Study area
We conducted a study at the Spree River, which is a sixth order lowland river located in north-eastern Germany. It flows through the Lusatian region, where lignite mining activities during the times of the German Democratic Republic have been extensive and are still conducted to date (Krümmelbein et al. 2012). In the 1960s, the Spree River was damned for flood control purposes and hydropower generation. As a consequence, the Spremberg Reservoir was built, which is one of the largest reservoirs in Germany. Nowadays, the reservoir plays a crucial role in securing the water quality of the Spree River water impacted from the mining activities. Due to the post-mining groundwater rerise, sections of the rivers are characterized by large amounts of iron, sulphate, trace metals, and aluminum (Friedland et al. 2021). To retain the iron load and to safeguard the drinking water supply, passive remediation technologies are applied. Sequential conditioning of the river water comprised the addition of lime to increase pH to accelerate iron (II) oxidation and the addition of a flocculation aid to enhance flocculation of iron hydroxide sludge (Uhlig et al. 2016). The precipitated iron sludge usually ranges between 1 000-4 000 kg, occasionally 150 000 kg on a daily base (Uhlmann et al. 2021). The visibility threshold of the dominant fraction of iron in the study area, iron (III) hydroxide, is about 2-3 mg L-1 (Benthaus and Totsche 2015); hence the conditioning of the iron-rich water lead to contrasting environments in river sections with turbid water of high iron concentrations upstream of the dam and clear water of lower iron concentrations downstream of the dam (Friedland et al. 2021, Uhlmann et al. 2021, Figure 1).