This paper presents experimental studies and first results of a pilot scale RAPS-treatment system to reduce the high iron content of an iron rich mine water. In 1985, economic reasons caused the closure of the fluorspar mine “Hohe Warte” near Gernrode in the German Harz Mountains. Though remediation works were conducted in the preceding years, mine water is currently flowing out freely of the dewatering adit and is impacting the environment and the receiving brook. The mine water is characterised by low pH-values, high conductivities and contains considerable amounts of iron, manganese, and arsenic. Annually, the mine discharges 0.7 Mio m³ of mine water with 7.8 tons of iron, 3.8 kg of manganese, 13 kg of arsenic, and 2,5 kg of uranium. Therefore, the discharging mine water has a great potential to affect the quality of the receiving streams and resulted in a decrease of biological diversity. Based on hydrogeological and hydrochemical investigations, a passive treatment system would be able to treat the mine water. In February 2003 a pilot system was installed near the dewatering adit consisting of a settlement pond, a RAPS system (reducing and alkalinity producing system) and a constructed wetland. It could be shown that this passive system was able to treat an aliquot of the mine water down to ecologically acceptable standards.