首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >Atmospheric noble gases as tracers of biogenic gas dynamics in a shallow unconfined aquifer
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Atmospheric noble gases as tracers of biogenic gas dynamics in a shallow unconfined aquifer

机译:大气稀有气体作为浅层无限制含水层中生物气动力学示踪剂

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摘要

Atmospheric noble gases (NGs) were used to investigate biogenic gas dynamics in a shallow unconfined aquifer impacted by a crude oil spill, near Bemidji, MN. Concentrations of ~(3,4)He, ~(20,22)Ne, ~(36,40)Ar, Kr, and Xe were determined for gas- and aqueous-phase samples collected from the vadose and saturated zones, respectively. Systematic elemental fractionation of Ne, Ar, Kr, and Xe with respect to air was observed in both of these hydrogeologic zones. Within the vadose zone, relative ratios of Ne and Ar to Kr and Xe revealed distinct process-related trends when compared to corresponding ratios for air. The degree of NG deviation from atmospheric concentrations generally increased with greater atomic mass (i.e., ?Xe > ?Kr > ?Ar > ?Ne), indicating that Kr and Xe are the most sensitive NG tracers in the vadose zone. Reactive transport modeling of the gas data confirms that elemental fractionation can be explained by mass-dependent variations in diffusive fluxes of NGs opposite to a total pressure gradient established between different biogeochemical process zones. Depletion of atmospheric NGs was also observed within a methanogenic zone of petroleum hydrocarbon degradation located below the water table. Solubility normalized NG abundances followed the order Xe > Kr > Ar > Ne, which is indicative of dissolved NG partitioning into the gas phase in response to bubble formation and possibly ebullition. Observed elemental NG ratios of Ne/Kr, Ne/Xe, Ar/Xe, and Kr/Xe and a modeling analysis provide strong evidence that CH4 generation below the water table caused gas exsolution and possibly ebullition and carbon transfer from groundwater to the vadose zone. These results suggest that noble gases provide sensitive tracers in biologically active unconfined aquifers and can assist in identifying carbon cycling and transfer within the vadose zone, the capillary fringe, and below the water table.
机译:在明尼苏达州贝米吉附近,使用大气稀​​有气体(NGs)来研究受原油泄漏影响的浅层无限制含水层中的生物成因气体动力学。分别测定了从渗流区和饱和区收集的气相和水相样品的〜(3,4)He,〜(20,22)Ne,〜(36,40)Ar,Kr和Xe的浓度。在这两个水文地质区中都观察到了相对于空气的Ne,Ar,Kr和Xe的系统元素分馏。在渗流区内,与空气的相应比例相比,Ne和Ar与Kr和Xe的相对比例显示出明显的与过程相关的趋势。 NG与大气浓度的偏离程度通常随原子质量的增加而增加(即,?Xe>?Kr>?Ar>?Ne),这表明Kr和Xe是渗流区内最敏感的NG示踪剂。气体数据的反应输运模型证实,可以通过与不同生物地球化学过程区之间建立的总压力梯度相反的NGs扩散通量的质量相关变化来解释元素分级。在地下水位以下的石油碳氢化合物降解的产甲烷区中也观察到了大气中NG的消耗。溶解度归一化的NG丰度遵循Xe> Kr> Ar> Ne的顺序,这表示溶解的NG响应气泡的形成和可能的沸腾而分配进入气相。观察到的Ne / Kr,Ne / Xe,Ar / Xe和Kr / Xe的元素NG比以及模型分析提供了有力的证据,证明地下水位以下生成CH4会导致气体逸出,并可能从地下水到渗碳带沸腾和碳转移。这些结果表明,稀有气体在具有生物活性的无限制含水层中提供了敏感的示踪剂,并且可以帮助识别碳在渗流带,毛细管边缘以及地下水位以下的循环和转移。

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