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During open-cast lignite mining in Lusatia (NE Germany), iron sulfide minerals associated with lignite, are exposed to air and weather. The weathering products iron (Fe), sulfate and acids subsequently enter the fluvial-lacustrine system Spree and its tributaries.
The aim of this work was to identify and explain the evolving biogeochemical signatures (spatio-temporal patterns) in River Spree. These signatures are significantly influenced by biogeochemical turnover processes triggered by iron input. To identify the signatures, a regional field study was conducted from the inland delta Spreewald to the mouth. This study was followed by two laboratory experiments under controlled redox conditions to investigate the effect of Fe on the long-term binding of phosphorus and the degradation of particulate organic material in the iron-enriched sediments.
Characteristic elements introduced by lignite mining (iron, sulfur, aluminum, cobalt, nickel) and by urban areas (zinc, lead, copper, chromium) could be determined by means of a principal component analysis. Based on these findings, the impact range of the open-cast mining was determined to be at least ~ 90 km downstream of the Spreewald. Within the urban-influenced systems Fürstenwalde and Berlin, the urban signature is more dominant than the mining signature. By means of sequential extractions and XRD analyses of the sediments, iron binding forms and characteristic iron minerals have been additionally identified (e.g., Fe(III) oxyhydroxides, pyrite, vivianite).
Sediment analyses and column experiments have shown that iron increases the phosphorus sorption and decreases the phosphorus release in the sediments of three studied lakes of the fluvial-lacustrine system. However, the application of different phosphorus retention models for Lake Neuendorfer See showed that Fe cannot reduce the eutrophic state of the lakes along the river, since the water retention times are too short and the external phosphorus loads are too high. Additionally, lake sediments with different Fe contents have been incubated with particulate organic material. Higher Fe contents resulted in reduced emissions of the greenhouse gases CO₂ and CH₄, which can be attributed to a reduced degradation of organic matter by Fe. Possible reasons include sorption processes of (dissolved) organic matter onto Fe oxyhydroxides and toxic effects of iron on microorganisms. Nevertheless, Fe (and sulfate) are available as electron acceptors during the decomposition of organic matter in River Spree.
The findings of this work deepen the understanding of the impact of iron from mining on sulfur, phosphorus and carbon turnover processes in fluvial-lacustrine systems. As a consequence of the Fe input from the Lusatian area, the signature will persist for decades in River Spree.