Abstract The Mount Vulture basin, which mainly consists of pyroclastic and subordinate lava flow layers, is one of the most important aquifers for drinking water and irrigation supply in southern Italy. In this study, we investigated the geochemical behaviour of the rare earth elements (REEs) in the groundwater of this aquatic system, assessing fractionation patterns and performing speciation calculations to elucidate the geochemical processes affecting the REEs' distribution. The groundwater collected was separated into two subsets corresponding to recharge and discharge water. Overall, the groundwater exhibited REE concentrations well below 1000 ng/l, and the average content measured in the discharge water is approximately three times higher than that of the recharge water. In the recharge water, the shale normalised patterns are characterised by a more negative average Ce anomaly, a more positive average Eu anomaly, and a larger average (La/Yb)PAAS. The observed relationship between Ce anomalies and dissolved oxygen (DO) supports the idea that Ce oxidative scavenging has a role in determining the size and shape of the Ce anomaly. Further, the less negative average size of the Ce anomaly in the discharge zone also suggests Ce desorption due to changes in redox conditions. As for the Eu anomalies, the largest Eu* are associated with the recharge zone, characterised by higher average Eh and DO values. The volcanics in the Mt. Vulture lithologies have post-Archean average shale (PAAS) normalised patterns showing positive Eu anomalies due to feldspars occurrence. This indicates that the Eu* of groundwater in the recharge zone is mostly produced by the dissolution of feldspars. In the discharge zone, most samples are in equilibrium with smectite and Eu2+ adsorption onto the clay phase surface or its inclusion in the mineral structure as exchangeable cations likely lead to the smaller average size of Eu*. The possible occurrence of secondary minerals preferentially scavenging light REEs may also explain the difference in the (La/Yb)PAAS index observed between the recharge and discharge zones. Speciation calculations indicate that dissolved REEs consist mainly of carbonato complexes (LnCO3+) whereas the percentage of dicarbonato complexes (Ln(CO3)2−) increases in samples with circa-neutral pH. At acidic pH, the Ln3+ species are numerous and, in samples with large amounts of dissolved sulphate, derived by feldspathoids dissolution, from the discharge zone, the percentage of LnSO4+ species is not negligible.

中文翻译：

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意大利南部秃鹰山火山地下水中稀土元素分布及地球化学行为

摘要 秃鹫山盆地主要由火山碎屑层和次级熔岩流层组成，是意大利南部最重要的饮用水和灌溉供应含水层之一。在这项研究中，我们调查了该水生系统地下水中稀土元素 (REE) 的地球化学行为，评估了分馏模式并进行了形态计算，以阐明影响 REE 分布的地球化学过程。收集的地下水被分成两个子集，分别对应补给水和排放水。总体而言，地下水的 REE 浓度远低于 1000 ng/l，排放水中测得的平均含量约为补给水中的三倍。在补给水中，页岩归一化模式的特征是更负的平均 Ce 异常、更正的平均 Eu 异常和更大的平均 (La/Yb)PAAS。观察到的 Ce 异常与溶解氧 (DO) 之间的关系支持 Ce 氧化清除在确定 Ce 异常的大小和形状中起作用的观点。此外，放电区中 Ce 异常的负平均尺寸较小也表明由于氧化还原条件的变化，Ce 解吸。对于 Eu 异常，最大的 Eu* 与补给区有关，其特点是平均 Eh 和 DO 值较高。山中的火山。秃鹰岩性具有后太古代平均页岩 (PAAS) 归一化模式，显示出由于长石的存在而导致的正 Eu 异常。这表明补给区地下水的Eu*主要是由长石溶解产生的。在放电区，大多数样品与粘土相表面上的蒙脱石和 Eu2+ 吸附或将其包含在矿物结构中处于平衡状态，因为可交换的阳离子可能导致 Eu* 的平均尺寸较小。次生矿物优先清除轻稀土元素的可能性也可以解释补给区和放电区之间观察到的 (La/Yb)PAAS 指数的差异。形态计算表明溶解的 REE 主要由碳酸根络合物 (LnCO3+) 组成，而二碳酸根络合物 (Ln(CO3)2−) 的百分比在 pH 值约为中性的样品中增加。在酸性 pH 值下，Ln3+ 种类很多，并且在含有大量溶解硫酸盐的样品中，