Using hydrogeochemical indicators to interpret groundwater flow and geochemical evolution of a freshwater lens on Majuro Atoll, Republic of the Marshall Islands
Using hydrogeochemical indicators to interpret groundwater flow and geochemical evolution of a freshwater lens on Majuro Atoll, Republic of the Marshall Islands
复制标题
DOI：
10.1007/s10040-019-02105-w
复制
发表时间：
2020-01
影响因子：
2.8
通讯作者：
Shuhei Yoshimoto;S. Ishida;Tsutomu Kobayashi;Kazuhisa Koda;T. Tsuchihara;K. Shirahata
中科院分区：
文献类型：
--
作者：
Shuhei Yoshimoto;S. Ishida;Tsutomu Kobayashi;Kazuhisa Koda;T. Tsuchihara;K. Shirahata
通过水文地球化学调查研究了太平洋环礁含水层中的地下水流和淡水透镜的地球化学演化。六氟化硫的测量结果表明，更深的地下水和透镜外围的地下水更古老，与向下和向外的地下水流动方案一致。这是太平洋环礁上的典型流程图，其中全新世-更新世不整合面限制了淡水透镜的形状。地下水中Mg/Ca的富集是较长停留时间的另一个指标，因为地下水和构成含水层的碳酸盐沉积物之间的接触导致Mg从高Mg方解石中释放，Ca作为低Mg方解石沉淀。地下水质量也受到人为氮负荷和地上有机物的影响，而这些氮负荷和有机物又因含水层中的反硝化作用和硫酸盐还原作用而发生变化，特别是在较老的地下水中。岛中心地下水的化学成分表明，淡水补给稀释了较老的盐水，随着时间的推移，这些盐水最终将被新补给的淡水所取代。
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.