At the Königstein mine uranium was extracted by an underground in situ leaching method. WISMUT developed a flooding concept which allows the reduction of pollutant concentrations and prevent pollutant migration into the aquifers above and downstream the mine. The development of the concept and the documentation for permit application were based on a multitude of scientific and engineering studies and prognoses on substance output using two different approaches (upscaling of a flooding experiment, Numeric box model). Both modelling tools provide similar prognoses of flooding. With the flooding in progress (start January 2001), the modelling tools will be validated and further improved.