Dissolved noble gas and isotopic tracers reveal vulnerability of groundwater in a small, high‐elevation catchment to predicted climate changes
Dissolved noble gas and isotopic tracers reveal vulnerability of groundwater in a small, high‐elevation catchment to predicted climate changes
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溶解的稀有气体和同位素示踪剂揭示了一个小型高海拔流域地下水对预测的气候变化的脆弱性
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发表时间：
2010
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通讯作者：
J. Moran
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作者：
M. Singleton;J. Moran
在内华达山脉一个小而高的集水区，地下水和溪水中稀有气体的浓度和多种同位素示踪剂限制了不同地下水成分的补给条件和地下停留时间。我们确定了三个对地下水流动有贡献的来源：(1)旅行时间短的季节性地下水补给，(2)经历了较长流动路径的放射性成因4He升高的水，以及(3)具有岩浆氦和碳同位素特征的深部流体上涌。我们的研究结果阐明了水文系统的两个重要方面，它们将对该系统如何应对气候变化产生直接影响：(1)冲积含水层的补给主要发生在集水区的下坡，因此对雪线高程的变化很敏感；(2)含水层西部的深层地下水非常年轻，提供的缓冲能力很小。尽管表观地下水年龄表明，停留时间从不到一年到几十年不等，但季节性补给的水主导着冲积含水层。稀有气体的补给温度接近年平均气温，比直接进入积雪的预期温度高5°-11°。过量的空气浓度表明在补给过程中气泡被困住，低于通过基岩裂隙进行补给的预期。相反，补给可能发生在较低坡度的植被区域，正如δ13C-溶解的无机碳值所表明的那样，这与土壤呼吸中的二氧化碳并入一致。
Noble gas concentrations and multiple isotopic tracers in groundwater and stream water at a small, high‐elevation catchment of the Sierra Nevada Mountains constrain recharge conditions and subsurface residence times of different groundwater components. We identify three sources that contribute to groundwater flow: (1) seasonal groundwater recharge with short travel times, (2) water with elevated radiogenic 4He that has experienced longer flow paths, and (3) upwelling of deep fluids that have “magmatic” helium and carbon isotope signatures. Results from our study illuminate two important aspects of the hydrological system that will have a direct impact on how this system responds to climate change: (1) recharge to the alluvial aquifer occurs primarily on the lower slopes of the catchment and is therefore sensitive to changes in snowline elevation and (2) deep groundwater in the western part of the aquifer is very young and provides very little buffering capacity. Although apparent groundwater ages indicate residence times range from less than a year to several decades, the water that recharges seasonally dominates the alluvial aquifer. Noble gas recharge temperatures are close to mean annual air temperature, and are 5°–11° higher than would be expected for direct influx of snowmelt. Excess air concentrations, indicating entrapment of air bubbles during recharge, are lower than would be expected for recharge through bedrock fractures. Instead, recharge likely occurs over vegetated areas on the lower slopes, as indicated by δ13C‐dissolved inorganic carbon values that are consistent with incorporation of CO2 from soil respiration.