Reservoir sediments in central Europe as archives of human-environmental interaction during the past 115 years - the example of the Urft Reservoir

The mid-European landscape has been influenced by humans since several millennia. In the Eifel Mountains in western Germany, mining and ore processing in combination with land-use changes considerably altered sediment composition and sediment fluxes. While there have been frequent studies to reconstruct changes in sediment fluxes on the long term, considerably less research focused on the past century. To decipher the recent human influence on the landscape, the sediments of the Urft Reservoir in the northern Eifel Mountains were analysed. The Reservoir started operation in 1905, and was the largest reservoir in Europe at this time. In November in 2020 the reservoir was drained for construction works, offering the unique possibility to analyse sediment volume and composition. A high resolution sediment budget for the past century was calculated using topographical maps with a scale of 1:1000 created prior to the construction of the reservoir. For the most recent topography the entire lake area was photogrammetrically surveyed using an uncrewed aerial system (UAS). Mean accumulation in the whole reservoir was around 1.54 m and regionally above 6 m. Additionally, 24 cores were retrieved from the bottom of the reservoir. A range of different sedimentological proxies including grain-size, heavy metals, geochemical ratios, sediment colour and microplastics were analysed. An absolute chronology was established based on 137Cs dating. Up to four different sedimentary units could be distinguished in the cores. The upper two units consist of reservoir sediments and were deposited between 1905 and 2020. The heavy metals content in these sediments show a strong connection to historical changes in the ore industry in the Urft valley. The decline of the metal processing industry as well as stricter environmental protection laws resulted in a reduced input of lead, copper and zinc from the 1960s to the 1980s. Since that time the content has remained relatively constant. Microplastic particles appear in the sediments since the mid-1960s. Furthermore, a distinct layer of high microplastic content was recorded in the cores. This event-layer could be traced back to a major fire in a glassworks and plastics factory in 1991 in the upper Urft catchment. In summer 2021, the northern Eifel Mountains were impacted by a catastrophic flooding event, resulting in massive destructions in the catchment of the Urft and strong relocation of sediments in the floodplain. To assess these geomorphologic changes in the Urft reservoir, the water level was lowered again in December 2021 and an additional UAS survey was conducted. Furthermore, additional sediment samples were taken. However, we could neither observe any significant changes in the heavy metal content in the flood sediment nor asses the sediment input by the flooding event. The topographic changes due to the flood were generally to low and within the error margins of our method (0.5 m).