Macroplastic emissions from the Odaw river, Ghana, into the ocean.

Rivers have been highlighted to play a key role in the transport of plastics into the ocean and especially, urban water systems act as a major source of plastic leakage into the marine environment.  However, observed data on plastic pollution in these urban water systems are scarce.  Our study focuses on the Odaw river basin which is the main drainage systems in Accra, Ghana, and is heavily polluted with macroplastics.  This is due to the high anthropogenic activities which have increased the indiscriminate dumping of waste into the water system.  These macroplastics in the water system accumulates over long periods, and a part of it is emitted into the ocean during high discharges.  Due to limited data on the quantification of macroplastics in the Odaw river, the seasonality and hotspot accumulation zones of these macroplastics in the river are unknown.  Such information is crucial for policymakers to prioritize future plastic debris monitoring and mitigation strategies.  We aimed to quantify the macroplastic emissions from the Odaw river into the ocean.  Using visual plastic counting from four bridges together with a hydrodynamic model, estimations for total yearly plastic fluxes through the Odaw river were made.  This model used rainfall data from 2016-2021 at the input nodes to simulate the discharges in the water system.  We estimated a total plastic flux from the Odaw river into the ocean between 2.6 x 10¹ and 1.7 x 10³ tons per year.  Low plastic fluxes were observed at the bridges during dry periods, but a 10 fold increase in plastic fluxes was observed during and after a rainfall event.  Except for two sampling days, negative plastic fluxes were observed at the two bridges closest to the ocean due to the effect of tides.  These observations made the estimation of the total plastic emissions into the ocean challenging because of the bidirectional flow dynamics of the macroplastics at these locations.  The findings of this study provide baseline data for macroplastic transport through the Odaw river into the ocean.  Future research could focus on investigating the accumulation zones of macroplastics at the bridges closest to the river mouths due to the role of tidal dynamics on the river plastic transport and export into the ocean.