Abstract

Volcanic aquifers have become valuable resources for providing water to approximately 2.5 million people in the Yogyakarta-Sleman Groundwater Basin, Indonesia. Nevertheless, hydrogeochemical characteristics at the basin scale remain poorly understood due to the complexity of multilayered aquifer systems. This study collected sixty-six groundwater samples during the rainy and dry seasons for physicochemical analysis and geochemical modeling to reveal the hydrogeochemical characteristics and evolution in the Yogyakarta-Sleman Groundwater Basin. The results showed that groundwater in the unconfined and confined aquifers exhibited different hydrogeochemical signatures. The Ca–Mg–HCO₃ facies dominated groundwater from the unconfined aquifer. The groundwater facies evolved into a mixed Ca–Mg–Cl type along the flow direction towards the discharge zone. Meanwhile, groundwater from the confined aquifer showed mixed Ca–Na–HCO₃, Na–HCO₃, and Na–Cl–SO₄ facies. The presence of Mg in the confined aquifer was replaced by Na, which was absorbed in the aquifer medium, thus showing the ion exchange process. The main geochemical processes can be inferred from the Gibbs diagram, where most groundwater samples show an intensive water–rock interaction process mainly influenced by the weathering of silicate minerals. Additionally, only groundwater samples from the confined aquifer were saturated with certain minerals (aragonite, calcite, and dolomite), confirming that the groundwater followed the regional flow system until it had sufficient time to reach equilibrium and saturation conditions. This study successfully explained the hydrogeochemical characteristics and evolution of a multilayer volcanic aquifer system that can serve as a basis for groundwater basin conservation.

中文翻译：

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印度尼西亚日惹-斯勒曼地下水盆地多层火山含水层水文地球化学演化及水岩相互作用过程

摘要

火山含水层已成为为印度尼西亚日惹-斯勒曼地下水盆地约 250 万人供水的宝贵资源。然而，由于多层含水层系统的复杂性，盆地尺度的水文地球化学特征仍然知之甚少。本研究在雨季和旱季收集了 66 个地下水样本，进行物理化学分析和地球化学建模，以揭示日惹-斯勒曼地下水盆地的水文地球化学特征和演化。结果表明，非承压含水层和承压含水层中的地下水表现出不同的水文地球化学特征。Ca-Mg-HCO ₃相主要来自无承压含水层的地下水。地下水相沿流向排泄区演化为Ca-Mg-Cl混合相。同时，承压含水层地下水呈现Ca-Na-HCO ₃、Na-HCO ₃和Na-Cl-SO ₄混合相。承压含水层中的 Mg 被 Na 取代，Na 被吸收到含水层介质中，从而显示出离子交换过程。主要的地球化学过程可以从吉布斯图推断出来，大多数地下水样品显示出强烈的水-岩相互作用过程，主要受硅酸盐矿物风化的影响。此外，只有来自承压含水层的地下水样品被某些矿物质（霰石、方解石和白云石）饱和，证实地下水遵循区域流动系统，直到有足够的时间达到平衡和饱和条件。这项研究成功地解释了多层火山含水层系统的水文地球化学特征和演化，可以作为地下水盆地保护的基础。