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

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 in the coastal plain of Hakodate, Hokkaido, in Japan using the B-Li-Cl dynamics to delineate water-rock interactions and diagenesis, and to classify SGW types. Major dissolved components and stable isotopes (δD, δ 18 O) 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 volcanic 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. • Saline groundwater (SGW) in the Hakodate area, Japan, can be classified into three types using B–Li–Cl geochemical signatures • Type 3 SGW enriched in B and Li linked to volcanic water–rock interaction • Type3 have undergone water-rock-interaction at 175±25°C using combination of multiple geothermometer. • Modelling reveals recharge via fractures near offshore volcanic vents • B and Li serve as effective tracers for SGW evolution in volcanic coastal zones