In 1961, SDAG Wismut began prospecting for uranium in the Elbe valley Cretaceous formation in Saxony. Uranium mining using conventional methods started at this site in 1967. Localised in different sandstones of the Cretaceous formation, the uranium deposit was developed across an area of approx. 6 km² and mined near the towns of Königstein – Bielatal – Langenhennersdorf – Struppen. As it was located in the immediate vicinity of the main shafts, the locality of Leupoldishain was almost completely undermined. Local and regional hydrology had significant impact on mining and subsequent mine rehabilitation. Reserves of the Königstein deposit totalled some 30,000 tonnes of uranium. The mine has some exceptional features which have to be taken into consideration when it comes to stabilisation and subsequent flooding. Given the relatively low grades of uranium mineralisation in the sandstone, investigations were conducted in the early 1970s to identify more efficient chemical production processes. In 1984 underground uranium mining was switched to chemical leaching. Various leaching technologies were used involving water diluted sulphuric acid. This leach solution was injected via boreholes into prepared sandstone blocks or blasted chambers. Following leaching, the pregnant liquor was collected and pumped to the surface for further treatment. Eventually, the uranium was separated in a processing plant.

By 1990, this technology had exposed more than 55 million tonnes of rock to sulphuric acid-bearing liquor. Part of this liquor remained trapped as pore water in the sandstone and caused further mobilisation of uranium and heavy metals. In the event of uncontrolled mine flooding, these elements would have dissolved and migrated into neighbouring aquifers or into receiving streams, respectively. In the light of this complex situation, outright flooding appeared to involve to great an environmental risk. For this reason, an independent concept for the flooding of the Königstein mine was developed back in 1991. Ten years later the stage was set to initiate controlled flooding of that mine.

The flood water is treated in a treatment plant. Following removal of the dissolved uranium, the mine water is fed to a water treatment plant prior to discharge into the Elbe river. It will probably take longer than 2012 to complete flooding of the mine complex.

Treatment of the flood water will be required beyond that point in time. Pumped via two newly drilled pumping boreholes reaching ca. 300 m down into the mine, the flood water contaminated with radionuclides and heavy metals will be delivered to the water treatment plant. This process will have to continue until such time as no more significant impact to aquifers and the Elbe River is to be anticipated.

Treatment of mine water, however, will have to continue beyond that timeframe. Wismut has put in place a comprehensive system of measurement points to monitor the flooding process and to document flood level and water quality.

Flooding of the Dresden-Gittersee mine site

Flooding of the mine which comprises the two mine fields of Dresden-Gittersee / Bannewitz and Heidenschanze was initiated in May, 1995. The flooding is aimed at restoring the natural ground water level in the Freital mining district. Sequence of the flooding is characterised either by the step-wise increase of the flood level or by its freeze made possible by two production wells installed in the mine fields. This measure is designed to prevent mine subsidence and other damage that might be caused by the abandoned mine.

When the flood level reached approx. 180 m a.s.l. in July, 2003, there was locally emergence of mine water in Freital-Potschappel. Thereupon, the flooding was suspended, and the flood level was lowered to some 156 m a.s.l. by the resumption of mine water pumping. Wismut, regulators, and advisers took months to determine the reasons of this emergence. It is assumed that some unknown water routing from century-old coal mining operations had played a major part. To achieve consolidated findings, an application was filed for the resumption of filling up to a level of 165 m a.s.l. At level rise > 156 m a.s.l. there was confirmation of the hydraulic connection existing between the Gittersee/Bannewitz and Heidenschanze mine fields. With a view to avoiding any further mine damage from such flood water levels, a connecting gallery (WISMUT-Stolln) is being driven at the geotechnically uncritical level of 120 a.s.l. starting from the Dresden/Gittersee/Bannewitz mine field to the Elbstolln to ensure the sustained safe discharge of flood water from the mine fields located on the right bank of the Weißeritz creek. This gallery will divert flood waters from the abandoned uranium mine fields to the Tiefer Elbstolln that ensures controlled discharge into the Elbe River. This will keep the flood water at a constant level of 120 m a.s.l.
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