Examining sampling protocols for microplastics on recreational trails

Hiking and trail running are increasingly popular and could present a significant source of microplastics on recreational trails in nature reserves, wilderness areas and conservation areas. Deposition may be concentrated on trail surfaces, however sampling techniques for microplastics on soil or rock surfaces have not yet been developed. In this study, sampling strategies were evaluated for microplastics on three types of recreational trail surfaces - asphalt, compacted soil, and a loose overlay of soil. We spiked trail surfaces with pink rubber microplastics and collected samples using a handheld vacuum, manual sweeping, and gel lifter tape. Spiked and in situ microplastics were extracted from soil samples using density separation (NaI, ρ = 1.6 g cm-3) with organic matter digestion (30% H2O2), then visualised and counted using stereomicroscopy. The gel lifter tape yielded the highest recovery of spiked and counts of in situ microplastics on asphalt (118% ± 15%, 3183 ± 830 microplastics per 40 cm2) and compacted soil (127% ± 7%, 333 ± 106 microplastics per 40 cm2). Sweeping produced quantitative recovery for spiked microplastics on compacted soil (88% ± 13%) but yielded significantly fewer in situ microplastics (148 ± 40 microplastics per 40 cm2) than the tape. Sweeping was the only technique to achieve quantitative recovery of spiked microplastics in the loose overlay of soil (110% ± 14%) when soil carbon was 0.8% ± 0.3%, however increasing soil carbon was associated with reduced microplastic recovery. Preliminary assessment indicated quantification of microplastics smaller than 100 μm was not possible with any of the methods tested. Sweeping and the gel lifter tape were both effective for evaluating microplastic deposition and spatial distribution on recreational trails, depending on the properties of the trail.