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Block 2.1: Hydrogeologie, Hydrologie und Wasserisotope
Dienstag, 28.09.2021:
9:30 - 11:00

Chair der Sitzung: Christoph Schüth, TU Darmstadt
Chair der Sitzung: Florian Einsiedl, TUM
Chair der Sitzung: Diana Burghardt, TU Dresden
Virtueller Veranstaltungsort: Block 2.1 - Meeting Link

Zeige Hilfe zu 'Vergrößern oder verkleinern Sie den Text der Zusammenfassung' an
9:30 - 10:00
Session Keynote

Oxygen is all around...

Johannes A.C. Barth1, Robert van Geldern2

1GeoZentrum Nordbayern, Deutschland; 2Friedrich-Alexander-Universität Erlangen-Nürnberg

Oxygen is the most abundant element within the earth system. At the earth´s surface and in its molecular form as O2 it is also one of the most important RedOx agents in both gaseous and dissolved forms. For such reactions, tracing of oxygen can identify fundamental processes including photosynthesis, respiration and exchange between the atmosphere and water. Moreover, as part of the water molecule oxygen can serve as a tracer of origin, mixing and movement of H2O. However, water can also release O2 via its splitting by for instance photosynthesis. Such processes also offer relationships to carbon cycling on various scales. While tracing of water with oxygen stable isotope ratios finds applications in a wide spectrum of fields including hydro(geo)logy, plant sciences and medicine, isotope tracing of gaseous and dissolved O2 is much less common. Future challenges and opportunities lie in combinations of water and molecular oxygen cycles via their stable isotope ratios to outline and constrain sources and sinks of this important element.

10:00 - 10:15

Impact of the 2018-2020 drought in Central Germany on the nitrogen cycling in a meso-scale catchment: Insights from hydrochemical and stable isotope investigations

Christin Mueller, Ronald Krieg, Ralf Merz, Kay Knoeller

Helmholtz Centre for Environmental Research UFZ, Deutschland

Recent investigations of the nitrate pollution in groundwater and surface water bodies in Germany confirm the well-known fact that the threshold value for nitrate concentrations (50 mg L-1) is exceeded in 18% of all sampling sites belonging to the nitrate monitoring network (German Federal Environmental Agency). Besides the input of excess nitrogen into the aquatic system by human activities, climate change related hydrological variability (extreme precipitation events or longer summer droughts) may also have an effect on nitrate loads in ground- and surface water.

Especially in the last years (2018-2020), a severe drought was observed in central Germany. In order to find out the potential impact of that drought on the catchment scale nitrogen cycling, we investigated the ground- and surface water compartments of the Holtemme watershed, a meso-scale river catchment in the Harz Mountains, Central Germany. The analysis of nitrate concentrations and corresponding isotopic signatures for groundwater and surface water samples were conducted during the entire dry period and were continued until discharge conditions went back to the long term mean in early 2021. The survey revealed decreasing nitrate concentrations for both compartments during drought conditions and a significant increase in the post-drought phase. Isotopic investigations allowed us to differentiate between distinct nitrate sources and microbial turnover processes. The time series analysis of δ15N-NO3 showed regular oscillations within the year, which illustrates a periodic fertilizer application. Corresponding δ18O-NO3 signatures show higher, seasonal-independent variations that can be explained by the normal isotopic variability of the ambient water that provides two thirds of the oxygen that is incorporated into the nitrate molecule during nitrification. However, flow paths for nitrate mobilization into the surface water seem to be unaffected by the drought because contributions of each nitrate source decreased equally during dry conditions. Nitrate concentrations increased after the dry period independently of recent nitrate supply. This implies that the soil system acts as a storage compartment, from which nitrate is easily released after the drought.

Our study confirms that hydrological variability is a highly important driver for nitrate mobilization at the catchment scale. Therefore, we suggest that the impact of changing hydrological conditions needs to be taken into consideration for management practices and policy actions.

10:15 - 10:30

Spatiotemporal analysis of Central European young water fractions

Michael Stockinger, Christine Stumpp

Universität für Bodenkultur, Wien, Österreich

The travel time of precipitation entering a catchment and leaving it as streamflow varies according to the flow paths precipitation takes. Fast precipitation travel times through catchments are especially interesting as they pose a high risk to river water quality. However, investigating influences on travel times is challenging due to complex water flow through heterogeneous landscapes. In this study, we investigated the fraction of streamflow younger than three months (Fyw) of nine major catchments in Central Europe and compared it to catchment characteristics and a teleconnection pattern that influences European large-scale weather: the North Atlantic Oscillation index (NAO). Adjacent catchments had similar long-term average and time-variable Fyw. These patterns were explained using catchment characteristics in a multiple regression analysis with prior collinearity removal, with grassland and 20-40% tree cover density explaining 84% of Fyw variability. Besides this spatial analysis, the annual changes in Fyw resembled each other in most catchments, leading to the hypothesis that a common, annually changing influence controls it. While total water storage in the catchments had no relationship to the time-variable Fyw, the NAO of the previous year was negatively correlated with this year’s Fyw (R² = 0.68). Three hypotheses are discussed as to how this inter-annual correlation could have happened, but no distinct explanation could be found. We recommend additional studies into this relationship as well as multiple regression with prior collinearity removal for future studies of the complex interplay of spatiotemporal variables on Fyw.

