Iron as a new water "pollutant" in Central Germany

The River Pleisse, the major tributary of the River Weisse Elster in the Central-German industrial region, used to be excessively polluted mainly by industrial wastewater from coal-processing, textile, and leather industries before the political change in 1989. Since then, anthropogenic pollution has significantly decreased due to close-down of industrial plants and upgrading of wastewater treatment plants. Nevertheless, the water in the lower course of the river has remained turbid and brown. This causes concern in the city of Leipzig, where efforts are made to reconvert the river branches and mill races that had been canalized underground into open rivers and canals.
The cause of this phenomenon is groundwater pumping in the large brown-coal open-cast mining district south of the city. Quantity, quality, and locations of these mine-drainage discharges are changing along with the operation and advance of the mines. Groundwater that is drained from tertiary horizons, where marcasite and pyrite are present, contains up to 80 mg/l of soluble iron. In the river, the iron is rapidly oxidized, bound to particles of suspended matter, and transported downstream. Previously, when the organic pollution was high, the mine-drainage water with its high level of dissolved iron even had a purifying effect. The iron then flocculated the suspended particulate matter and precipitated it into sediment with a high content of sapropel.
Sediments sampled between 1991 and 2008 upstream of the mining area had iron contents between 4 and 5%. In the lower course of the river, the iron levels ranged between 24% in 1991, 8% in 1994, 20% in 2004, and 17% in 2008. The geogenic iron background level in floodplain soils on the lower River Pleisse is 4.3 % (Müller et al. 2003). Since iron is a common geogenic element it has accumulated with the mine drainage in the lower course of the River Pleisse for long. Suspended particulate matter, sampled between 2000 and 2008 in the lower course of the river at Leipzig, contains 6 to 26 mg/kg iron. This corresponds to 2 to 69 mg/l total iron. The wide variability is due to the fact that the sampling campaigns were made during different hydrological events (flood, drought).
Most of the iron in the river is particle-bound. That is why the total iron content of the river water is influenced by the amount of suspended solids in the water and by the concentration of iron on these particles. It depends on direct inputs as well as on the hydrological situation. Heavy rain erodes soils, and flood events mobilise old sediments (secondary sources), so that concentration peaks may occur.
The mean daily iron load is 1.2 t/d, during mean streamflow of 3.36 m³/s (gauged upstream of the mine-drainage input) and with a total iron concentration of 1 mg/l. Since January 2007, the mine-drainage inputs have been limited to 0.3 m³/s. This corresponds to an iron load of 0.8 t/d, calculated with a mean concentration of 30 mg/l. So, the total iron concentration in the river increases up to 3 mg/l downstream of the mine-drainage input.
It is a paradox that the decrease in pollution in the River Pleisse (heavy metals and organics) has also a negative side-effect. The fine iron-oxide particles remain longer in suspension, so that the riverbed on the whole lower course becomes covered with a brownish encrustation.
As a consequence, the mine operators are planning to build a mine-drainage treatment plant.