Abstract The groundwater chemistry of the semi-arid volcanic island of Porto Santo, part of the Madeira archipelago, Atlantic Ocean, was investigated. Generally, the groundwater was brackish, containing 2–10 mol % seawater. Groundwater with up to 20 mM alkalinity and a Na enrichment of up to 30 mM, as compared to the Na concentration predicted by the seawater Na/Cl ratio, was found in the main aquifer. Also notable are the high concentrations of F (up to 0.3 mM), B (up to 0.55 mM), As (up to 0.35 μM), all in excess of WHO recommendations, as well as up to 6 μM V. Geochemical modeling, using the PHREEQC code, was used to explore different scenarios that could explain the genesis of the observed bulk groundwater chemistry. First, a model for aquifer freshening with the displacement of resident seawater from the aquifer by infiltrating freshwater, was tested. This scenario leads to the development of NaHCO3 waters as observed in many coastal aquifers. However, the measured alkalinity concentration in the groundwater was far higher than the concentration predicted by the freshening model. In addition, the behavior of modelled pH and PCO2 were at variance with their distributions in the field data. The second model explored the possible effect of volcanic glass leaching on the groundwater chemistry. Using insight derived from studies of volcanic glass surface alteration as well as experimental work on water-volcanic glass interactions, a geochemical model was developed in which the exchange of H+ for Na+ on the volcanic glass surface is the main mechanism but the exchange of other cations on the volcanic glass surface is also included. The uptake of H+ by the glass surface causes the dissociation of carbonic acid, generating bicarbonate. This model is consistent with the local geology and the field data. It requires, however, volcanic glass leaching to occur in the unsaturated zone where there is an unlimited supply of CO2. The exchange reaction of H+ for Na+ is confined to the surface layer of volcanic glass as otherwise the process becomes limited by slow solid state diffusion of H+ into the glass and Na+ out of the glass. Therefore, volcanic ash deposits, with their high volcanic glass surface areas and matrix flow, are the aquifers where this type of high NaHCO3 waters can be expected, rather than in basalts, which predominantly feature fracture flow. The trace components F, B, As and V are believed to originate from hyaloclastites, consisting of predominantly (90%) of trachy-rhyolite volcanic glass. Although stratigraphically older than the main calcarenite aquifer, topographically they are often located at higher altitudes, above the phreatic level and located along the main recharge flow path. In addition, the semi-arid climate conditions provide a long groundwater residence time for the reactions as well as limited aquifer flushing.

中文翻译：

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半干旱大西洋波尔图桑托岛火山玻璃浸出和地下水地球化学

摘要 研究了大西洋马德拉群岛的一部分 Porto Santo 半干旱火山岛的地下水化学。一般来说，地下水是微咸的，含有 2-10 mol% 的海水。在主含水层中发现了碱度高达 20 mM 且 Na 浓度高达 30 mM 的地下水，与海水 Na/Cl 比率预测的 Na 浓度相比。同样值得注意的是高浓度的 F（高达 0.3 mM）、B（高达 0.55 mM）、As（高达 0.35 μM），都超过了世卫组织的建议，以及高达 6 μMV。地球化学建模，使用 PHREEQC 代码，用于探索可以解释观察到的大量地下水化学起源的不同场景。首先，通过渗透淡水将常驻海水从含水层中置换出来的含水层淡水模型，被测试。正如在许多沿海含水层中所观察到的那样，这种情况导致了 NaHCO3 水域的发展。然而，测得的地下水碱度浓度远高于清新模型预测的浓度。此外，模拟的 pH 值和 PCO2 的行为与其在现场数据中的分布存在差异。第二个模型探讨了火山玻璃浸出对地下水化学的可能影响。利用火山玻璃表面蚀变研究以及水-火山玻璃相互作用实验工作的见解，开发了地球化学模型，其中火山玻璃表面上 H+ 与 Na+ 的交换是主要机制，但其他阳离子的交换火山玻璃表面也包括在内。玻璃表面吸收 H+ 导致碳酸分解，生成碳酸氢盐。该模型与当地地质和野外资料一致。然而，它需要在不饱和区域发生火山玻璃浸出，在该区域有无限的 CO2 供应。H+ 与 Na+ 的交换反应仅限于火山玻璃的表层，否则该过程会受到 H+ 进入玻璃和 Na+ 缓慢固态扩散到玻璃外的限制。因此，具有高火山玻璃表面积和基质流的火山灰沉积物是可以预期这种高 NaHCO3 水的含水层，而不是主要以裂缝流为特征的玄武岩中的含水层。微量成分 F、B、As 和 V 被认为来自透明碎屑岩，主要由 (90%) 粗面流纹岩火山玻璃组成。尽管在地层上比主要方钙石含水层更古老，但从地形上看，它们通常位于更高的海拔，高于潜水面，并位于主要补给流路径沿线。此外，半干旱气候条件为反应提供了较长的地下水停留时间以及有限的含水层冲洗。