Study on discrimination methodology to identify the origin and area of influence of deep-seated fluids in terms of dissolved chemicals

Abstract

  For selecting the HLW geological disposal site, it is necessary to evaluate the groundwater chemical and physical stability and inflowing of deep-seated fluids which is idiomatically classified as slab-derived fluid, fossil seawater, and oil-field brine. Among them, the slab-derived fluid is found in some areas in Japan, they often have high temperature, high CO₂ gas, and low pH characters. The features may have adverse impact for geological disposal systems, hence, consolidation of the methodology for finding the fluid is required. However, according to previous studies, the terminology of deep-seated fluids is somewhat ambiguous, and chemical characteristics of each deep-seated fluids does not well organized. In this study, the authors classify the terminology of deep-seated fluids by the formation mechanisms and the reduction of common chemical features of deep-seated fluids are conducted based on the own field surveys over 30 springs and previous studies. The origin of the deep-seated fluids could be classified by dissolved chemicals such as concentration of Cl, He isotope ratio and stable isotope ratio combined with concentrations of other dissolved ions and gases. Dissolved chemicals could be also useful tracer to find out the area of influence of the deep-seated fluid.