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Export pathways of biosolid derived microplastics in soil systems – Findings from a temperate maritime climate
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This field study tracked how microplastics from sewage sludge (biosolids) spread once applied to agricultural land, finding that particles can migrate horizontally through soil to surface water via overland and interflow runoff, as well as potentially leaching vertically toward groundwater. The results are significant because biosolid application is a major and growing pathway through which microplastics enter farmland soils across many countries.
The environmental fate of microplastics (MPs) added to agricultural soils remains poorly understood, particularly regarding their mobility in soils. Here we investigate the potential for MP export from soil to surface waters and groundwater in two agricultural settings with a 20-year history of biosolid treatment. A third site where biosolids had never been applied served as a reference (Field R). The potential for MP export along overland and interflow pathways to surface waters was determined from MP abundances in shallow surface cores (10 cm) along ten down-slope transects (five each for Field A and B), and through MP abundances in effluent from a sub-surface land drain. The risk of vertical MP migration was assessed from 2 m cores, and from MP abundances in groundwater sampled from the core boreholes. XRF Itrax core scanning was conducted on two of the deep cores to capture high-resolution optical and 2-D radiographic imaging. Results suggest limited MP mobility at depths >35 cm, with MPs largely recovered in surface soils characterised by lower compaction. Furthermore, abundances of MPs across the surface cores were comparable, with no evidence of MP accumulations observed. Average MP abundance in the top 10 cm of soil across Field A and B was 365 ± 302 MP kg-1, with 0.3 MP l-1 and 1.6 MP l-1 recovered from the groundwater and field drainpipe water samples, respectively. MP abundances were significantly higher in fields treated with biosolids than in Field R (90 ± 32 MP kg-1 soil). Findings suggest ploughing is the most significant driver of MP mobility in upper soil layers, however the potential for overland or interflow movement cannot be excluded, particularly for fields that may be artificially drained.
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Researchers investigated whether surface water runoff transports microplastics from sewage sludge-amended agricultural soils to aquatic ecosystems in a semi-arid region of Spain, providing evidence that runoff is a relevant pathway for microplastic redistribution.
Microplastics from Agricultural Soils to Freshwaters: Pathways, Loads, and Controls
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Researchers synthesized field surveys and modeling studies on microplastic transfer from agricultural soils to freshwaters, finding that while only a small fraction of soil MPs is exported during typical rain events, extreme storms and biosolid applications can generate fluxes rivaling other major emission pathways, with particle size, density, and shape strongly controlling mobilization.
Biosolid amendments as drivers for microplastic pollution in soil: Measurements and insight from multiple analytical methods in an agricultural field study
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Farms sometimes spread "biosolids" (treated sewage sludge) on fields as fertilizer, and this study measured microplastic particles in soil that had received these applications over a decade compared to untreated farmland. Using detailed chemical testing, researchers confirmed that soil treated with sludge contained identifiable plastic contamination, supporting concerns that this common farming practice can be a source of microplastics building up in agricultural land. Since these plastics can end up in the crops we eat and potentially in our food and water supply, understanding where they come from is an important step toward reducing our overall exposure.
Investigation and analysis of microplastics in sewage sludge and biosolids: A case study from one wastewater treatment works in the UK (article)
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This UK case study tracked microplastics through an entire sewage sludge treatment stream at one wastewater works, from raw influent through to biosolids applied to agricultural land. Microplastics were present at every stage and concentrated in biosolids, suggesting land application of biosolids is a significant pathway for introducing microplastics into agricultural soils.
Investigation and analysis of microplastics in sewage sludge and biosolids: A case study from one wastewater treatment works in the UK
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Researchers investigated microplastic contamination in sewage sludge and biosolids at a UK wastewater treatment plant and found significant quantities of microplastics persisting through the treatment process. Since millions of tonnes of biosolids are applied to farmland annually in the UK, this represents a major pathway for microplastic pollution entering agricultural soils. The study highlights the need for better monitoring and treatment methods to reduce microplastic transfer from wastewater to the terrestrial environment.
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