Settling Velocities of Environmentally Weathered Plastic Fibers from the Mekong River in Southeast Asia

The influences that weathering processes may have on plastic settling have not been well documented or quantified. In this study, the physical characteristics and settling velocities of environmentally weathered micro- and macroplastic fibers collected in the Mekong River in Southeast Asia were measured in a laboratory setting for the first time. Size and shape ranges were larger than previous studies in which lab-generated particles were employed for experimentation. Measurements were used to evaluate several current drag equations. Out of four tested drag equation candidates, the Zhang & Choi (2022) equation was best at predicting the behaviors of weathered particles with an average error of 18% and root-mean-square error of 2.4%. The impacts of settling on estimating plastic loads were demonstrated using a plastic transport model parametrized for the Mekong directly upstream of Phnom Penh. Results suggest that a much larger fraction of plastic particles move toward the bottom of the water column than estimated with the conventional settling velocity method (97% vs 1%). These findings provide essential information on the settling characteristics of environmentally weathered microplastics, which will subsequently allow transport models to better represent the behaviors of plastics found in the aquatic environment.