Abstract: An assessment of the potential use of a typical Japanese marine clay, Ariake clay, as a soil barrier material in landfills was presented. Two types of heavy metals, Cd2+ and Pb2+, were selected as key contaminants for laboratory diffusion test. Using commercially available contaminant transport software, the effective diffusion coefficients and partition coefficients of both Cd2+ and Pb2+ were back-calculated from the results of the diffusion test. The effective diffusion coefficients were found to be consistent with previously published values, indicating that the presented testing method was acceptable. The high retained percentage of Cd2+ and Pb2+ in the Ariake clay specimen indicated that the Ariake clay might be used as a good barrier material in landfills. With the derived effective diffusion coefficients and partition coefficients, the influences of Cd2+ and Pb2+ on the underlying aquifer in an assumed landfill were numerically assessed using the 1D diffusion transport theory.