As a contribution of safety assessment of the underground repository for radioactive wastes (ERA) in Morsleben isotope investigations in the hydrosphere has been carried out. The measured tritium concentrations of brines infiltrating into the mine cannot be interpreted in a conventional way due to contamination of mine air with tritium. However, modelling the isotope exchange allows conclusions regarding the water balance of the dripping brines. A complex interpretation which includes hydrogeochemical data results in a qualitative assessment of the infiltrating brines in regard to their hazard potential. An acute danger cannot be derived from the data available up to the present. The natural input of cosmogenic radionuclides (tritium, radiocarbon) into the aquifers above the salt level permits to study radionuclide migration at the ERA site. Tritium from the nuclear weapon tests is detectable up to a depth of 50 m below groundwater level with a maximum in about 20 m. From these data infiltration velocities of 1.6 m/a at maximum and 0.9m/a in avarage arc derived. The14C measurements of samples from more than 100 m depth yield model ages in the order of 104years. This indicates a significantly reduced groundwater dynamic in the deeper horizons.