Temporal and spatial variations of microplastics in roadside dust from rural and urban Victoria, Australia: Implications for diffuse pollution

Highlights

• Microplastics in roadside dusts were firstly confirmed in rural and urban Australia.

• Small microfibers (<1 mm) and polyesters were dominant in all samples.

• Urbanized streets with high population size are prone to accumulate microplastics.

• Microplastics can be flushed away during heavy storm.

• Roadside dust monitoring are useful in microplastic diffuse sources managements.

Abstract

Microplastic pollution is an important issue for environmental management as their ubiquity in marine and freshwaters has been confirmed. Pollution sources are key to understanding how microplastics travel from land to open oceans. Given that information regarding microplastic transport from diffuse sources is limiting, we conducted a study on roadside dust from rural and urban Victoria, Australia, over two seasons. Any deposited fugitive dust and particulate matter may also be present in our samples. The average microplastic abundance over two seasons ranged from 20.6 to 529.3 items/kg (dry weight based), with a predominant portion of fibers and items less than 1 mm. Polyester and polypropylene were the dominant polymer types (26%) while cotton and cellulose were the most common non-plastic items (27%). Sampling sites displayed consistent microplastic abundances over time and shared similar patterns in size, shape and polymer composition. Multiple correlation and principal component analysis suggest that urbanization and rainfall are important influences to roadside microplastic accumulation. The observed microplastic hotspots were generally located within close vicinity of areas where urban intensive land use and regional population sizes are high. Microplastics accumulated on roads and road verges during periods of dry weather and were flushed away during heavy storms while the corresponding trigger value was unknown. Monitoring roadside dust can be considered as an initial and cost-effective screening of microplastic pollution in urban areas. Further efforts should be made to optimize the methodologies and we advocate prolonged sampling schemes for roadside dust monitoring.

Introduction

Microplastic pollution of aquatic environments has been intensively documented within the past decade (Law, 2017; Sutherland et al., 2019). It was estimated that 79% of the plastics ever made will end up in landfills or natural environments, being significant sources for microplastics via the process of fragmentation (Geyer et al., 2017). Microplastics were suggested as a marker of the anthropocene and participating in environmental or biogeochemical cycles (Bank and Hansson, 2019; Galloway et al., 2017). Henceforth, new insight into the transportation of land-based microplastics is needed to understand where and how microplastics are most likely to enter the environment.

The transport pathways of microplastics, generally from land-based sources to the open ocean, are gradually becoming clearer thanks to recent global investigations. Point sources from wastewater treatment plants (WWTPs), plastic industries and fishing activities have been identified as major sources of microplastic pollution from the catchments (Carr et al., 2016; Estahbanati and Fahrenfeld, 2016; Imhof et al., 2013). As a result, formative efforts have been made to reduce microplastic emission from WWTPs and focus on microbeads from personal care products (Conkle et al., 2018; Lares et al., 2018). However similar studies elsewhere have found high amounts of microplastics in water bodies which cannot be explained by point sources (Barrows et al., 2018; Lahens et al., 2018). In such cases, it has been suggested that other anthropogenic influences (e.g. high population densities and intensive urban land use) may be responsible for the microplastic hotspots observed (Dris et al., 2015; Peters and Bratton, 2016).

Pollution from diffuse sources is an important factor which must also be considered, although the mechanisms involved in their transport are less clear (Dris et al., 2016; Rodrigues et al., 2018). In urban areas, diffuse pollution stems from rainfall or any surface runoff moving over the ground, transporting contaminants into waterways, lakes and even open oceans (Carpenter et al., 1998; Deletic, 1998). Diffuse pollution can be a major source of nutrient and metal pollution to surface water in instances where point pollution sources were well managed (Carpenter et al., 1998; Maguire et al., 2019).

Compared to point sources, managing microplastic pollution from diffuse sources is a difficult endeavor because it is hard to regulate the intermittent pollution inputs and to locate sources. Microplastics can enter the environments via multiple pathways including surface runoff, aerial deposition and drainage; owing to the complexity of diffuse source discharge rates, the relationship between ambient pollution levels and diffuse pollution is uncertain (Segerson, 1988). Roadside dust or road dust/street dust is an example of a diffuse source; they are composed of a diverse mixture of materials containing atmospheric deposits, litter, and soil-derived particulate matter, all of which have been influenced by diffuse sources prior to joining dusts (Khan and Strand, 2018; Murakami et al., 2008). In our work, the “roadside dust” was defined as a mixture that includes a part of fugitive dust and deposited particulate matter as well, which is slightly different to the terminology “road dust” (Khan and Strand, 2018). If pre-emptive treatments are not made, roadside dust can be carried by street runoff and become a significant source of pollutants which may ultimately enter estuaries, embayments and the open ocean by storm drains (Spies et al., 1987).

Despite a wide range of pollutants having been confirmed in roadside dust, few reports on microplastic abundance in such dusts exist, which may be an important phase of their transport. Preliminary studies of street dust in Iran revealed up to 1.6 × 10⁵ items/kg of microplastics (Abbasi et al., 2017). Weather conditions, such as accumulated rainfall and wind force, may aid in the further transport of these particles, as has been shown for other diffuse pollutants (Allen et al., 2019; Bergmann et al., 2019; Hurley et al., 2018). At its current stage, roadside dust monitoring provides a complementary means of assessing the ambient levels of soil and atmospheric microplastic deposition. Given that roadside dust is composed of more microplastics than other environmental mediums, they may be considered sentinels of polymer pollution at low concentrations. Dust monitoring could be a valuable initial screening for microplastic pollution levels in urban areas. A more detailed diagnosis of primary pollution sources might not be easily achieved using this method alone as dusts are complicated mixtures. In order to overcome these limitations, more specific isolation and microplastics identification should be used in sample assessment so that long-term data is more useful for risk assessment and environmental management.

Unfortunately, the current understanding of regional microplastics emission, from land sources to sinks, is incomplete due to a lack of sufficient baseline data for land-based pathways. In the current work we selected Australia as a study area, where microplastics monitoring is limited in terrestrial environments, and sought to answer: 1) the abundance and type of microplastics in roadside dust and likely diffuse pollution sources; 2) compare the spatial distribution of microplastics in roadside dust between rural and urban areas; 3) identify important factors which influence microplastic abundance in streets (i.e. weather conditions/urbanization).