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).