Controls on microplastic flux during sand bed evolution

Microplastic contamination of river sediments has been found to be pervasive at the global scale however, the physical controls governing the storage, remobilization and pathways of transfer in fluvial sediments remain largely unknown. This is particularly so in sand bed rivers where the migration of bedforms has the potential to both store and release any microplastics contained within the sediment bed.  Without detailed experiments to model the movement of microplastics through, and storage within, sand bedforms it is impossible to understand what the environmental legacy of our excessive plastic pollution will be.We report a series of mobile-bed laboratory flume experiments designed to explicitly quantify the relationship between sand bed surface development and microplastic flux characteristics. Experiments were performed within a glass sided, flow recirculating flume of rectangular cross section (8m x 0.5m x 0.5m). A uniform sand bed (D50 of 450μm) was seeded with either PVC pellets (d=1.4g/cm3), Nylon pellets (d= 1.2 g/cm3), Polycarbonate fragments (d=1.2 g/cm3), Acetal beads (d = 1.4g/cm3) or Nylon fibres (d = 1.15g/cm3). Plastics were mixed into the sediment bed at either 0.1% or 0.5% concentration by mass and sediment beds were exposed to a flow rate of either 0.6 or 0.8 ms-1. Experiments were run until equilibrium conditions were attained as measured by bedform migration rate. During each experiment aerial photographs were taken every 2.5 minutes and videos shot through both the side walls and from top down to track bedform migration and plastic flux. Transported sediment and plastics were captured at the downstream end of the flume in a sediment trap to allow fluxes to be calculated. At the end of each run photographs were taken of the drained bed surface with photogrammetry then used to model the 3D bed surfaces.Controls on microplastic flux as a result of bed evolution is discussed in terms of both flow rate flow rate and microplastic type.