Groundwater quality can be deteriorated by leachate that seeps through landfills. The placement of a low permeable barrier at the bottom of the landfill would protect the soil and groundwater by restricting or delaying the leachate migration to the subsurface environment. Clay liners have traditionally been widely used to construct engineered barrier systems. The use of composite materials, particularly waste materials modified with clay, as landfill liners has increased in recent years. This study investigated the use of natural soil from the lower Himalayan region and fly ash, a waste product from thermal power plants, as a potential landfill liner material. The most important parameters in landfill design, hydraulic conductivity (< 10-7 cm/s) and unconfined compressive strength (0.20 MPa), have been evaluated. Further, a set of contaminant transport experiments with reactive tracer were carried out for the composites to understand the governing transport mechanisms of leachate through a liner. The experimental data obtained was analyzed using the breakthrough curves. Different physical and chemical equilibrium and non-equilibrium transport models were employed to analyze the experimental data. The descriptive analysis of the BTCs suggests the contribution of multiple porosity domains in the transport of the contaminant through a liner. Factors like clay dispersion, swelling in clay, and immobile fractions behaving as source/sink terms contribute to complex transport behavior through the composite. The findings of this study provided a scientific understanding of the processes involved in the plume movement and can add to the effectiveness of the design of low permeable barriers.