Microplastic transport processes in freshwater environment

Abstract:

Microplastics particles (size usually below 5 mm, MP in the follows) captured widespread attention after being found in the world’s great oceanic gyres. The concern is about the possible negative impacts on both abiotic and biotic life, both in the marine and freshwater environments, in particular due to the prediction that the amount of microplastics in some ocean compartments should double by 2030 (Hale et al., 2020).

The understanding of MP transport in freshwater environments is a key process because the rivers are the main vector of plastic litter towards the oceans.

We concentrated our work on experimental investigations of motion of MP particles with specific gravity greater than 1 when transported by water flow over a sand bed. The experiments were carried out in a tilting laboratory flume, 0.43 m wide and 10.5 m in length; the MP particles were the same used in Francalanci et al. (2020). We observed the MP particles from the incipient motion condition to bedload condition, while suspension occasionally occurred. The flow conditions were characterized by means of flow discharge, water depth and near bed velocity profile, which allows estimating the corresponding shear velocity. The image recording of the solitary moving particle on flume side allowed evaluating the mean particle velocity along its trajectory.

The results showed that plastic particles have a threshold for incipient motion in terms of Shields number lower than that of sand sediment. Velocity of plastic particles is well interpreted by formulas in the Literature, at low values of the bed shear stress; a hiding correction factor is probably needed at higher values of the shear stress.

These experiments constitute a reference for developing suitable closure relationship for transport processes of MP particles in rivers.

Future developments will consider grain mobility in the case of bulk transport and in the case of mixtures of plastic-sand sediments.

S. Francalanci, A. Menci, E. Paris, F. Ruggero and L. Solari. (2020) “Plastic delivery from river to sea: experiments on transport processes”, Proceedings of River Flow IAHR Conference 7-10 July 2020.

Hale, R. C., Seeley, M. E., La Guardia, M. J., Mai, L., & Zeng, E. Y. (2020). A Global Perspective on Microplastics. Journal of Geophysical Research: Oceans, 125, e2018JC014719. doi.org/10.1029/2018JC014719.