Initial situation

The former Borna Ost/Bockwitz opencast mine pits to the north-east of Bornas have been rehabilitated and made available for after-use. Basically these are the former main pit, the Bockwitzer Lake, which includes dammed water storage, and the remaining Südkippe landfill, the main water reservoir, and the wetland. After the water levels have been reached, excess water is drained off through a drainage system. Discharges into the drainage system must comply with the official discharge parameters: a pH value of between 6.0 and 8.0, and a maximum iron content of 3 mg/l.

Approach

After depth-oriented sampling and testing of the former opencast mine pit, the results of the monitoring were assessed, and the expected water quality determined using a lake model. To guarantee the required discharge conditions, the Bockwitzer Lake had to be neutralised. The results of a field test led to the initiation of a pilot test to neutralise the lake by adding soda: Since the completion of the technical installations in March 2004, soda has regularly been discharged into the lake. At the same time an extensive monitoring programme has been conducted to optimise the conditioning measures.

Result

The continuing neutralisation measures have improved the quality of the water in the former opencast mine pit, and it has proved possible to discharge surplus water from the Bockwitz Lake into the drainage system while complying with the discharge criteria mentioned above.
The pilot test is being continued in order to reach a stable water quality in the former mine pit.