The Active Storage Facility for Residuals (Aktives Lager fu¨r Ru¨cksta¨nde – ALfR) — an open-air controlled area — was used for the storage of radioactive waste (liquid and solid waste) at the site of the NPP Rheinsberg in the former GDR. Liquid waste was cemented and dumped for near surface final disposal. During the period of operation of the ALfR some incidents happened. Residues of evaporated cooling water of unknown amount (ca. 1–10 m3), the drop-out from small leakages and the wash-off from contaminated open-air storage barrels contaminated the soil. Since 1997 the buildings of the ALfR have been decommissioned and the site will be remediated. From the beginning of decommissioning up to now the groundwater has been monitored. This paper shows different steps of decommissioning and explains the results of groundwater monitoring. Currently 33 sampling points are being used for sampling groundwater from different layers. The radiological parameters under examination are Cobalt-60, Cesium-137, Strontium-90 and Tritium. Non-radiological parameters like Bore, Potassium, Sodium, Nitrate, pH value and TOC (total organic carbon) are also being investigated. Since 1997 the activity concentration of Cobalt-60 and Tritium in the groundwater has been decreasing. The activity concentration of Strontium-90 had increased from 1997 to 2007. Since 2007 activity concentration of Strontium-90 has been decreasing. 350 m away from the ALfR, in the direction of groundwater flow, the Lake Stechlin is located in a nature protection area. The groundwater beyond the ALfR-site is not used as drinking-water and up to now there has been no detectable contamination of Lake Stechlin. Therefore there is no radiological impact on humans by contaminated groundwater. The hypothetic effective dose of Cobalt-60, Tritium and Strontium-90 resulting from postulated consumption of groundwater was determined using consumption rates according to the German Radiation Protection Ordinance. In case of using the groundwater as drinking-water the annual effective dose is 0,4 mSv/a at a postulated drinking-water well on the ALfR-site and 2,5 μSv/a at a postulated drinking water well in a distance of 142 m. Results of the observations: The observations of direction and velocity of the flow of the ground-water principally confirms the transportation model made at the beginning of decommissioning of ALfR. In vertical direction, the real spread of contamination in the groundwater is smaller than assumed. The dilution of Co-60 is higher than assumed. In contrast to the assumptions of the transportation model, Cesium-137 has a lower mobility.

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