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.

Microplastics in brown trout (Salmo trutta Linnaeus, 1758) from an Irish riverine system.

Rivers play an important role in the overall transport of microplastic pollution (1 µm to 5 mm), with fluvial dynamics expected to influence biotic interactions, particularly for fish. So far, there have been few assessments of microplastics in freshwater salmonids. The prevalence (i.e. percentage occurrence) and burden (i.e. abundance per fish) of microplastics were assessed in the gastrointestinal tracts (GITs) and stomach contents (SCs) of 58 brown trout Salmo trutta Linnaeus, 1758 sampled at six sites along the River Slaney catchment in south-east Ireland. Sites were divided into two classifications (high and low exposure) based on proximity to microplastic pollution sources, comprising three sites each. Analysis of biological traits (e.g. fish length) and diet was performed on the same fish to determine possible factors explaining microplastic burden. Microplastics were found in 72% of fish having been recovered from 66% of GITs (1.88 ± 1.53 MPs fish⁻1) and 28% of SCs (1.31 ± 0.48 MPs fish⁻1). Fibres were the dominant particle type recovered from GITs (67%) and SCs (57%) followed by fragments. No difference in median microplastic burden was observed between fish collected in high and low exposure sites. Microplastic burden was unrelated to fish fork length, while microplastic size distribution (100 ≤ 350 µm, 350 µm to ≤ 5 mm) was unrelated to S. trutta age class estimates. Furthermore, microplastic burden was not explained by dietary intake. Though further research is necessary, this study showed the presence of microplastics in wild S. trutta collected from an Irish riverine system, which could have further implications for top-level consumers that feed on the species, including humans. Further analysis is required to determine possible trophic linkages for the species, with respect to microplastics, and to assess the suitability of S. trutta for monitoring microplastics in river systems.