Plastics in rivers - understanding the behavior of plastic particles through hydrodynamics

Plastic in the environment is finally receiving the attention that an emerging pollutant of global concern requires, but there remain many knowledge gaps in the field. Moving forward, research efforts should focus on understanding the dynamics of plastic pollution; rivers play a key role as the primary conduit between land and sea. From a hydrological perspective, a key question is which processes drive the transport and fate of river plastics. The objective of this study is to link plastic particles and their properties to river hydrodynamics and transport processes.

We conducted a detailed analysis of three months (Jun-Aug 2018) in an ongoing field data collection in the Hillsborough River in Florida. For each month, we have five surface and water column samples that were collected in the cross-section with a 500 μm mesh neuston net and a detailed flow profile created by an Acoustic Doppler Current Profiler (ADCP). Samples are processed in the laboratory, and a subsample of plastics is analyzed with a microRaman spectrometer. Particles are categorized by mass, count, size, shape, and polymer type. With the samples taken at different positions, we investigate the impact of particle properties on transport, and the ADCP data provides high-quality river hydrology data. Combining these field datasets, we are examining how hydrology and transport processes such as advection, dispersion and settling affect the transport of plastics. The samples show different distribution and movement patterns at changing flow conditions. Concentrations ranged from 0.3 to over 3 counts/m3 with particle sizes between 0.1 and 80 mm. In June, the plastic was concentrated at the surface, and the plastic was passing down through the water column due to settling and vertical mixing. In July and August, with higher flows and shear stress, the sediment resuspended particles as well. Turbulence and hydrology are also causing uneven horizontal distributions.

This work contributes to the understanding of transport mechanisms and variation of plastic in rivers, particularly in coastal and urbanized waterways. This is one of the first studies using and analyzing detailed hydrologic field data to look at transport and retention of plastic loads. This work is essential to enhance the development of mitigation strategies and to forward modelling efforts.