Compacted clay liners are common, major, components of landfill leachate (fluid) containment systems. This is sensible, but knowledge and understanding of the longterm performance and behaviour of day mineral/landfill leachate systems remain very limited. The authors studied the reactions of day soil with leachate simulants related to three different climates and waste cultures. The day came from a Tertiary sequence near Melbourne, Australia, a type in common use locally for landfill engineering. X-ray diffraction was used to observe mineralogical change in 3 mm clay plugs caused by reactions with the leachate simulants. Changes in hydraulic conductivity were also measured. The results show both that the different leachates have distinct effects on the clay minerals, and that the leachate/day reactions have direct measurable and distinct impacts on hydraulic conductivity. The laboratory studies were completed at the University of Melbourne. The X-ray diffraction work was completed at The Natural History Museum in London. The experimental results are discussed here and indications given of some potential implications.