Effect of tidal current on the settling and accumulation of microplastics in the Chao Phraya River estuary, Thailand.

The deposition of MPs in a water column and surface sediment during a mixed spring tidal cycle of the Chao Phraya River estuary was investigated. The settling MPs during flood and ebb tides were collected by deploying traps at 3 m below the surface, while the settled MPs throughout the tidal cycle were collected by deploying traps at 1 m above the bottom. The settling rate of MPs was 2168 pieces/m²/h during highest to low tide, and 639 pieces/m²/h during high to lowest tide. The deposition rate of MPs after the end of the tidal cycle was 3172 pieces/m²/day, while the accumulation rate of MPs in the surface sediment was 1515 pieces/m²/day. The settling MPs tended to decrease inversely to the suspended solids and salinity. The major types of the deposited MPs were polyethylene (36 %) and polyamide (33 %), while that of the surface sediment was epoxy resin (80 %). [Display omitted] • The deposition rate of microplastics collected by sediment traps was 3,172 pieces/m²/day, while the accumulation rate of microplastics in the surface sediment was 1515 pieces/m²/day. • The settling of microplastics is inverse to salinity and suspended solids. • Low-density plastics were the majority of plastics that settled at the sediment trap 1 m above the bottom, but denser plastics highly accumulated in the surface sediment. • The result is evidence of a resuspension of microplastics during a tidal current in a river estuary. [ABSTRACT FROM AUTHOR]