Long-term service performance of cutoff walls considering chemical compatibility of soil-bentonite

Considering the high concentrations of Na⁺ and Ca²⁺ in the leachate of landfills, the hydraulic conductivity of soil-bentonite will increase significantly, thereby accelerating the high-risk pollutants to break through the soil-bentonite cutoff wall. A numerical model for the transport of organic contaminants in the soil-bentonite cutoff wall-aquifer system under the action of Na⁺ and Ca²⁺ is established. This model can consider the changes in parameters such as hydraulic conductivity and hydrodynamic dispersion coefficient with the concentrations of Na⁺ and Ca²⁺. The numerical solution of the model is obtained by the finite difference method. The correctness of the model is verified by comparing its calculated results with the existing analytical solutions and those of the COMSOL software. Finally, based on the established numerical model, the effects of chemical compatibility of soil-bentonite, water head difference and bentonite content on the long-term service performance of the cutoff wall are analyzed and discussed with toluene as a representative organic pollutant. The results show that after considering the chemical compatibility of soil-bentonite, the cutoff wall changes from never breaking down to breaking down after 75 years of service, and the service life is significantly reduced. Reducing the water head difference at both sides of the cutoff wall can effectively delay the breakthrough time of contaminant. The lower limit of the bentonite content of the cutoff wall should be appropriately increased on the basis of the original anti-seepage requirements to deal with the decrease of anti-fouling barrier performance of the cutoff wall under the action of contaminants.