Mining and ore processing of “copper shale” in the Mansfeld region, Germany, were stopped in 1990 as a consequence of the obsolete level of the applied technology. One of the residues of the former pyrometallurgical activities is a flue dust, which was scrubbed out of the process gases. The scrubbing resulted in an extremely fine-grained sludge, which was stored in “ponds” where it settled down as sediment. The need to assess the risk of heavy metal emissions posed by such sludge deposits when they dry up and become exposed to weathering prompted the question over the depth to which heavy metals can possibly be mobilized. A drill hole was bored into the dried-up sediment of such a “pond” and the sediment material was analysed. The results show a substantial influence of weathering in the upper 0.5 m of the material, In particular the oxidation of the heavy metal sulphides abundant in the sediment and the subsequent dissolution of the sulphates is nowadays a major problem for the quality of the nearby ground- and surface waters.