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Sitzungsübersicht
Sitzung
Block 2.2: Hydrogeologie, Hydrologie und Wasserisotope
Zeit:
Dienstag, 28.09.2021:
11:30 - 12:15

Chair der Sitzung: Christoph Schüth, TU Darmstadt
Chair der Sitzung: Florian Einsiedl, TUM
Chair der Sitzung: Diana Burghardt, TU Dresden
Virtueller Veranstaltungsort: Block 2.2 - Meeting Link

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Präsentationen
11:30 - 11:45

Neue Einblicke in die Infiltrationsbedingungen des Oberjura Thermalwassers in der südlichen Bayerischen Molasse mit Hilfe der 14CDOC Methode

Theis Winter, Florian Einsiedl

Technische Universität München, Deutschland

Der Oberjura Thermalwasser Aquifer im Süddeutschen Molassebecken ist der Hauptexplorationshorizont für die geothermische Energiegewinnung in Bayern. Der karbonatische Oberjura Aquifer zeigt sowohl klein- als auch großräumig starke Heterogenitäten hinsichtlich seines strukturgeologischen Aufbaus auf. So sind im Oberjura Aquifer Karsterscheinungen und tiefreichende Störungssysteme zu finden. Die neusten Literaturarbeiten unterscheiden, anhand von statistischen Clusteranalysen, zwischen drei unterschiedliche Wassertypen hinsichtlich der hydrochemischen und isotopengeochemischen Beschaffenheit im zentralen Teil des Süddeutschen Molassebeckens in Bayern und lassen ein alpennahes Infiltrationsgebiet im Süden des Süddeutschen Molassebeckens vermuten.

Anhand hydrochemischer und isotopengeochemischer Parameter untersuchen wir die Infiltrationsbedingungen des Oberjura Thermalwassers und grenzen den zeitlichen Rahmen der Infiltration mithilfe Piston Flow Grundwasseralter ein.

Prinzipiell ist das Oberjura Thermalwasser in diesem Gebiet vom Na-HCO3-Cl Grundwassertyp, zeigt aber an den südlichen Standorten Ca2+ und Mg2+ Gehalte unter den zu erwartenden Gehalten. Die Werte der stabilen Wasserisotope liegen zwischen -10,6 und -11,7 ‰ für δ18O und -85,0 bis-86,0 ‰ für δ2H und deuten auf eine Infiltration insbesondere während kaltzeitlicher Bedingungen hin.

Die Kombination aus den Ergebnissen der Hydrochemie, stabilen Wasserisotopen und den Piston Flow Grundwasseraltern aus den 14CDOC und 14CDIC Altersbestimmungen in Verbindungen mit 81Kr-Altern aus der Literatur bestätigen die Ergebnisse der stabilen Wasserisotope und geben nun Hinweise darauf, dass sich das Thermalwasser des Oberjura im Süden aus mindestens zwei Grundwasserkomponenten zusammensetzt. Auf Grund der ermittelten Piston Flow Alter der untersuchten Grundwässer ist eine Infiltration nur über „Bypass“-Strukturen, die in der überlagernden Molasse zu finden sind, zu erklären.



11:45 - 12:00

Supporting the interpretation of δ¹⁵N values of dissolved NH₄⁺ and NO₃⁻ in the water column of a seasonal stratified lake using Numerical modeling and PCR gene amplification

Gisela Alejandra Pena Sanchez1, Florian Einsiedl1, Clara Duffner2,4, Stephanie Schulz2,4, Bernhard Mayer3, Anja Wunderlich1

1Technical University of Munich; 2TUM School of Life Sciences; 3University of Calgary; 4Helmholtz Centre Munich

Occurring simultaneously with denitrification in anaerobic environments, other pathways like anaerobic oxidation of ammonium (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) may play an important role for the removal of fixed nitrogen (N) to gaseous N₂ from freshwater ecosystems. Recent studies that were conducted in freshwater ecosystems showed that the relevance of anammox, denitrification and DNRA, significantly changes depending on site conditions, and thus significance of anammox in freshwater is still largely unknown.

Here we investigate the pathways and relevance of nitrogen losses at the seasonal stratified lake Fohnsee during the development of the vertical redox stratification. Our approach combines concentration profiles and corresponding stable isotope compositions of NO₃⁻ and NH₄⁺, with qPCR of the hydrazine synthase gene (hzsB), nitrite reductase (nirK and nirS) genes, and numerical modeling of δ¹⁵N-NH₄⁺ values during the stratification period.

