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Sitzung
Poster Kurzvorträge - Themenblock 2: Hydrogeologie, Hydrologie und Wasserisotope
Zeit:
Dienstag, 28.09.2021:
11:00 - 11:15

Virtueller Veranstaltungsort: Poster Kurzvorträge - Meeting Link

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Präsentationen
11:00 - 11:04

Stable water isotopes as natural tracers in a hydrogeological study of riverbank filtration in N’Djamena, Chad

Maike Gröschke, Lilli Witt, Paul Königer, Sara Vassolo

Federal Institute for Geosciences and Natural Resources (BGR), Groundwater Resources - Quality and Dynamics

To assess the potential of riverbank filtration along the Chari River in the Lake Chad basin, a hydrogeological test site at a large water supply well located in close proximity to the river was established in N’Djamena, Chad. At the site, the aquifer is composed of Quaternary sediments with a total thickness of about 50-55 m and consist of alternating layers of sand and clay or silt. There are two high permeable layers at depths between ̴ 23-27 m and ̴ 45-50 m separated by a fine sand/silt layer. To date, the site consists of five deep and five shallow observation wells located around the supply well, and a shallow and a deep observation well on the opposite riverbank in Cameroon. Since 2019, samples from the Chari River, the supply well, and the observations wells on the Chadian side were collected every two weeks for hydrochemistry and water stable isotopes - with some interruptions - whereas groundwater levels were recorded with automatic data loggers.

The water level data shows that aquifer discharge conditions prevail throughout the year and that the hydraulic head in the shallow aquifer is always higher than in the deeper one. The largely fluctuating stable isotope compositions from the river can be traced in the shallow aquifer, which allow for an estimation of groundwater flow velocities ( ̴ 0.9 m/d), while the isotopic compositions in the deep observations wells remain constant throughout the year. The isotopic composition of the water supply well lies between that of the shallow and the deep observation wells without any seasonal variations. It is still unclear whether different flow paths and travel times towards the well in the shallow aquifer dampen the isotopic signal, or if a significant groundwater flow component in the fine sand/silt separating the medium sand layers contributes to the water budget of the supply well.



11:04 - 11:08

Isotopen-hydrobiogeochemische Untersuchungen an anoxischen Grundwässern off- und on-shore der norddeutschen Küste

Michael Ernst Böttcher1,2,3, Anna-Kathrina Jenner1,3, Cátia M.E. von Ahn1,2, Iris Schmiedinger1, Andreas Roskam4

1Leibniz Institute of Baltic Sea Research (IOW), Deutschland; 2University of Greifswald, Deutschland; 3University of Rostock, Deutschland; 4NLWKN Aurich, Deutschland

Die Leybucht liegt im Einzugsgebiet der Osterems und wird durch die vorgelagerten ostfriesischen Inseln vor der offenen Nordsee geschützt. Seit dem Mittelalter steht die Leybucht unter dem Einfluss von Verlandungserscheinungen (NLÖ, 2001). Im Jahre 2017 wurde im Küstenvorfeld in größtenteils fein- und mittelsandigen Sedimenten eine Bohrung (R144) abgeteuft, die für die Grundwasserbeobachtung in 57 bis 61 m unter Geländekante verfiltert wurde. In der vorliegenden Studie wurden das Grundwasser dieser Bohrung sowie zwei on-shore Grundwässer (Leybuchtsiel 1 und 2) in den Jahre 2019 und 2020 beprobt und hydrochemisch auf Haupt- und Spurenelemente analysiert. Diese Messungen wurden ergänzt um die Vermessung der stabilen Wasserisotope (2H, 18O), der C-Isotopenzusammensetzung des DIC sowie der S- und O--Isotopenzusammensetzung des gelösten SO4. Die Daten wurden darüber hinaus einer physikochemischen Analyse mit PHREEQ-C unterzogen.

Die Untersuchungsergebnisse zeigen, dass alle Wässer anoxisch sind und sich durch hohe gelöste Eisengehalte auszeichnen. Die drei Grundwässer unterscheiden sich hinsichtlich ihrer Wasserisotopen-Signatur, liegen aber auf der für Cuxhaven etablierten lokalen meteorischen Wasserlinie und ähneln Süßwasser-beeinflussten Porenwässer, die in tiefen Sanden im Rückseitenwatt der Insel Spiekeroog gefunden wurden (Böttcher et al., 2014). Die Bohrungen R144 und Leybuchtsiel 1 verzeichnen den Einfluss von Nordseewasser. Die Isotopenzusammensetzung des DIC und Sulfats verweist auf die Mineralisierung von gelöstem organischen Kohlenstoff hin unter Verwendung der Elektronenakzeptoren z.B., Sauerstoff, Nitrat, und Sulfat, z.T. hat bereits Methanogenese stattgefunden. Die gelösten Eisengehalte können der dissimilatorisch-mikrobiellen Reduktion von Eisenoxiden und/oder der mikrobiell-katalysierten Zersetzung von Eisensulfiden stammen (z.B., Zhang et al., 2012). Ein Austritt dieser Wässer in der Nordsee hätte eine Erhöhung des Entgasungpotenzials für CO2 zur Folge.

