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Methanemissionen aus staugeregelten Fließgewässern: Von Messungen zu Modellen

Fachliche Zuordnung Hydrogeologie, Hydrologie, Limnologie, Siedlungswasserwirtschaft, Wasserchemie, Integrierte Wasserressourcen-Bewirtschaftung
Förderung Förderung von 2010 bis 2018
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 179857136
 
Erstellungsjahr 2018

Zusammenfassung der Projektergebnisse

Inland waters transport and transform significant amounts of carbon and account for ~18% of the global emissions of the greenhouse gas methane. Recent estimates point towards a significant contribution of man-made water bodies and reservoirs to these emissions. High emission rates have mainly been attributed to tropical reservoirs. In a reference study at six impoundments at the River Saar in Central Europe, we found that also small river impoundments in the temperate zone can be significant and unexpectedly strong sources for atmospheric methane, comparable in magnitude to tropical reservoirs. In continuation of this study, we investigated the representativeness of the findings from the Saar and the environmental conditions that potentially affect methane emissions from impounded rivers. To validate the short-term observations made during the first project phase, we continued ebullition measurements at selected sites to cover a complete annual cycle. In addition, we sampled the sediments, which are accumulating in the impoundments and estimated their methane production potential in laboratory experiments. The results confirmed that impounded river zones can produce and emit methane at high rates, although methane fluxes were up to one order of magnitude higher in summer than during winter, with temperature being the key driver. In comparison to other long-term ebullition measurements made in various aquatic systems on three different continents, the emissions rates measured at the Saar were highest, but all aquatic systems showed a nearly consistent temperature dependence. Measurements in other impoundments of the Rhine River system showed similar and consistent properties of methane formation but also showed that not all river impoundments are methane emission hot spots. Methane production rates in the sediments were found to be related to organic carbon content, but the main differences in methane emissions among impoundments was related to sediment delivery and deposition patterns. In areas with sediment depositions, ebullition was found to be the most important pathway for methane emission in all studied impoundments. Large amounts of gas that can be released in response to changing water level or atmospheric pressure suggested that significant amounts of gas are stored in the sediments. In a series of laboratory experiments, we found that the volumetric gas content in sediments of productive aquatic systems, like the studied river impoundments, can be up to 20% by volume. This unexpected high gas content leads to qualitative changes of the physical properties of the sediments, similar to the the well-known changes in soil properties occurring during the transition from saturated to unsaturated conditions. Also similar to soil structure in terrestrial environments, we observed dense network of connected macropores in aquatic sediments. They are formed by migrating methane bubbles and constitute preferable flow paths for bubbles and porewater. The gas storage capacity and the vertical extent of this network is related to grain size and other mechanical properties of the sediment. Ongoing research aims at combining the findings on methane formation and gas bubble dynamics in sediments with models, which are capable of describing and predicting the temporal dynamics of methane emissions. In addition, continuing research efforts include the expansion of study sites to include river impoundments across different climatic zones as well as the development of sediment management techniques, which eventually are capable of reducing methane emissions from these man-made aquatic systems.

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