Concentration-Discharge Relationships of Plastic Pollutants in Streams

Abstract:

Previous observations suggest that discharge may play a significant role in controlling the concentration of plastic in streamflow, but this relationship has not yet been explicitly quantified. We know that discharge magnitude and mode (i.e., dominant contributing hydrological processes) can be a significant control on the concentration of natural mineral sediments in streams. Nevertheless, commonly applied power-law relationships between mineral sediment concentration and water discharge typically display multiple orders of magnitude of residual variability, particularly in the small mountainous rivers that export the majority of terrestrial sediments to the coastal oceans. This variability is due in part to stochastic processes controlling water and sediment delivery and routing through the channelized system. Temporal structure to this variability can manifest in time-dependent concentration-discharge relationships, from hydrograph hysteresis (i.e., different rising vs. falling limb concentration-discharge relationships) to interdecadal scale trends. Investigation of temporal patterns in concentration-discharge relationships can provide insight into the transport and supply processes and be used to refine flux estimation. Additionally, differences in the particle size distribution of suspended load during runoff and baseflow can elucidate sources and transport pathways of mineral sediment. We set out to use these analysis techniques to investigate plastic pollution concentration-discharge relationships in a small mountainous river. We determined that a power-law rating curve best described the plastic concentration-discharge relationship with a positive slope. We found evidence for clockwise hysteresis at the event scale, and indications that plastic may not exhibit early storm flushing relationships. Plastic particle size distributions were also insensitive to hydrologic mode, which may indicate the importance of plastic pollution storage and resuspension dynamics in the channel, particularly in small, mountainous rivers.