Analytical model for non-isothermal diffusion of contaminants in unsaturated composite liner

Barriers consisting of a composite geomembrane (GMB) and compacted clay liner (CCL) are the most widely used systems for the landfill bottom liner. It is anticipated that for the case of intact GMB, the CCL remains unsaturated under the thermal gradient sourcing from the decomposition of the wastes. The existing knowledge on the effects of such coupled thermo-hydraulic conditions on the leakage of organic compounds are rather limited. An analytical solution is proposed to the non-isothermal diffusion of organic compounds in the unsaturated GMB/ CCL composite liner. The solution is based on the separation of variables for one dimensional problem which is tested against two benchmarks to demonstrate the accuracy of the solution. The influences of thermal diffusion, temperature dependent parameters and unsaturated water distribution on the overall transport of contaminant in the system were investigated. The results show that the thermal gradient can affect the contaminant diffusion in two ways: (i) by the Soret effect and (ii) by the temperature dependent coefficient in GMB. It is indicated that the contaminant concentration and flux can be significantly reduced when the state of underlying liner transfers from saturated to unsaturated. The effect of unsaturated water distribution should be considered when the unsaturated exponent is larger than 0.1 m(-1).