Export pathways of biosolid derived microplastics in soil systems - Findings from a temperate maritime climate

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.