Literatur:

Böttcher M.E., Lipka M., Winde V., Dellwig O., Böttcher E.O., Böttcher T.M.C., Schmiedinger I.(2014) Multi-isotope composition of freshwater sources for the southern North and Baltic Sea. Proc. 23rd SWIM conference, Husum, 46-49.

NLÖ (2001) Beweissicherung Küstenschutz Leybucht. Morphologisch-sedimentologische Untersuchungen. Forschungsstelle Küste. Abschlußbericht.

Zhang Y.-C., Slomp, C.P., Broers H.P., Passier H.F., Böttcher, M.E., Omoregie E.O., Lloyd J.R., Polya D.A. & van Cappellen P. (2012) Isotopic and microbiological signatures of pyrite-driven denitrification linked to pyrite oxidation in a sandy aquifer. Chem. Geol., 300-301, 123-132.



11:08 - 11:12

Measuring high-resolution geochemical depth-profiles in hyporheic stream sediments with focus on aerobic and anaerobic methane oxidation

Tamara Michaelis, Anja Wunderlich, Florian Einsiedl

Technische Universität München, Lehrstuhl Hydrogeologie, Deutschland

A large part of carbon and nutrient cycling in rivers and streams takes place in the hyporheic zone, a hotspot of biogeochemical activity in the river bed where surface- and groundwater meet. Typically, redox zones with different dominant electron acceptors form sequentially, sorted by the energy yield of the respective redox reactions. This zonation is subject to the influence of multiple parameters such as sediment composition, river temperature, surface- and groundwater interaction, water and sediment chemical composition and others. In anaerobic sediments, methane production takes place as last step in this so-called redox ladder making many rivers net carbon emitters. Understanding sources and sinks of methane, the second most important greenhouse gas in the world, is crucial in times of climate change, but due to high spatial and temporal heterogeneity of redox processes in hyporheic sediments the relationship between methanogenesis and microbial methane oxidation is not yet fully understood.

To help filling this knowledge gap, our research group is measuring high-resolution depth-depending geochemical profiles at different locations across a stream bed. A sediment peeper is used to obtain pore water samples with a 1 cm depth-resolution. Concentration gradients of dissolved oxygen, nitrate, nitrite, sulfate, ammonia and methane show depth and width of the different redox zones. To explain the dominant methane production pathway and to interpret concentration gradients, stable carbon isotopes in methane (δ13C-CH4) are measured. The δ13C-CH4 in the methanogenic zone can be used to distinguish between aceticlastic and hydrogenotrophic methanogenesis. An isotopic enrichment in δ13C values of methane towards the sediment surface may indicate microbial degradation while measurements of dissolved oxygen can be used to separate aerobic from anaerobic methane oxidation. In the absence of oxygen, other electron acceptors may become relevant for microbial methane oxidation, for example nitrate, nitrite or sulfate. In addition, the performance of a simple 1D diffusion model will be compared with the results of a 1D diffusion-reaction model in their ability to capture the measured methane concentration gradients. This will be used as a supporting evidence of the findings from concentration and isotope measurements.

In a poster, we want to present geochemical profiles in combination with modeled concentration gradients and carbon stable isotope measurements in methane for different locations across the stream. The different sampling sites are compared in terms of sediment composition and location in the stream bed.



11:12 - 11:15

Karstic springs as strong emitters of CO2

Kern Lee1,2, Robert van Geldern1, Johannes Barth1

1Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), Deutschland; 2Alberta Environment and Parks, Edmonton, Canada

Inland aquatic systems play an important role in the global carbon cycle. They not only transport terrestrial carbon to the oceans but also emit carbon in the form of carbon dioxide to the atmosphere. Among inland water types rivers are significant sources of CO2 to the atmosphere, and karstic watersheds are particularly important in this respect due to their large availability of inorganic carbon.

In this context, few studies have quantified CO2 fluxes from temperate source springs and headwaters in karstic basins yet. Our study addressed this information gap by investigating CO2 outgassing from streams and groundwater-fed springs in a karstic terrain in central Germany, known as the Franconian Alb. We used DIC concentration and stable isotope (δ13CDIC) data to separate biological and geological factors that might be responsible for temporal and spatial variations and gradients in CO2 losses. This study examined characteristics of dissolved inorganic carbon (DIC) and excess partial pressures of CO2 (epCO2) in the source springs and headwaters of four watersheds in a Central European karstic region, via dissolved inorganic carbon concentration and stable carbon isotope measurements.

Our results showed the most 13C-depleted δ13CDIC values at the source springs, which become rapidly enriched downstream due to CO2 degassing. Concurrently, epCO2 values, while consistently in excess of atmospheric concentrations at the spring sources, show decreases of up to 92% within only 50 m downstream. In conjunction with the large observed flux estimates of up to 88 g C m–2 day–1, these findings suggest that karstic springs can act as focussed major CO2 sources to the atmosphere. Because headwater streams constitute the bulk of the surface area of most watersheds, they may provide a disproportionately large contribution to CO2 effluxes in carbonate-dominated basins, in which source springs play a particularly important role.

Reference

Lee, van Geldern and Barth (2021), Extreme gradients in CO2 losses downstream of karstic springs. Science of the Total Environment, 778, 146099, https://doi.org/10.1016/j.scitotenv.2021.146099



 
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