Leaching of microplastics by preferential flow in earthworm (Lumbricus terrestris) burrows

Summary

This laboratory study examined how Lumbricus terrestris earthworms mediate the transport and leaching of low-density polyethylene microplastics through sandy soil columns. Using a controlled five-treatment experimental design with replicates, the authors demonstrated that earthworms ingest surface microplastics and transport them to deeper soil layers, with a tendency to preferentially move smaller particles (<250 µm). Microplastics were subsequently detected in leachate only from treatments containing earthworms, suggesting that bioturbation creates preferential flow pathways that enhance vertical microplastic migration and potential groundwater contamination.

UK applicability

The findings are relevant to UK soil and water quality management, given widespread earthworm populations in UK agricultural soils and the potential for microplastic contamination of groundwater resources. The study suggests that earthworm-mediated transport may be a significant but overlooked pathway for microplastic mobilisation in UK arable and pastoral systems, particularly in areas with shallow water tables. However, UK soil types and hydrogeological conditions vary considerably from the sandy soil tested, so local validation would be needed to assess groundwater contamination risk in clay-dominant or freely draining regions.

Key measures

Microplastic particle distribution by size class (<150 µm, 150–250 µm, 1–250 µm) across soil depths; saturated hydraulic conductivity (Ksat); microplastic detection in leachate; soil layer concentration profiles; earthworm biomass and activity

Outcomes reported

The study quantified how earthworm activity transports low-density polyethylene (LDPE) microplastics to deeper soil layers and determined whether microplastics can be flushed from soil columns via water leaching. Results demonstrated that microplastics were detected in leachate only from treatments containing earthworms, indicating earthworm-mediated transport facilitates microplastic mobilisation. Earthworms transported microplastics to lower soil layers with smaller particles preferentially mobilised.

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