The role of turbulence in the deposition of intrinsically buoyant MPs

Intrinsically floating microplastics (MP) such as polyethene (PE) or polypropylene (PP) are among the most common MPs found in aquatic sediments. There must hence be mechanisms that cause lighter-than-water MPs to deposit despite them being buoyant. How these MPs end up in the sediment bed is only partly understood. This study explores how turbulence in the water can affect the vertical movement of buoyant MP and bring them in contact with the bed. The deposition of PE (995 kg m-3) in slow-flowing water (average flow velocities of 1.85 and 4.17 cm s- 1) was measured by tracking them and analyzing their motion in an open, rectangular, glass-sided flume. Flow characteristics in terms of turbulent kinetic energy and shear velocity were measured by particle image velocimetry. Experiments were conducted at a water depth of 27 cm and at various hydraulic conditions created by adjusting inflow speeds and using different bed materials: medium gravel, fine gravel, medium sand, cohesive sediment, and glass. The results showed that the vertical velocity of the MPs in the turbulent flow regimes varied over 4 orders of magnitude from their predicted rising velocity in quiescent water (laminar flow). Turbulence mixing resulted in distribution throughout the water column with a substantial quantity consistently subject to downward vertical transport, which in turn increased the chance of the PE particles encountering the bed and potentially getting immobilized. This work provides a plausible explanation and further experimental validation for the concept of mixing induced transfer of MPs from the water surface to the sediments of shallow waters.