||Tailings basins within mining areas may represent potential sources of environmental contamination for soil and underground water. In fact, the disposed muds are typically characterized by high concentration of heavy metals and other possibly dangerous compounds. The tailings basins built in Europe before the legal implementation of the EU Directive on the landfill of waste (Directive 99/31) were not provided with impermeable barriers. In such conditions, during the initial period of the basin life the liquid phase in the disposed residue filtrates throughout the solid phase under a unit vertical hydraulic gradient, reaching the soil underneath; afterword, when the accumulated mud forms an impermeable layer at the bottom of the basin, the same mud, under the load of the superimposed new strata, consolidates, ejecting liquids throughout the bottom. The article discusses the implementation criterion aimed at evaluating the conformity of old tailing basin to the new regulation on landfill of waste and a method for the calculation of the rate of polluted liquids released through the bottom of a tailing basin during its operative life and after its closure.