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Polymer-Specific Vertical Migration of Microplastics in Soil Beneath Landfill Leads to a Risk-Depth Decoupling: Attenuated Abundance Does Not Mean Reduced Ecological Risk
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Researchers studied soil and groundwater at a landfill site, finding microplastics present throughout 6 meters of soil depth. While total microplastic amounts decreased deeper underground, mobile plastic types like polyethylene became more common at depth, keeping ecological risk levels steady rather than declining alongside abundance.
Microplastics (MPs) generated from plastic degradation in municipal solid waste landfills pose a persistent threat to subsurface soils and groundwater, yet their vertical distribution and migration mechanisms in deep soils remain poorly understood. We established nine soil and five groundwater sampling locations across an informal landfill site. Soil cores were collected at ∼1 m intervals to 6 m depth to analyze MP distribution and ecological risks. The results indicate pronounced MP abundance in both deep soils and groundwater, with average abundances of 57,740 ± 42,355 particles/kg dry weight and 55 ± 51 particles/L, respectively. The dominant polymers in soil were polytetrafluoroethylene (PTFE) and polyethylene (PE), accounting for 60.14% and 22.58%, respectively. In contrast, groundwater was primarily characterized by PE and polypropylene (PP), with PE representing the highest proportion at 58.82%. MP abundance in soils decreased significantly with increasing depth, indicating attenuation during vertical migration. Notably, distinct polymer-specific migration behaviors were observed: PTFE was preferentially retained in shallow soils, whereas PE exhibited greater vertical mobility and became relatively enriched in deeper soils and groundwater. Ecological risk assessment revealed that although the abundance of MPs was observed to decrease significantly in depth, the potential ecological risk index remained relatively stable across soil depths due to the increasing proportion of mobile polymers such as PE and HDPE in deeper layers. This indicates that risk assessments based solely on abundance may systematically underestimate the MP threat in deep subsurface environments. This work elucidates that polymer-specific migration is a key mechanism controlling the subsurface transport and fate of MPs from landfills, providing a scientific basis for developing polymer-informed risk management strategies.
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