Estimation of flow dynamics of saline groundwater in a coastal geothermal region based on B–Li–Cl classification and geochemical measurements

Abstract

Coastal geothermal saline groundwater (SGW) in Quaternary volcanic regions represents an important geothermal resource, but its complex chemical compositions often obscure its origin and dynamics. This study aims to elucidate the SGW dynamics in the coastal plain of Hakodate, Hokkaido, in Japan using the B–Li–Cl to delineate water-rock interactions and diagenesis, and to classify SGW type. Major dissolved components and stable isotopes (δ²H, δ¹⁸O) are susceptible to freshwater dilution, limiting their diagnostic utility. The B–Li–Cl composition allowed for the classification of SGW into three types (Type 1, 2, and 3). Types 1 and 2 were influenced by direct seawater contributions and other origins such as diagenesis and volcanic gas, respectively. Type 3, with higher B (Boron) and Li (Lithium) concentrations than the other SGW, is distributed in the geothermal region, and reflects seawater modified by interaction with rocks at 175 ± 25 °C. Integration of chemical equilibrium modeling and geothermometer supports this geochemical evolution of the SGWs. Additionally, hydrogeological analyses have estimated that the flow dynamics of Type 3 waters are recharged through fractures near offshore submarine volcanic vents that migrate to deeper subsurface regions. These findings offer new insights into coastal hydrothermal systems in the study area. The Li and B concentrations serve as effective tracers for elucidating the origin of SGW in coastal geothermal regions, and their application is expected to lead to a more comprehensive understanding of SGW flow dynamics.