The objective of this paper is to propose my insight into the relationship between “environmental geochemistry” and “environmental chemistry” via introduction of my consecutive researches. Although both research field uses the term “environment,” “environmental geochemistry” is a fundamental science while “environmental chemistry” is an applied science. I started my research career from environmental geochemistry of arsenic in groundwater, then worked for various different topics in environmental chemistry laboratory: (i) manganese and arsenic dynamics in the Lake Biwa, (ii) application of advanced speciation method for environmental monitoring, and (iii) bioaccumulation of trace elements in high predator marine organisms. What I realized through couple of studies is the goal of environmental geochemistry and environmental chemistry are completely different. However, I also realized that education of environmental geochemistry must be fruitful to supply skilled people to environmental chemistry. Bridging scientists probably play crucial role to sustain both academic fields into the future.

The δ¹⁸O_sw (seawater δ¹⁸O) and its relationship with salinity are principal information for analyzing mixing of water masses in the ocean, as well as reconstructing paleo-salinity from biogenic carbonate. Although δ¹⁸O_sw–salinity equations have been reported in the Sea of Japan, it's unclear whether such equations are applicable in the Tsushima Current area. Here, we report 74 δ¹⁸O_sw data in the Tsushima Current area. For shallow seawater samples (≤100 m),data were plotted along the mixing line that connects High Salinity Tsushima Current Water (HSTCW) and low-salinity/low-δ¹⁸O_sw water. In the Tsushima Strait, the source of low-salinity/low-δ¹⁸O_sw water was the Changjiang Diluted Water (CDW).Yet, in the Sea of Japan, the Tsushima Current surface waters exhibited more δ¹⁸O-depleted freshwater sources than the CDW (δ¹⁸O_sw=-9.1+0.27×Salinity).Analysis of the freshwater budget with their δ¹⁸O values in the Tsushima Current surface water reveals that terrestrial water inputs from the Japanese Islands, precipitation, and evaporation were attributable to more δ¹⁸O-depleted freshwater sources. Further, we found that a mixing model for HSTCW, freshwater, and Northern Japan Sea Surface Water can be useful for analyzing provenance and mixing ratios of each surface water mass in the Sea of Japan.

Chemical compositions previously obtained from 333 hot springs in Ishikawa Prefecture were firstly examined for the geochemical characteristic. The Cl^- concentration varies from 9 to 17,490 mg/L. Two geothermometers (Na⁺/K⁺ratio and SiO₂ concentration) have been applied to estimate the subsurface temperatures with the chemical data and show that there are several high temperature areas (>150℃).Secondly, 29 hot springs and three river waters were newly collected from selected areas and analyzed for their hydrogen and oxygen isotopic compositions to also examine the source. The chemical and isotopic compositions indicate that the hot spring waters are a mixture of meteoric water, sea water (fossil sea water in a Green Tuff region) and volcanic fluid. The high temperature geothermal resources on the basis of the estimated subsurface temperature (ca. 200℃) may exist in the Hakusan area, the southern part of Ishikawa. The estimated geothermal resources in this area are ca. 60 MW. During geothermal power generation in this area, calcite is expected to precipitate in the reservoir and production wells according to the saturation indices for the hot spring waters and should be prohibited the precipitation.

The long-term safety of geological disposal systems for high-level radioactive wastes will depend in part on the retention behavior of any elements that are released from the waste in the surrounding host rocks. The retention of rare earth elements (REE),thorium (Th),and uranium (U) in deep sedimentary formations of the Horonobe area, Hokkaido, Japan (the Koetoi and Wakkanai Formations) was investigated in the present investigation as a case study. The REE and Th in the Koetoi and Wakkanai Formations appear to be retained in clay, aluminosilicate and phosphate minerals. The distribution of these elements is relatively homogeneous in both formations. Uranium was sorbed by organic matter or was precipitated in secondary minerals under reducing conditions during initial sedimentation and/or later diagenesis. Changes in hydrogeological or rock facies conditions appear to have no impact on the retention behavior of the REE, Th, and U. These observations suggest that such retention behavior can be effective over very long periods of time. The results of the present study are thus consistent with those of previous investigations, which indicate that sedimentary formations in the Horonobe area have effectively attenuated radionuclide transport due to a reducing environment in a chemically closed system.