The natural clayey ground below three old waste sites in the Slovak Republic was mineralogically, chemically and geotechnically investigated. Concentration profiles of both the heavy metal and chloride ions in the soil below the waste sites were established. From these, together with the knowledge about the age of the waste site, the migration velocity of the ions into the clay was estimated. For heavy metal ions the velocity is in the range of a few centimetres per ten years. The migration velocity of chloride ions is five to ten times larger. From all the calculated apparent diffusion coefficients, the values for zinc about 1.2 x 10^-11 m²/s, for lead about 1 x 10^-11 m²/s and for chloride about 3 x 10^-10 m²/s seem to be the most realistic. With regard to the mineralogical composition, the most significant retardation processes were estimated. However, a sequence analysis indicated that sometimes the role of the ion exchange can be overestimated. The study also demonstrated the difficulties in gaining undisturbed samples of sufficient quality by drilling, as well as in the interpretation of the established concentration profiles.