A lab in the field: high-frequency analysis of water quality and stable isotopes in stream water and precipitation

作者：von Freyberg Jana; Studer Bjorn; Kirchner James W

来源：Hydrology and Earth System Sciences, 2017, 21(3): 1721-1739.

DOI：10.5194/hess-21-1721-2017

<jats:p>&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; High-frequency measurements of solutes and isotopes (&lt;sup&gt;18&lt;/sup&gt;O and &lt;sup&gt;2&lt;/sup&gt;H) 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 &lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;18&lt;/sup&gt;O and &lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt;H 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&lt;span class="thinspace"&gt;&lt;/span&gt;‰ for &lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;18&lt;/sup&gt;O and 0.17&lt;span class="thinspace"&gt;&lt;/span&gt;‰ for &lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt;H in 10&lt;span class="thinspace"&gt;&lt;/span&gt;min average readings taken at intervals of 30&lt;span class="thinspace"&gt;&lt;/span&gt;min. Carryover effects were less than 1.2&lt;span class="thinspace"&gt;&lt;/span&gt;% between isotopically contrasting water samples for 30&lt;span class="thinspace"&gt;&lt;/span&gt;min sampling intervals, and instrument drift could be corrected through periodic analysis of secondary reference standards. The precision of the ion chromatograph was typically  ∼ &lt;span class="thinspace"&gt;&lt;/span&gt;0.1–1&lt;span class="thinspace"&gt;&lt;/span&gt;ppm or better, with relative standard deviations of  ∼ &lt;span class="thinspace"&gt;&lt;/span&gt;1&lt;span class="thinspace"&gt;&lt;/span&gt;% or better for most major ions in stream water, which is sufficient to detect subtle biogeochemical signals in catchment runoff. &lt;br&gt;&lt;br&gt; 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&lt;span class="thinspace"&gt;&lt;/span&gt;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 &amp;amp;lt;&lt;span class="thinspace"&gt;&lt;/span&gt;21&lt;span class="thinspace"&gt;&lt;/span&gt;% based on isotope tracers but were significantly overestimated (40 to 82&lt;span class="thinspace"&gt;&lt;/span&gt;%) 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 &lt;q&gt;pre-event&lt;/q&gt; 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.&lt;/p&gt;
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• 出版日期2017-3-23