A Geomorphic Framework for the Analysis of Microplastics in Riverine Sediments

Abstract

The wide-spread use and persistence of plastics in the environment have placed them on the list of significant emerging pollutants. In contrast to marine environments, the analysis of plastic debris, including microplastics (particles <5 mm in maximum diameter), in freshwater systems is limited, and even fewer studies have examined microplastics in riverine sediments. Nonetheless, it has become clear that microplastics are now a ubiquitous component of riverine ecosystems and their distribution is dependent on anthropogenic inputs and the physical and chemical processes that control their transport, transformation, and deposition along the drainage network. In many ways, the transport and fate of microplastics will parallel that of other particulate matter that has been extensively studied for at least the last 50 years. Here, we briefly explore the application of a geomorphic approach to the assessment of sediment-contaminated rivers to the microplastic problem, and argue that future studies can significantly benefit by incorporating the principles of this approach into their analyses.

A critical review of environmental factors influencing the transport dynamics of microplastics in riverine systems: implications for ecological studies

“…Owing to biofilms, MPs usually considered less dense than water may become denser and then sink to the riverbed. The development of biofilms tends to increase the efficiency of forming aggregates of more prominent and denser particles (Miller and Orbock Miller 2020), thus impacting vertical movement. Although buoyant, turbulent forces also transport MPs vertically in the water column by Zhang (2017); vertical movement generally favours denser MPs in biotopes with sub-critical to critical tranquil flow where both the level of disturbance (i.e.…”

“…are impacted by different hydraulic patterns and channel morphology, especially within the flow-dependent hydraulic biotopes. For instance, Miller and Orbock (2020) stated that MPs concentrations, transport and storage in river sediments are highly susceptible to changes in flow conditions over a range of temporal scales. Wang et al (2021) observed a positive correlation between river channel width and MPs larger than 2 mm, and a negative correlation with smaller MPs.…”

“…It is clear that at the reach scale, hydraulic biotopes reflect substrate type, depth, and flow velocity that influence MPs particle and biota distribution (Jowett et al 1991;Wadeson and Rowntree 1998;Thomson et al 2001). Miller and Orbock (2020) stated that a critical finding of using a geomorphological approach for assessing rivers affected by hydrophobic contaminants is that sediment-associated contaminants are not randomly distributed within the channel bed but are partitioned into specific depositional features within the channel, creating variations in spatial concentrations of these contaminants between channel bed features (e.g. pools, glides, riffles) as a function of their physical attributes and hydromorphological variations.…”