Environmental Risks Posed by Microplastics in Urban Waterways

Microplastics (MPs) have been classified as any plastic particle with a diameter less than 5 mm. MPs have been identified as a prominent environmental concern due to the fact that they associate with a wide range of toxic contaminants, and also because of their non-degradability and long lifespan. Urban waterways are important natural resources that support human life, economic development, as well as natural ecosystems, which are closely linked with human and aquatic ecosystem well-being. Urban waterways are also the first places to suffer from MPs pollution derived from terrestrial environments. Therefore, it is essential to fully understand the presence and spatial distribution patterns of MPs in urban waterways to further evaluate the hazardous consequences resulting from their presence. This chapter provides a detailed discussion on MPs pollutants in relation to the definition, occurrence, distribution patterns and morphological characteristics to contribute knowledge on MPs pollution in urban waterways, and provides baseline data for further analysis of MPs from a risk assessment perspective.

The presence and distribution of various MP particles exhibit inconsistent patterns in urban waterways. Factors including land use type and anthropogenic activities are regarded as the dominant contributor of MPs in urban waterways as these are the major emission sources of plastic pollution. In addition to the diverse origins of MP pollution, environmental factors such as hydrodynamic conditions, photothermal degradation and the interactions between MPs and natural systems influence the presence of MPs by altering their properties such as size, shape and density. This chapter discusses the influence of emission sources, namely, land-based, aquatic-based and atmospheric-based sources, and environmental factors, namely, hydrodynamic conditions, photothermal degradation, and microorganism colonisation on the presence of MPs abundance and types in urban waterways. Such understanding will guide environmental risk management and critical decision-making related to source tracking, and impact mitigation of MP pollutants in urban aquatic systems.

MPs are complex environmental contaminants as they bind to a wide range of other contaminants. MPs associated contaminants include toxic chemical compounds that are used as additives during the plastic manufacturing process and various contaminants such as organic compounds, metals and microorganisms that are adsorbed from the surrounding environment. The organic and inorganic contaminants attached to MPs will exert direct and indirect impacts on aquatic environments due to the presence and migration processes of MPs. This chapter provides a detailed discussion on the environmental risks of MPs in urban aquatic environments arising from the release/sorption mechanisms between MPs and common contaminants, namely, metals and organic compounds. Additionally, this chapter also identifies the impacts of MPs on aquatic microorganisms from a risk assessment perspective.

This chapter presents the outcomes of a case study which focused on understanding the environmental risks and transport processes of microplastics (MPs) in urban river sediments. Sediment samples obtained from twenty-two sampling points along the Brisbane River, Australia, over four different climatic seasons revealed relatively high concentrations of MPs in the sediments, with abundance ranging from 0.18 to 129.20 mg kg⁻¹. The environmental risk posed by MPs was assessed by analysing their impacts on other pollutants present in the sediments, namely, nutrients (TC, TN, TP) and fourteen metals (Al, As, Cr, Co, Cu, Fe, Mg, Mn, Ni, Cd, Se, Sr, Zn, Pb) commonly present in urban aquatic environments. All three nutrient parameters were found to have a strong positive relationship with MPs concentration, while negatively correlated to MPs hazard index. The tested metals in Brisbane River sediments were observed to be correlated more with MPs hazard index rather than MPs concentration. Additionally, a three dimensional hydrodynamic and particle transport modelling framework developed showed that sedimental MPs with lower density and slow settling velocity (such as PE and PP) have higher mobility, while dense MP particles (such as PA and PET) are more likely to accumulate in the river sediments close to source points.

This chapter provides a consolidated summary of the research study outcomes on the investigation of the environmental risks in relation to MPs occurrence and migration in urban waterways. The knowledge created is expected to provide practical guidance and recommendations for the formulation of effective management and mitigation strategies for MPs in urban aquatic environments. This chapter also identifies key areas where significant knowledge gaps exist in relation to MPs source tracking, interaction mechanisms (release and sorption capacity) with metals and organic contaminants, as well as the environmental consequences to urban waterways from a risk perspective.