Using hydrogeochemical indicators to interpret groundwater flow and geochemical evolution of a freshwater lens on Majuro Atoll, Republic of the Marshall Islands

Yoshimoto, Shuhei; Ishida, Satoshi; Kobayashi, Tsutomu; Koda, Kazuhisa; Tsuchihara, Takeo; Shirahata, Katsushi

Using hydrogeochemical indicators to interpret groundwater flow and geochemical evolution of a freshwater lens on Majuro Atoll, Republic of the Marshall Islands | Utilisation d’indicateurs hydrogéochimiques pour interpréter l’écoulement des eaux souterraines et l’évolution géochimique d’une lentille d’eau douce sur l’atoll de Majuro, République des îles Marshall Utilización de indicadores hidrogeoquímicos para interpretar el flujo de agua subterránea y la evolución geoquímica de una lente de agua dulce en el atolón de Majuro (República de las Islas Marshall) 利用水文地球化学指标解释Marshall群岛共和国Majuro 礁淡水透镜体的地下水流动和地球化学演化水文地球化学的指標を用いたマーシャル諸島共和国マジュロ環礁の淡水レンズにおける地下水流動と水質進化の検討 Usando indicadores hidrogeoquímicos para interpretar o fluxo de águas subterrâneas e a evolução geoquímica de lentes de água doce no Atol Majuro, República das Ilhas Marshall

2020

Groundwater flow and the geochemical evolution of a freshwater lens in an aquifer on a Pacific atoll were investigated by hydrogeochemical surveys. Sulfur hexafluoride measurements showed that deeper groundwater and groundwater at the periphery of the lens are older, consistent with a downward and outward groundwater flow scheme. This is the typical flow scheme on Pacific atolls where a Holocene–Pleistocene unconformity restricts the shape of the freshwater lens. Enrichment of Mg/Ca in the groundwater is another indicator of a longer residence time, because contact between the groundwater and the carbonate sediments composing the aquifer leads to the release of Mg from high-Mg calcite and the precipitation of Ca as low-Mg calcite. Groundwater quality was also affected by anthropogenic nitrogen loading and aboveground organic matter, which were altered by denitrification and sulfate reduction in the aquifer, especially in the older groundwater. The chemical composition of the groundwater in the center of the island, where saline water is up-coning, implies that freshwater recharge dilutes the older saline water, which as time passes will eventually be replaced by newly recharged freshwater.

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