Abstract Low-enthalpy geothermal systems are widely spread around the world. Their exploitation for geothermal energy requires an accurate knowledge of the system. Detailed hydrogeochemical characterization is of pivotal importance since geothermal systems are often associated to the occurrence of toxic gases and elements which may contaminate any connected water resources. In this study we demonstrate how increased knowledge of a low-enthalpy geothermal system (Cimino-Vico, Central Italy) can be acquired from the analysis and interpretation of major chemistry and 87Sr/86Sr of local spring waters and groundwaters. With a model-based approach, we assess the main processes governing the major ion composition and arsenic (As) mobility in the system. The occurrence of high concentration of arsenic in the groundwater of the study area is a severe problem for the inhabitants that use the resource for domestic purposes. The system's hydrogeology consists of a shallow highly permeable aquifer, composed of alkaline-potassic volcanic rocks and characterized by fresh waters, a semi-confining layer at the base of the fresh water aquifer, and a deeper thermal aquifer in Mesozoic carbonates and Triassic evaporites. Upwelling of hot waters (up to 63 °C) to the shallow aquifer is related to the presence of faults and fractures in the semi-confining layer. The major chemistry of the deep thermal waters was found to be controlled by dedolomitization, while the fresh waters chemistry is governed by the interaction with the volcanic rocks and the mixing with the upwelling CO2-rich thermal waters. The outcomes of geochemical modeling are consistent with a conceptual model positing that arsenic bound to iron-(hydr)oxides becomes mobilized in the shallow, volcanic aquifer when thermal waters ascend into such shallow aquifer, where they promote the desorption of arsenic in favor of bicarbonate.

中文翻译：

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低焓热液系统中水文地球化学和砷迁移率的模型解释

摘要 低焓地热系统在世界范围内广泛分布。他们对地热能的开发需要对系统有准确的了解。详细的水文地球化学表征至关重要，因为地热系统通常与可能污染任何相关水资源的有毒气体和元素的出现有关。在这项研究中，我们展示了如何通过对当地泉水和地下水的主要化学成分和 87Sr/86Sr 的分析和解释来获得对低焓地热系统（意大利中部 Cimino-Vico）的更多了解。通过基于模型的方法，我们评估了控制系统中主要离子组成和砷 (As) 迁移率的主要过程。研究区地下水中高浓度砷的出现对于将资源用于生活的居民来说是一个严重的问题。该系统的水文地质包括浅层高渗透性含水层，由碱性钾火山岩组成，以淡水为特征，淡水含水层底部的半封闭层，以及中生代碳酸盐岩和三叠纪蒸发岩中的较深热含水层。热水（高达 63 °C）上升到浅层含水层与半封闭层中断层和裂缝的存在有关。发现深部热水的主要化学成分受脱云石化控制，而淡水化学成分受火山岩的相互作用以及与上涌的富含 CO2 的热水的混合控制。