10:30 - 10:45

Stable water isotope analysis and improved lumped-parameter modeling for characterizing unsaturated subsurface flow

Anne Imig, Fatemeh Shajari, Florian Konrad, Florian Einsiedl, Arno Rein

Chair of Hydrogeology, Faculty of Civil, Geo and Environmental Engineering, Technical University of Munich, Germany

The characterization of water flow in the unsaturated zone is an important task, e.g., for evaluating water resources and stresses imposed by climate change and for protecting groundwater resources. In a 3-year field study, we have measured stable water isotopes (δ2H and δ18O) in precipitation and the outflow of two vegetated lysimeters situated in Wielenbach, Germany. The lysimeters contained soil cores of different textures, i.e. sandy gravel (Ly1) and clayey sandy silt (Ly2). Maize has been cultivated on top of the lysimeters, and four different herbicides have been applied to the maize plantation.

Lumped-parameter modeling was applied for interpreting stable water isotope observations in lysimeter outflow and for determining the mean transit time of water in the subsurface and the dispersion coefficient. Usually, lumped-parameter model (LPM) approaches consider steady-state flow, which is due to their model structure that implements analytical solutions (with constant coefficients) for simulating stable water isotope transport. In this work, we have extended this approach by subdividing the simulation time into hydrologically relevant sub-periods. Flow and transport parameters vary between these sub-periods, so that temporally varying flow is mimicked (keeping constant coefficients in each sub-period). Furthermore, preferential flow paths were considered and implemented in the model. For validation, numerical modeling of unsaturated flow and stable water isotope transport was carried out using HYDRUS-1D.

Application of the extended LPM approach could significantly improve the simulation of stable water isotopes observed in lysimeter outflow, by considering seasonal changes of flow and transport parameters. In general, LPM results corresponded well to numerical modeling results. Observations were more difficult to describe for Ly2, where the seasonal fluctuation of stable water isotopes seems not fully met by numerical modeling. The consideration of a constant δ18O upshift could improve simulations, i.e. representing, in a simplified assumption, the influence of immobile (isotopically enriched) water as an additional component that contributes to the isotopic signature of lysimeter outflow water. Both the LPM and numerical approach are hence considered to be well suited for decision support. As an advantage of the LPM approach, less input data and fitting parameters (with associated uncertainties) are required, making it a powerful tool for groundwater management methodologies.

10:45 - 11:00

Identifizierung von Oberflächenwasser-Grundwasser-Interaktionen anhand von charakteristischen Isotopensignalen

Michael Engel1,2, Simon Mischel1, Sabrina Quanz1, Dirk Radny1, Francesco Comiti2, Lars Düster1, Axel Schmidt1

1Bundesanstalt für Gewässerkunde, Deutschland; 2Freie Universität Bozen-Bolzano, Italien

Die Verwendung stabiler Wasserisotope kann entscheidend zum Prozessverständnis von Oberflächenwasser-Grundwasser-Interaktionen an Flüssen beitragen. Flusswasser weist im Gegensatz zu Grundwasser eine höhere Variabilität in der Isotopenzusammensetzung auf, die durch Abflussereignisse wie Schneeschmelze oder Hochwasser nach Starkregenereignissen gesteuert wird. Die kontrastierende Isotopenzusammensetzung von Flusswasser und Grundwasser kann verwendet werden, um ein Mischungsmodell aus den Eingangsgrößen Flusswasser und Grundwasser vor sowie während des Ereignisses aufzustellen. Hierzu liefert der folgende Beitrag zwei Beispiele mit sehr unterschiedlich großen Einzugsgebieten aus der wissenschaftlichen Praxis.

An der Ahr, einem 53 km langen Gebirgsfluss mit einem Einzugsgebiet von 629 km² (in Südtirol, Italien) wurden monatliche Wasserproben von Flusswasser und Grundwasser von 2016 bis 2018 genommen und auf stabile Wasserisotope (δ18O und δ2H) analysiert. Die durch Schneeschmelzereignisse ausgelöste, isotopisch leichtere Flusswassersignatur (z.B. δ2H: -88,2 bis -101 ‰) konnte als ein charakteristisches Isotopensignal verwendet werden, um ein Mischungsmodell von Flusswasser und Grundwasser aufzustellen. Die Analyse zeigt, dass das Ereigniswasser ca. 41 Tage ± 10 bis zum Brunnen benötigt. Dies entspricht einer Fließgeschwindigkeit von ungefähr 0,2 bis 0,3 m d-1. Der maximale Anteil an Flusswasser im Grundwasser konnte auf ca. 4% ± 1, 15% ± 2, 19% ± 4 and 51% ± 4 für die Ereignisse im Juni 2016, Mai 2017 sowie Mai und Juli 2018 geschätzt werden.

Im deutschen Teil der Mosel, einem 232 km langen Abschnitt des zweitlängsten Nebenflusses des Rheins, werden am Schleusenstandort Lehmen seit Sommer 2020 monatliche Wasserproben von der Mosel im Ober- und Unterpegel der Stauhaltung sowie Grundwasser von 4 Grundwassermessstellen genommen. Die Analyse auf stabile Wasserisotope zeigt, dass hier das Oberflächenwasser isotopisch etwas leichter ist (δ2H: -62,3 ‰) als das Grundwasser (δ2H: -51,3 ‰). Erste Ergebnisse deuten einen monatlichen Versatz der Flusswasser-Isotopensignatur im Grundwasser an. Eine Schleusenwartung im September 2020 führte zu einer Absenkung des Flusswasserspiegels um 1,5 m und damit zu einem veränderten hydraulischen Gradienten zwischen Fluss- und Grundwasser. Es konnten während dieser Zeit nur geringfügige Veränderungen in der Isotopenzusammensetzung des Grundwassers nachgewiesen werden.

Beide Beispiele bestätigen den Nutzen stabiler Wasserisotope bei grundlagenorientierten, wie auch angewandten hydrologischen Fragestellungen, um komplexe Vorgänge bei der Oberflächenwasser-Grundwasser-Interaktion zu verstehen.

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