Abstract In a volcanic-sedimentary aquifer in central Italy, we investigate the co-existence of arsenic and fluoride in groundwater, aiming at identifying the most probable processes deductible at regional/groundwater body scale leading to the observed co-contamination in groundwater. Further, the areas at risk for human health where high concentrations can produce a significant risk to human health have been investigated. The study area is located in Latium (Central Italy) where silica-undersaturated alkali-potassic formations of Plio-Pleistocene age largely outcrop above marine and continental sand and clay deposits (Neogene) and continental alluvial deposits (Lower Pleistocene–Middle Pleistocene). Geochemical data from groundwater at 322 wells and 76 springs have been analyzed through statistical methods including clustering/PCA and geostatistical analysis. The results show exceedances of the drinking water standards for F (1.5 mg/L) and As (10.0 μg/L) in 29% and 55% of the sampled groundwater, respectively. Multivariate statistics suggest a widespread process of water-rock interaction with the K-alkaline volcanic formations releasing As, F, K, Si, V, Rb and PO4 to the groundwater. As and F show a good correlation (Pearson's r = 0.61, Spearman's rs = 0.59) and define a separate PCA component, confirming that their background in groundwater might be governed by a common process. Kriging interpolations have been used to study the spatial distribution of the two parameters, identifying areas with the highest concentrations and highest probability of exceeding the standards for human consumption. Moreover, by resampling the As-F data with the jackknife technique it was possible to identify the variations of their correlation index in the study area, due to specific As or F anomalies. While in the peripheral areas of the volcanic districts, dominated by sedimentary deposits, the As-F correlation index does not present important fluctuations, Indicator Kriging shows specific As or F correlation anomalies within the volcanic groundwater bodies and along the Tyrrhenian coastline. These anomalies seem to correspond to the zones with the highest thermal flux and/or are located near important structural lineaments. Fluoride correlation anomalies close to mining sites (fluorite) have also been observed. We hypothesize that, unlike the regional co-contamination, these local anomalies are related to the upwelling of geothermal fluids along fracture/fault systems that mix with cold groundwater, or to the interaction with mineral deposits particularly enriched of these elements.

中文翻译：

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地下水中的砷-氟化物共污染：意大利中部火山-沉积含水层的背景和异常

摘要 在意大利中部的火山沉积含水层中，我们调查了地下水中砷和氟化物的共存情况，旨在确定在区域/地下水体尺度上最有可能导致观察到的地下水共污染的过程。此外，已经调查了高浓度会对人类健康产生重大风险的人类健康风险区域。研究区位于 Latium（意大利中部），上新世-更新世的二氧化硅不饱和碱-钾地层主要出露在海相和陆相砂和粘土沉积物（新近纪）和陆相冲积沉积物（下更新世-中更新世）之上。通过聚类/PCA和地质统计分析等统计方法分析了322口井和76个泉水的地球化学数据。结果显示，抽取的地下水中，分别有 29% 和 55% 的 F（1.5 mg/L）和 As（10.0 μg/L）超过了饮用水标准。多变量统计表明，水-岩相互作用与 K 碱性火山岩层相互作用的广泛过程将 As、F、K、Si、V、Rb 和 PO4 释放到地下水中。As 和 F 显示出良好的相关性（Pearson 的 r = 0.61，Spearman 的 rs = 0.59）并定义了一个单独的 PCA 组件，确认它们在地下水中的背景可能受一个共同的过程控制。克里金插值已被用于研究两个参数的空间分布，确定浓度最高和超过人类消费标准的可能性最高的区域。此外，通过使用折刀技术对 As-F 数据进行重新采样，可以识别研究区域中由于特定 As 或 F 异常而导致的相关指数的变化。而在以沉积沉积为主的火山区外围地区，As-F相关指数没有出现明显波动，Indicator Kriging显示火山地下水体内部和第勒尼安海岸线沿线存在特定的As或F相关性异常。这些异常似乎对应于具有最高热通量的区域和/或位于重要的构造线附近。还观察到靠近矿区（萤石）的氟化物相关异常。我们假设，