Hydrogeochemistry of submarine groundwater discharge (SGD) in a volcanic coastal area : Mabini Peninsula, the Philippines

Submarine Groundwater Discharge (SGD) delivers water and transports chemicals to the ocean. It is essential in the global geochemical cycle and coastal water quality. Mabini Peninsula in the Philippines is a volcanic coastal area that has been noted as the center of the Coral Triangle, home to the world’s most biodiverse reef ecosystem. It is important to study SGD in the area that hosts these vital ecosystems. I aim to understand the hydrogeochemistry of SGD and identify the processes behind it. I collected a total of 11 SGD samples at various depths, ranging from the intertidal zone to ~60 m underwater. I find that the springs emit thermal, acidic waters (~50°C, pH ~5.5-6.5) with high dissolved CO₂ (P [subscript CO2] ~14,700-54,000 ppm). The warmer temperature suggests that thermal convection driven by a volcanic heat source is a significant driver of SGD in the area. Meanwhile, high P [subscript CO2] is consistent with magmatic component. The hydrogen and oxygen isotope composition (δD = -24‰ to -2‰ and δ¹⁸O = -2.4‰ to +1.8‰) indicate that the spring waters are a mixture of three end-members: terrestrial groundwater, recirculated seawater, and hydrothermal water. Overall, the SGD waters are rich in major ions compared to the average groundwater (Ca²⁺ = 394.55–550.55 mg/L; K⁺ = 365.12–512.94 mg/L; Na⁺ = 5720.49–10269.01 mg/L; Mg²⁺ = 312.80–1258.55 mg/L, Cl⁻ = 10497.09–18652.77 mg/L, and SO₄²⁻ = 665.41–2585.57 mg/L). The SGD waters are also significantly enriched in Si³⁺ (6.94–87.72 mg/L), As³⁺ (60–3222 µg/L), and B³⁺ (4.04–74.95 mg/L). All three indicate magmatic water contribution. I interpreted major ions using various plots which discriminate processes, including: mixing diagrams with the three end-members and Giggenbach diagram for geothermal solute equilibria. The mixing plots show that most spring waters are outside the mixing triangle, implying the presence of low-temperature water-rock interaction. The geothermal solute equilibria diagram reveals that the spring waters consist of weakly to moderately mineralized solutions. Understanding the unique properties and hydrogeochemistry of SGD water in this site can help us to understand how it impacts and contributes to the biodiverse ecosystems in the Coral Triangle.