The 14C age of groundwater around saltwater-freshwater interface in deep coastal aquifer

Abstract:

This study is focused on determining the groundwater age in mixing zone around saltwater-freshwater interface in deep confined aquifer by ¹⁴C methods. A drilling up to 160m in depth was carried out with sampling geologic cores at a distance of 350m from coastal line. The elevation of the ground surface in the drilling site is about 4m, locates on the foot of Mt. Fuji, the highest mountain in Japan. The geology consists mainly of Quaternary, brittle volcanic rocks and fan deposits, and occasionally silt layers. The mixing zone of saltwater-freshwater was detected under 85m depth at the drilling site. A screen interval of the borehole was set from 151 to 157m depth and sampled the groundwater (hereafter called as FGB-water). The Cl- concentration of the FGB-water was 4,990 mg/L with NaCl type (approximately a quarter of salt water), indicating that FGB-water was taken from landward side of mixing zone around saltwater-freshwater interface. In addition to the drilling, the sampling of groundwater from more than 50 wells in the basin was carried out.

The adjusted ¹⁴C age of FGB-water is estimated at approximately 10,000 years by fitting calculations. The ages of inland groundwater taken from almost same depth to FGB-water are 200-3,000 years. Although it is unclear why the FGB-water is much older than others, a likely explanation is due to the groundwater around saltwater-freshwater interface is moving upward roughly along the landward sloping interface (Cooper, 1964). Such movement would bring deeper and older groundwater to shallow zone.

At the drilling site, the freshwater with low electric conductivity was obtained from surface to 84m depth. And they have much lighter stable isotopes (D and ¹⁸O) compared to those in inland. That is to say, the groundwater recharged at higher elevation distribute at shallow zone at the FGB. This result supports the upward moving of groundwater at the coastal area.