Increasing stable isotope values of nitrate (δ¹⁵N and δ¹⁸O) together with the identification of two specific marker genes for denitrification (nirK and nirS) indicate that denitrification is occurring in the anoxic water column of Fohnsee. However, stable isotope values of δ¹⁵N of ammonium and water chemistry results were ambiguous about the occurrence of anaerobic ammonium oxidation within the water column. Modeling results and qPCR analysis using hzsB marker gene for anammox showed that the observed stable isotope shift in δ¹⁵N of ammonium may be the result of mixing processes between ammonium remaining from nitrification in the oxic water column with strongly enriched δ¹⁵N values of 25‰, and ammonium that is formed by degradation of organic carbon during methanogenesis in the lake sediments with δ¹⁵N values of around 11‰.

We concluded that in a highly dynamic ecosystem such as seasonal stratified lakes, anammox bacteria only overcome limiting conditions occasionally, and may be active at most at low rates and on a temporal scale, thus, denitrification is suggested to be the principal path of nitrogen loss at the seasonal stratified lake Fohnsee. Finally, this study highlights the importance of solid understanding of the various transformations and isotope effects within a natural ecosystem for the successful interpretation of isotopic data.



12:00 - 12:15

Investigation of chloroform degradation mechanisms using multi-dimensional isotope fractionation analysis: Hydrodechlorination over Pd and Rh vs reductive dechlorination with Fe0

Berhane Abrha Asfaw1,2, Kaori Sakaguchi-Söder2, Christoph Schüth1

1Hydrogeology, Institute IAG, TU Darmstadt; 2Material Flow Management and Resource Economics, Institute IWAR, TU Darmstadt

Multi-dimensional isotope fractionation analysis is a useful tool to investigate degradation mechanisms of organic substances. In this study, we investigated the chlorine, carbon and hydrogen isotope fractionation patterns of chloroform during catalytic hydrodechlorination using two commercial catalysts palladium-on-alumina (Pd/Al2O3) and rhodium-on-alumina (Rh/Al2O3). The fractionation patterns of catalytic reaction were further compared to the pattern of reductive dechlorination of chloroform with zero valent iron (Fe0).

100 ml of deionized water in a reactor was saturated with hydrogen, which serves as an electron donor. The initial concentrations of chloroform and catalysts were designed to be 100 mg/l and 0.04 g/l, respectively. At specified time intervals, 1 ml of the water phase was taken from the reactor and placed in a 2 ml vial. Chlorine isotope analysis was implemented by GC-qMS (Agilent Technologies) coupled to a purge and trap sampler (PTA 3000, IMT GmbH) using the water phase from each 2 ml vial. For the carbon and hydrogen isotope analyses, a certain volume of the headspace in the 2 ml vial was injected into GC-C/Py-IRMS (Thermo Fisher Scientific).

The dual isotope slopes (ΛC-Cl = Δδ13C/Δδ37Cl) during the catalytic dechlorination of chloroform over Pd and Rh were determined to be 2.3 ± 0.12 and 2.1 ± 0.14, respectively. These values are significantly lower than the reported ΛC-Cl value of 8 ± 2 (Torrentó et al., 2017) and 8 ± 1 (Rodríguez-Fernández et al., 2018) for the reductive dechlorination of chloroform with ZVI. Our results indicate that the degradation mechanism of chloroform over Pd and Rh differs from the mechanisms with ZVI.

We further investigated the apparent kinetic isotope effects (AKIE) of carbon and chlorine isotopes. Whereas no significant difference was observed in AKIEC between degradation with catalysts and ZVI, AKIECl with Pd and Rh (1.037 and 1.032, respectively) are much higher than the theoretical limit for C-Cl bond cleavage (KIECl = 1.013) by Elsner et al., 2005. The large AKIECl with Pd and Rh suggest non-concerted, step-wise reaction, i.e. no intramolecular competition among the three chlorine atoms exists during the degradation process of chloroform, unlike concerted reaction that is known to take place during reductive dechlorination of chloroform with ZVI.

In summary, our study showed that 1) dual isotope slope and AKIE are useful tools to investigate degradation mechanisms and 2) chloroform degradation mechanism over Pd and Rh is assumed to be none-concerted, step-wise reaction based on the AKIE of Cl.

Key words: chloroform hydrodechlorination, Multi-dimensional CSIA, Palladium, Rhodium, Fe0



 
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