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An one-dimensional model for contaminant advection and dispersion through a GCL/AL liner system was developed to analyze the equivalence between a GCL (geosynthetic clay liner) and a CCL (compacted clay liner). The continuity of contaminant concentration and flux at the interface between the GCL and the underlying attenuation liner (AL) are obeyed in the model, and background concentrations in the soil liner are also considered. Based on the assumption that contaminant transport through a GCL was a steady state process, an analytical solution was obtained. Increasing the leachate head from 0.3 m to 10 m results in a reduction of the breakthrough time of benzene by a factor of 2.7. The breakthrough time of benzene increases by a factor of 7.0 when the hydraulic conductivity of GCL decreases by one order of magnitude. The breakthrough curves are more sensitive to the hydraulic conductivities of the GCL and AL (attenuation layer) than to the thickness of the AL. The standard 75 cm CCL can be replaced by a combination of a GCL and a 1.0-4.0 m thickness of AL. The proposed method can be used for preliminary design of GCL composite liners, assessing the equivalence between GCL and CCL, preliminary design of a remediation method for contaminated soils, and evaluating experimental results.

Marine sediments have been considered to be a major sink for microplastics, yet the pollution history of microplastics recorded in these sediments remains poorly understood. Using a combination of 210 Pb chronology and quantification of microplastics in undisturbed sediment cores, here we established the forty-year pollution history of microplastics in the northern South China Sea (SCS), the largest marginal sea of the western Pacific. We found that the pollution of microplastics in the northern SCS commenced in the 1980s. A dramatic increase of microplastic abundance in about 1998 marked an important breakpoint for microplastic contamination. Since then, microplastic abundances in the sediments have continued to increase and reached the highest level in 2018. This was well in line with the increasing trend of plastic output in the local industries. Reconstructing regional pollution history further revealed the shift of microplastic depocentres in the northern SCS over the past forty years. We estimated that the microplastic abundances in the sediments at nearshore stations will double by 2028. Our results provide the first example of the reconstruction of microplastic pollution history in marine sediments and new insights into how microplastics contaminated the marginal sea.

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