A lab in the field: high-frequency analysis of water quality and stable isotopes in stream water and precipitation
A lab in the field: high-frequency analysis of water quality and stable isotopes in stream water and precipitation
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DOI：
10.5194/hess-21-1721-2017
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发表时间：
2016-11
影响因子：
6.3
通讯作者：
J. Freyberg;B. Studer;J. Kirchner
中科院分区：
文献类型：
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作者：
J. Freyberg;B. Studer;J. Kirchner
对降雨和水流中溶质和同位素（18O和2H）的高频测量可以为流域流动路径和旅行时间提供重要信息，但收集、运输和分析数千瓶瓶装样品所涉及的工作量和样品存储工件严重限制了采用传统采样方法的流域研究。然而，最近朝着更紧凑和坚固的分析仪的发展，现在已经可以在较长时间内以亚小时频率测量现场的化学和水同位素。在这里，我们提出了一种膜蒸发连续水采样器的实验室和现场测试，耦合到一个腔衰落光谱仪，用于实时测量δ18O和δ2H，结合双通道离子色谱仪（IC），用于同步分析主要阳离子和阴离子。同位素分析仪在每隔30分钟进行10分钟平均读数时，δ18O和δ2H的精度分别优于0.03‰和0.17‰。在30 min采样间隔内，同位素对比水样之间的携带效应小于1.2%，仪器漂移可以通过二级参考标准的周期性分析来纠正。离子色谱仪的精度通常为~ 0.1-1 ppm或更好，对溪流中大多数主要离子的相对标准偏差为~ 1%或更好，这足以检测集水区径流中微妙的生物地球化学信号。我们将耦合同位素分析仪/IC系统安装在一个小型集水区溪流旁边的非绝缘小屋中，并在28天内每30分钟分析一次溪流和降水样本。这些高频测量通过化学和同位素示踪剂的端元混合分析，促进了事件水组分的详细比较。对于两个先前湿度条件相对干燥的事件，同位素示踪剂的事件水组分< 21%，但化学示踪剂明显高估了（40%至82%）。这些观测结果，再加上降水同位素输入的暴雨到暴雨模式以及相关的水流同位素响应，得出了流域径流生成的概念性假设。根据这一假设，由降水事件动员起来的事件前水，取决于先前的湿度条件，可能比供给基流的较深、较老的水明显更浅、更年轻、矿化程度更低，从而定义了用于水线分离的事件前端元。这项概念验证研究说明了在跨越多个水文事件的长时间内以高频率捕获同位素和水化学行为的潜在优势。
High-frequency measurements of solutes and isotopes (18O and 2H) in rainfall and streamflow can shed important light on catchment flow pathways and travel times, but the workload and sample storage artifacts involved in collecting, transporting, and analyzing thousands of bottled samples severely constrain catchment studies in which conventional sampling methods are employed. However, recent developments towards more compact and robust analyzers have now made it possible to measure chemistry and water isotopes in the field at sub-hourly frequencies over extended periods. Here, we present laboratory and field tests of a membrane-vaporization continuous water sampler coupled to a cavity ring-down spectrometer for real-time measurements of δ18O and δ2H combined with a dual-channel ion chromatograph (IC) for the synchronous analysis of major cations and anions. The precision of the isotope analyzer was typically better than 0.03 ‰ for δ18O and 0.17 ‰ for δ2H in 10 min average readings taken at intervals of 30 min. Carryover effects were less than 1.2 % between isotopically contrasting water samples for 30 min sampling intervals, and instrument drift could be corrected through periodic analysis of secondary reference standards. The precision of the ion chromatograph was typically ∼ 0.1–1 ppm or better, with relative standard deviations of ∼ 1 % or better for most major ions in stream water, which is sufficient to detect subtle biogeochemical signals in catchment runoff. We installed the coupled isotope analyzer/IC system in an uninsulated hut next to a stream of a small catchment and analyzed stream water and precipitation samples every 30 min over 28 days. These high-frequency measurements facilitated a detailed comparison of event-water fractions via endmember mixing analysis with both chemical and isotope tracers. For two events with relatively dry antecedent moisture conditions, the event-water fractions were < 21 % based on isotope tracers but were significantly overestimated (40 to 82 %) by the chemical tracers. These observations, coupled with the storm-to-storm patterns in precipitation isotope inputs and the associated stream water isotope response, led to a conceptual hypothesis for runoff generation in the catchment. Under this hypothesis, the pre-event water that is mobilized by precipitation events may, depending on antecedent moisture conditions, be significantly shallower, younger, and less mineralized than the deeper, older water that feeds baseflow and thus defines the pre-event endmember used in hydrograph separation. This proof-of-concept study illustrates the potential advantages of capturing isotopic and hydrochemical behavior at a high frequency over extended periods that span multiple hydrologic events.