Weathering alters the profile of trace metals and organic compounds in leachates and bioavailability extracts from microplastics of trail running shoes.

Microplastics (MPs) from rubber outsoles of trail running shoes may contribute significantly to contamination in protected areas. In the natural environment, weathering processes can damage MP molecular structure and alter the mobility of inorganic and organic compounds used as additives in rubber. In this study, we characterised changes in the surface morphology, functional groups, and thermal stability of MPs weathered on and below the soil surface over 12 weeks, and analysed inorganic and organic additives in leachates (0.01M CaCl2) and bioaccessibility extracts (ethyl acetate). Weathering conditions included UVC irradiation at 25 °C and 80% soil moisture. Microplastics on the soil surface exhibited cracking, fragmentation, and increased extractability of zinc, sulphur, titanium and fatty acids. Microplastics below the soil surface were not significantly physically or chemically altered, however zinc leachability increased following extended weathering by up to 155%. Bioaccessibility of thiol, aromatic and cyclic organic additives decreased from both surface and sub-surface MPs over the 12 week weathering period, but there was evidence of an increase in transformation by-products. Microplastic toxicity may be significantly altered by environmental conditions and MP weathering. It is critical ecotoxicological studies use weathered MPs to assess impacts on rare and endemic species found in protected spaces.

Microplastic pollution on hiking and running trails in Australian protected environments.

Microplastics (MPs) are ubiquitous worldwide, present even in remote areas of the natural environment. Hiking and trail running are a source of MPs on recreational trails in protected environments, which are characterised by high biodiversity and natural, ecological or cultural significance. Our understanding of the risks of microplastic pollution is impeded however by a lack of information on MPs present in the soil environment in such areas. This study characterised the quantity and physicochemical characteristics of MPs in two conservation areas in south-eastern Australia: 1) the adjacent Duval Nature Reserve and Dumaresq Dam Reserve, and 2) the Washpool and Gibraltar Range National Parks. We measured atmospheric deposition over a six-month period in the Reserves, and baseline amounts of MPs on recreational trails in the Reserves and National Parks. Atmospheric deposition averaged 17.4 MPs m-2 day-1 and was dominated by fibres, comprising 84 % of MPs. Microplastics detected on trail surfaces ranged from 162.5 ± 41.6 MPs/linear metre to 168.7 ± 18.5 MPs/linear metre and exhibited a very wide range of physical and chemical characteristics. The majority of MPs on the trail surfaces comprised polyurethane, polyethylene terephthalate and polystyrene, and 47-71 % were fibres. Microplastics were attributed to clothing, footwear, litter, and diffuse sources. Minimising and preventing MP pollution, however, is complex given there are multiple direct and diffuse sources, and several factors influencing increased MP deposition and retention in the environment.

Trail running events contribute microplastic pollution to conservation and wilderness areas.

Clothing and footwear designed for trail running shed microplastics (MPs) during use. Trail running events may therefore present a significant source of MP pollution in conservation and wilderness areas. Microplastics may present long-term risks to biodiversity and endemic plant and animal species in such areas. In this study, we used a before-after-control-impact approach to quantify and characterise MP emissions from clothing and shoe outsoles during trail running events. Microplastic deposition on trail surfaces was assessed using both a controlled study and during two public trail running events in New South Wales, Australia (the Duval Dam Buster and the Washpool World Heritage Trail Race). Microplastics were present on trails after all events and included fibres and rubber fragments. Microplastic counts varied considerably depending on trail surface hardness and gradient, and clothing and footwear properties. The controlled study showed running tights (leggings) and shoes with soft rubber outsoles produced more MPs than shirts and hard rubbers. In the trail running events, abrasive wear to shoe outsoles produced an average of 0.3 ± 0.1 to 0.9 ± 0.2 MPs/linear metre/runner, and clothing produced 0.7 ± 0.3 to 2.0 ± 0.3 fibres/linear metre/runner, with fibres accounting for 63-69% of MPs. Microplastic deposition from both footwear and clothing was higher on sloped and rock trail surfaces than flat and soil surfaces. Laser Direct Infrared (LDIR) Imaging indicated the main types of MPs present on trails were polyurethane, polyethylene terephthalate and polyamide. Trail running is increasing in popularity and large-scale events may cause a rapid and significant input of MPs in protected areas. Land managers, event coordinators and outdoor apparel manufacturers could mitigate MP impacts however, by diverting foot traffic around ecologically sensitive areas, capping participant numbers, and developing abrasion resistant clothing and footwear.

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.