Prediction of hydro-geochemical characteristics in a deep groundwater at the coastal area

In terms of hydrological and biophysical cycling in nature, groundwater characteristics in a coastal area have attached attention over recent decades. Previous hydrological studies as the represented by submarine groundwater discharge (SGD) studies have gradually clarified the interaction between present seawater and shallow groundwater. On the other hands, for the deep groundwater, there is little available information such as the groundwater flow system, chemical formation process, and relationship with fossil water. To understand groundwater flow system and geochemical characteristics in a coastal area, large scale core drilling up to a depth of 1000 m is carried out at Hamasato area, Horonobe town of Hokkaido, Japan. The drilling site is located 300m apart from the shoreline. The geology is characteristic by unconsolidated sandstone and half-consolidated siltstone. The sandy layer is predominant up to 500m depth, while silty layer is predominant below 500m depth. Geochemical analysis for the inorganic ion chemistry and stable isotopic composition (deuterium and oxygen-18) of pore water samples extracted from the geological core was also conducted at the laboratory. Vertical profile of geochemical data suggests the existence of five hydrological zones with different groundwater flow system up to a depth of 1000m. The chloride concentration and isotopic data indicate that a diffusion phenomenon has been occurred between 502 and 782m bgs, and connate water with high chloride concentration and heavy isotopic composition exist below 782m bgs. Furthermore, the high correlation between Br- and NH4+ indicate that methanation process contributed to water chemistry formation mechanism of deep groundwater in study area. From these geological and geochemical data, it is considered that the groundwater boundaries are not necessary correspond to geological boundaries and various processes contribute to the decision of water chemistry.