up to 16800 mg/L. Below 782m bgs, the concentration stays around 17000 mg/L or less. The high chloride concentration, 16800 mg/L is close to that of recent sea water but lower d-value. From these results, it is considered that the geologic layer below 782m bgs contains a kind of connate water and the chloride ion has been diffusing toward 502m bgs.
The groundwater age of primary aquifer obtained by stable isotopes was supported by particle tracking model (MODPATH). Constructed model was simple but based on the results of electromagnetic and seismic explorations and some hydraulic properties reported. With respect to the profile of chloride concentration in diffusion zone, between 502-782m bgs, the calculation using 1D Fick’s second low found that the effective diffusion coefficient from 1.0×10-10 to 1.0×10-9 m2/s are the best for fitting to the actual profile, assuming the diffusion started since 1.3 Ma, which is the age of geologic layer at 502m bgs.