Tidal plastic recycling: net river plastic transport limited by tidal dynamics

The processes controlling the transport in tidal rivers and estuaries, the interface between fluvial and marine systems, remain largely unresolved. For this reason, current estimates of riverine plastic pollution and export into the ocean remain highly uncertain. Hydrodynamics in tidal rivers and estuaries are influenced by tides and freshwater discharge. As a consequence, flow velocity direction and magnitude can change diurnally. In turn, this impacts the transport dynamics of solutes and pollutants, including plastics. 

Plastic transport dynamics in tidal rivers and estuaries remain understudied, yet the few available observations suggest that plastics can be retained here for long time periods, especially during periods of low net discharge. Additional factors such as riparian vegetation and riverbank characteristics, in combination with bidirectional flows and varying water levels, can lead to even higher likelihood of long-term retention.

Here, we provide a first observation-based estimation of net plastic transport on daily time scales in tidal rivers. For this purpose, we developed a simple Eulerian approach using sub-hourly observations of floating plastic transport and discharge during full tidal cycles. We applied our method to the Saigon river, Vietnam, throughout six full tidal cycles in May 2022.

We show that the net plastic transport is about 27-32% of the total plastic transport. We found that plastic transport and river discharge are positively and significantly correlated (Pearson's r = 0.87, R²= 0.75). The net plastic transport is higher than the net discharge (27-32% and 18%, respectively), suggesting that plastic transport is governed by other factors than water flow. Such factors include wind, plastic concentrations in the water, and entrapment of plastics downstream of the measurement site. The net plastic transport rates per tidal cycle alternate between positive (seaward) net transport and negative (landward) net transport, as a result of the diurnal inequality in the tidal cycles. We found that soft and neutrally buoyant items had considerably lower net transport rates than rigid and highly buoyant items (11-17% vs 31-39%), suggesting the retention time strongly depends on item characteristics.

Our results demonstrate the crucial role of tidal dynamics and bidirectional flows in net plastic transport. We emphasize the importance of understanding fundamental transport dynamics in tidal rivers and estuaries to ultimately reduce the uncertainties of plastic emission estimates into the ocean.