[U-02] 人新世・第四紀の気候および水循環

[U02-P01] Origins of groundwater in eastern Tongatapu Island, Kingdom of Tonga: Insights from radiocarbon and hydrochemical data

*福與 直人¹、Geoffrey Clark²、平林 頌子³、宮島 利宏³、横山 祐典³ (1.産業技術総合研究所地質調査総合センター、2.オーストラリア国立大学、3.東京大学大気海洋研究所)

キーワード：放射性炭素、地下水、石灰岩

Groundwater constitutes a vital freshwater resource on Tongatapu Island (21.21° S, 175.15° W), within the Kingdom of Tonga. However, this resource is under increasing pressure from overexploitation and pollution. This study examined the dynamics of groundwater in eastern Tongatapu, utilizing radiocarbon concentration (Δ¹⁴C) of dissolved inorganic carbon (DIC) to elucidate groundwater movement, mixing processes, and the origin of groundwater. To supplement the interpretation of radiocarbon data, stable isotopes (δ¹⁸O, δD), and water quality analysis, including major elements such as Ca and Mg, were also conducted. Radiocarbon data revealed a wide range of Δ¹⁴C values (from approximately -150‰ to 30‰). Δ¹⁴C values were generally lower in less saline, low-conductivity waters, indicating a greater influence of old carbon derived from carbonate rocks. Conversely, higher Δ¹⁴C values were observed in more saline waters, reflecting the contribution of modern carbon from seawater. Stable isotope ratios ranged from approximately -5.5‰ to 1‰ for δ¹⁸O and from -30‰ to 5‰ for δD, while Ca concentrations varied from approximately 100 mg/L to 450 mg/L and Mg concentrations from 10 mg/L to 1200 mg/L. These values further supported the findings from radiocarbon data. The findings were further supported by stable isotope ratios and trace element concentrations. Additionally, major elements and radiocarbon in freshwater and lagoon water were found to be less influenced by seasonal variations, while those in seawater showed significant seasonal changes. Major elements, stable isotopes, and radiocarbon generally reflect a mixture of seawater and freshwater, with these two water sources acting as end-members. Finally, the data for tap water and water collected in a nearby cave exhibited strong similarity, suggesting they share a common source, which is consistent with the fact that tap water is pumped from groundwater. These results enhance our understanding of the recharge processes and the current state of water resources on the island.