Flooding of approximately
20 million m³ of open mine space
Following termination of uranium mining, activities at the Ronneburg site in East Thuringia switched to mine decommissioning and close-out, just as at any other mining or milling site operated by SDAG Wismut. Approximately 20 million cubic metres of open mine workings and 40 shafts had to be prepared for flooding. This involved removal of grease/oils and of hazardous chemicals from underground mines, backfilling and sealing of day shafts and near-surface mine voids, and construction of about 120 hydraulic barriers.
Flooding of mine workings occurs in two hydraulically separate systems: flooding of the mine fields located south of the A 4 motorway was initiated at the turn of the year 1997/98, that of the mine fields located north of the A 4 began in January 2000. By the beginning of June, 2004, the flood water level in the southern mine fields had risen to approx.
200 m a.s.l. and that in the mine fields located north of the A 4 to approx. 210 m a.s.l.
High contaminant levels in the flood water south of the A 4 are well in excess of standards for direct discharge into receiving streams. This necessitated the construction of a water treatment plant, of flood water collection and feeding systems, and of discharge units.
Anticipated flood water emergence points are controlled by a comprehensive flood surveillance system (flood monitoring) to allow remedial action to be taken in case of need. Being the anticipated main discharge area, the Gessen valley was provided with a basic water catchment system designed to collect emerging flood water spilling over at a level of 240 m a.s.l. and to channel it to the Ronneburg water treatment plant. Initial flood water discharges were observed in the Gessen valley in November 2006. Optimisation and the extension of water catchment capabilities in the Gessen valley will allow to cope with further flooding challenges. Somewhat smaller discharge locations to the north of the federal motorway BAB 4 occurred late in 2008.
At present, the flood water has risen to a level of ca. 235 to 240 m a.s.l.
Based on the current state of knowledge it is estimated that water treatment will have to be continued for a period of up to 25 years.