Abstract. Studying the response of streamwater chemistry to changes in discharge can provide valuable insights into how catchments store and release water and solutes. Previous studies have determined concentration–discharge (cQ) relationships from long-term, low-frequency data of a wide range of solutes. These analyses, however, provide little insight into the coupling of solute concentrations and flow during individual hydrologic events. Event-scale cQ relationships have rarely been investigated across a wide range of solutes and over extended periods of time, and thus little is known about differences and similarities between event-scale and long-term cQ relationships. Differences between event-scale and long-term cQ behavior may provide useful information about the processes regulating their transport through the landscape. Here we analyze cQ relationships of 14 different solutes, ranging from major ions to trace metals, as well as electrical conductivity, in the Swiss Erlenbach catchment. From a 2-year time series of sub-hourly solute concentration data, we determined 2-year cQ relationships for each solute and compared them to cQ relationships of 30 individual events. The 2-year cQ behavior of groundwater-sourced solutes was representative of their cQ behavior during hydrologic events. Other solutes, however, exhibited very different cQ patterns at the event scale and across 2 consecutive years. This was particularly true for trace metals and atmospheric and/or biologically active solutes, many of which exhibited highly variable cQ behavior from one event to the next. Most of this inter-event variability in cQ behavior could be explained by factors such as catchment wetness, season, event size, input concentrations, and event-water contributions. We present an overview of the processes regulating different groups of solutes, depending on their origin in and pathways through the catchment. Our analysis thus provides insight into controls on solute variations at the hydrologic event scale.
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 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.
A field experiment was conducted to investigate preferential transport of herbicides and to explore processes that cause rapid movement. Bromide, chloride, and three herbicides (triasulfuron, atrazine, and terbuthylazine) with different mobility characteristics were applied to six 1.4 x 1.4 m field plots on a loamy and a sandy soil. At both sites, three of the plots were covered with a plastic roof 1 month before the beginning of the experiment to achieve different initial water contents between the plots. Two days before the beginning of the tracer experiment, crops were removed, and the soil surface was homogenized with a spade to a depth of 15-20 cm. One day after application of the chemicals the plots were irrigated with a sprinkling apparatus. The cumulative amounts of infiltration until the time of sampling were 30, 60, and 90 mm within 1, 2, and 3 days, respectively. A trench was excavated, and soil cores were taken horizontally from a 1 x 1 m profile in a regular 0.1 x 0.1 m grid. The loamy and the sandy soil showed completely different transport patterns. In the loamy soil the bulk mass of herbicides remained in the top layer; however, considerable amounts of herbicides were transported below the root zone. A few percent for triasulfuron and atrazine and <1% for terbuthylazine were detected below 0.5 m depth after 90-mm cumulative infiltration. Traces of all herbicides were found down to 0.9 m. The depth distributions for anions and all herbicides were similar. These results show that the herbicides were only partly adsorbed by the soil matrix. A fraction of these chemicals was transported with or without minor adsorption along cracks or fissures. In the sandy soil, chemical movement was confined to the top 0.4 m, and the penetration depth of the herbicides was consistent with their mobility characteristics: triasulfuron showed greatest mobility, atrazine was moderately mobile, and terbuthylazine was the least mobile of all three.
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