An Examination of Microplastic Pollution in Protected Areas from Outdoor Recreation

Microplastics (MPs) (plastic particles 1 μm - 5 mm) are an emerging pollutant of global concern. Research on MP pollution in soils of conservation and wilderness areas is largely lacking, despite these locations being valued for possessing high biodiversity and natural, ecological and cultural significance. Outdoor recreation may be a leading source of MPs in such areas, which typically lack other types of land use. In particular, trail running may be a major contributor to MP pollution, with participation rapidly increasing worldwide and runners reliant on technical apparel and footwear that comprise abrasion-prone plastic materials. This study examined MP occurrence from outdoor recreation in protected areas. Investigations ranged from sampling protocol development, through to analysis of MPs on trails in two Australian conservation areas with recreational and competitive trail running, the 1) Duval Nature Reserve and adjoining Dumaresq Dam Reserve, and 2) the World Heritage Listed Washpool and Gibraltar Range National Parks. Microplastic deposition from trail running events and diffuse sources were empirically examined, and MP movement in the natural environment was assessed using rainfall simulation. Effects of weathering on additive leachability and bioaccessibility were investigated using simulated weathering of MPs from trail running shoe outsoles. Microplastics were detected on all trail surfaces sampled in the Reserves and National Parks, averaging 162.5 ± 41.6 MPs/linear metre and 168.7 ± 18.5 MPs/linear metre respectively. Microplastics were sourced to clothing, shoes, litter and diffuse sources, and the chemical profile differed between and within the study sites depending on trail topography, vegetation type, meteorological factors and proximity to urban areas. Trail running events represented episodic, large-scale sources of MP pollution on trail surfaces, with 67 ± 10 to 278 ± 77 rubber MPs/linear metre and 139 ± 76 to 636 ± 110 fibres/linear metre deposited during the Washpool World Heritage Trail Race and Duval Dam Buster due to shoe outsole and clothing abrasion. Microplastic release was significantly higher on inclines and rock surfaces compared to flat areas and soil. Atmospheric deposition sourced MPs to non-trafficked areas and averaged 17.4 MPs m-2 day-1 in the Duval Nature Reserve and Dumaresq Dam Reserve, increasing during periods of high frequency, low-intensity rainfall. Short storm events were not sufficient to substantially re-distribute MPs deposited on soil surfaces, with 85 – 99% of particles retained on trails. Microplastic retention was highest in resource accrual areas, where vegetation, loose soil and surface roughness reduced runoff velocity and entrapped particles. Microplastics from local and diffuse sources were predominantly fibres, and the majority of MPs comprised polyurethane, rubber, polyethylene terephthalate, polyamide, polystyrene or polypropylene. Microplastics from trail running shoe outsoles were shown to be a source of potentially toxic trace elements and organic compounds, including zinc, bis(2-ethylhexyl) phthalate, 1- octanethiol, and butylated hydroxytoluene. Photo-oxidative weathering to MPs on soil surfaces increased leachability of zinc, sulphur, titanium and fatty acids. Microplastics weathered below the soil surface did not exhibit significant physical or chemical changes, however zinc leachability increased following extended weathering by up to 155%. Bioaccessibility of organic thiol, aromatic and cyclic additives decreased in all parts of the soil profile, but the bioaccessibility of an extensive range of transformation by-products likely increased. This work represents the first steps of research on MP pollution from outdoor recreation and significantly advances our understanding of the potential implications for protected places. Initial measurements of MPs on trails indicate MP pollution may pose a risk to biodiversity, wildlife, vegetation and ecosystem resilience in these environments. Impacts of MPs specifically from recreation activities may be most apparent in accrual areas on and adjacent to trails, where MPs are likely to accumulate. Ecotoxicology studies assessing effects of both freshly abraded and weathered MPs are critical in order to understand potential impacts on rare and endemic species found in protected areas. Policy changes are urgently needed to protect these unique environments. Land managers may take steps to address local sources of pollution – primarily by capping visitor numbers and diverting foot traffic around ecologically sensitive areas – however the abundance of MPs from long-range transport and atmospheric fallout indicates global action is required to prevent MP pollution in protected spaces. Improvements to clothing design and manufacture may be an important step in reducing global microfibre emissions.