a lab in the field: high-frequency analysis of water quality and stable isotopes in stream water and precipitation
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ID: 228567
2017
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Abstract
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
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-eventendmember 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.
| Reference Key |
freyberg2017hydrologya
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|---|---|
| Authors | ;J. von Freyberg;B. Studer;J. W. Kirchner |
| Journal | materials research bulletin |
| Year | 2017 |
| DOI |
10.5194/hess-21-1721-2017
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| URL | |
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