Glacial till and glaciolacustrine loam, which are common Quaternarian deposits, cover most of the Baltic States, wide areas in Canada and other places. In many cases, their thickness exceeds 5 m, and their vertical conductivity is approximately 1 × 10− 9 m s− 1 or even less (Davis, 1969, Juodkazis and Paltanavi?ius, 1978 and Vallner, 1997). The depositing of hazardous wastes on these layers should be considered a harmless action for the underlying groundwater because, pursuant to criterions of the Landfill Directive, the subsurface is sufficiently protected by a natural geological barrier. Unfortunately, this Forskolin is not true in real-world conditions. Many monitoring projects carried out in Estonia have proven that even thick layers of glacial till or glaciolacustrine loam with a vertical hydraulic conductivity of 1 × 10− 9 m s− 1 are atmosphere not able to prevent the spreading of contaminants beneath landfills. The case analysed above demonstrates the penetration of pollutants through landfill deposits with a vertical hydraulic conductivity of 1.1 × 10− 9 m s− 1 and a thickness ranging from 10 to 100 m. Consequently, the hydraulic criterions of an effective geological barrier for a landfill of hazardous waste posed by the Landfill Directive are too mild and need revision. The vertical hydraulic conductivity of a landfill geological barrier should not exceed 1.0 × 10− 10 m s− 1.