威尼斯平原地下含水层中的砷:地球化学模型和砷硫化物矿物的出现,对布伦塔中部地区(意大利)数据的审查

Fabio Tateo, Paolo Fabbri, M. C. Dalconi, Luca Peruzzo
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引用次数: 1

摘要

威尼斯平原是众所周知的地下水砷(As)浓度较高的地区(最高超过 400 微克/升;在某些地区甚至高达 647 微克/升)。我们选择了帕多瓦北部的一个研究地区,该地区具有与大多数西方国家相似的典型居住、工业和农业特征,缺乏热液、火山或人为砷来源。试点地区是多项研究的重点,本说明将对这些研究进行回顾。这些研究的目的是核实地下水和沉积物中砷浓度的分布情况(对地下水沉积物以及过滤和未过滤地下水进行矿物学和地球化学分析),并模拟水与岩石相互作用产生的砷流动性。含水层储层的粒度包括砾石、砂、粉砂和粘土。与世界其他平原相比,研究区域含水层沉积物中的有机物含量似乎比较特殊(较高);它会影响氧化还原电位和地下水中砷的相对浓度。该地区地下水中的砷污染和氧化还原条件差异很大。氧化和高度还原条件下的地下水与中等还原条件下的地下水相比,砷浓度要低得多。含水层沉积物中存在砷矿物质(如雷汞-帕拉雷汞),在意大利平原地区首次通过不同的分析技术记录了研究材料中的砷矿物质。由于这些矿物在全世界未受火山或热液活动影响的平原沉积物中都很罕见,因此它们的出现是威尼斯平原含水层的一个显著特征。在研究区域的泥炭沉积物中发现了这些砷矿物,这与地球化学建模的结果一致,即它们需要在高度还原的条件下才能从地下水中析出。建模结果表明,在氧化和轻微还原条件下(从 200 mV 到 -50 mV),砷被吸附在固相上,但氧化还原电位的进一步降低会导致砷从固相上解吸,从而造成地下水污染(从 -50 mV 到 -250 mV)。如果氧化还原电位变得更负(低于 -250 mV),地球化学条件将有利于砷硫化物的形成。根据地球化学模型的预测,雄黄-赤松石相的沉淀是通过从地下水中提取砷进行的,这也从数量上说明了在研究区高度还原的地下水中测得的砷浓度较低的原因。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Arsenic in the groundwater aquifers of the Venetian Plain: geochemical modelling and occurrence of As-sulfides minerals, a review of data from the medio Brenta domain (Italy)
The Venetian Plain is known for areas with high concentrations of arsenic (As) in groundwater (up to more than 400 μg/L; exceptionally 647 μg/L, in selected areas). A study area was chosen, north of Padua, which exhibits typical residential, industrial, and agricultural characteristics similar to most Western countries and lacks hydrothermal, volcanic, or anthropogenic sources of arsenic. The pilot area was the focus of several studies which are reviewed in this note. The objectives of the studies were to verify the distribution of As concentrations in groundwater and sediments (mineralogical and geochemical analysis of groundwater sediments and of filtered and unfiltered groundwater) and to model the mobility of arsenic arising from water-rock interaction. The grain size of aquifer reservoirs includes gravel, sand, silt, and clay. The amount of organic matter in the aquifer sediments of the study area seems peculiar (higher) compared to other plains in the world; it influences the redox potential and the relative concentration of As in groundwater. Arsenic contamination in groundwater and redox conditions varied greatly in the area. Groundwater under oxidizing and highly reducing conditions had much lower arsenic concentrations compared to groundwater under intermediate reducing conditions. Arsenic minerals (such as realgar-pararealgar) occur in aquifer sediments and they were documented in the studied materials by different analytical techniques for the first time in the context of the Italian plains. Since these minerals are rare throughout the world in plain sediments not affected by volcanic or hydrothermal activity, their occurrence is a distinctive feature of the Venetian Plain aquifer. These arsenic minerals were found in peat sediments of the study area, consistent with geochemical modeling results, which require highly reducing conditions for their precipitation from groundwater. Modeling suggests that under oxidizing and up to slightly reducing conditions (from 200 mV to -50 mV), arsenic is adsorbed on solid phases, but a further decrease in redox potential leads arsenic desorption from solids and consequent groundwater contamination (from -50 mV to -250 mV). If the redox potential becomes even more negative (below -250 mV), geochemical conditions are favorable to the formation of arsenic sulfides. The precipitation of the realgar-pararealgar phases, predicted by the geochemical model, proceeds by extracting arsenic from the groundwater and quantitatively accounts for the lower arsenic concentration measured in the highly reducing groundwater of the study area.
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