GEOCHEMICAL PROCESSES CONTROLLING BRACKISH THERMAL WATERS AT A COASTAL AREA (BUSAN CITY, SOUTH KOREA) FROM COMBINED HYDROCHEMICAL AND ISOTOPIC DATA, AND THERMODYNAMIC MODELING

Brackish thermal waters (up to 12 g/L and 68°C) have been observed from a coastal area of Busan city on the southeastern tip of Korean peninsula. In this study we evaluated a variety of geochemical processes controlling the hydrochemistry of thermal waters by using chemical and stable isotopic data (δ¹⁸O, δD, ³H, δ³⁴S, δ¹³C and ⁸⁷Sr/⁸⁶Sr). The observed brackish compositions are the result of seawater inflow into thermal waters, with the decreasing seawater fractions toward inland (from 0.4 to 0.02). Hydrogen and oxygen isotopic data of water indicate that deep groundwater from an underlying heat source ascends through the seawater/groundwater interface. During the upflow, significant modifications of water chemistry in addition to simple mixing occurred. Calculations of the mass transfer, coupled with stable isotope data (δ³⁴S, δ¹³C, and ⁸⁷Sr/⁸⁶Sr), suggest that a number of water-rock interactions (i.e., silicates weathering, precipitation of carbonate/sulfate minerals, cation exchange, and sulfate reduction) also occur in thermal waters. Based on geochemical modeling using the PHREEQC code with LLNL thermodynamic database, we could reproduce the observed salinity front. The results could quantitatively demonstrate major geochemical processes as follows: 1) plagioclase hydrolysis intrinsically controlling compositions of thermal waters, in conjunction with the precipitation of secondary clays (Ca-montmorillonite and calcite), during deep circulation of fresh groundwater down to the underlying heat source (~150 °C) in granitic rocks, and 2) significant hydrochemical modification at the initial stage of seawater intrusion via several processes such as cation exchange, gypsum precipitation (and sulfate reduction), and dolomitization; this process possibly occurred at low temperatures. Flushing and cooling back of saline thermal waters whose chemistry were controlled by the above mentioned processes also occur by recently recharged fresh groundwater.