ABSTRACT
The Ronneburg mining district in the eastern part of Thuringia (Germany) was between 1950 and 1990 one of the largest uranium mining sites in the world. Since 1990 the remediation of the mining site is realized by the Wismut GmbH with financial support of the German government. To reduce the concentrations of radionuclides and heavy metals in ground and surface waters, the remediation activities include two main projects: (1) flooding of the underground mine; (2) backfilling and covering of the former open pit mine with material from the waste rock dumps. The waste rock mainly consists of black shales, metabasaltic rocks and carbonate of Ordovician to Devonian age. These metasedimentary and volcanic rocks contain up to 7 wt% sulfides, 5-9 wt% organic carbon, 30-60 ppm uranium and a series of trace elements. The aim of this case study is to support the application of effective remediation techniques in a catchment area of a small creek, which was effected by acid rock drainage (ARD) generated from a nearby waste rock dump and which is expected to be the most important exfiltration area of flooding waters after the groundwater rise. Since 1997 the influence of highly mineralized, low pH seepage waters on the unsaturated valley sediments as well as on ground and surface waters has been investigated. About 160 drill holes and geophysical investigations (e.g. resistivity measurements) allow a detailed characterization of the present day distribution of different valley sediments. With hydrochemical methods and geochemical modeling, flow paths of seepage water e.g. in the unsaturated valley sediments were detected. The influence of ARD on surface water and groundwater was assessed. Sources and sinks for heavy metals and uranium are revealed. The use of heavy metals and rare earth elements (REE) as tracers, allows the identification of several processes along different flowpaths e.g. dilution and (co)precipitation. These results are completed by a series laboratory investigations.
