Estimating plastic pollution levels in rivers through harmonized monitoring strategies

Abstract

Plastic pollution in freshwater environments is an emerging threat to ecosystem health and human livelihood. Plastics that accumulate in rivers and lakes over long time periods have direct local impact, and may eventually also reach the world’s oceans. Monitoring river plastic pollution is therefore key to quantify, understand and reduce plastics in all aquatic ecosystems. Unfortunately, global river plastic monitoring is still in its early stage. The lack of harmonization between ongoing monitoring efforts compromises the direct comparison and combination of available data. Recently, the United Nations Environment Programme (UNEP) launched guidelines on plastic monitoring in rivers and lakes, to provide a starting point for practitioners and scientists towards harmonized data collection, analysis, and reporting. Here, we demonstrate the application of those guidelines through a practical workflow to support to design effective plastic monitoring strategies. The workflow is a five-step iterative approach that aims to match the defined research goal or question to the available resources and river basin characteristics. To test the relevance of the workflow in different settings, we applied the workflow to three rivers across relevant gradients, including geography, hydrology, and plastic pollution levels. In the Rhine basin (Switzerland, France, Germany, the Netherlands) we showed that urban areas and tributaries may act as entry points of plastic pollution. For the Odaw basin (Ghana) we compared floating, terrestrial and riverbank plastics, identifying source and accumulation zones. Finally, in the Mekong river (Cambodia) we quantified the contribution of the city of Phnom Penh to plastic pollution through quantifying the plastic mass balance in the Mekong-Tonle Sap-Bassac region. We show that despite the simplicity of the selected methods and the limited duration of the data collection, a harmonized approach can provide crucial insights in the state of plastic pollution in very different river basins. Although each case study focused on one specific goal, each one is directly transferable to any river globally. Data collection is key in reaching local, regional and global goals on reducing plastic pollution. With our paper we aim to provide a simple workflow to support the design of effective and comparable river plastic monitoring strategies.

Large variation in Mekong river plastic transport between wet and dry season

“…In the Seine, plastic transport increased with a factor ten when discharge increased only a factor three (van Calcar and van Emmerik, 2019). A recent study in the Meuse found a power law relation between discharge and plastic transport, suggesting a non-linear response of plastic transport to discharge (van Emmerik et al, 2022b). The disproportional increase of plastic transport to discharge is generally explained by additional mobilization of plastic towards the river, and remobilization of accumulated plastics on the riverbanks and floodplain due to increased water level and flow velocities.…”

“…Not all methods are however suitable to be applied during extreme flow conditions. During floods, large debris and debris patches can make net sampling challenging and dangerous, both from boats or bridges (van Emmerik et al, 2022b). Visual counting from bridges or imagebased techniques provide a safe alternative.…”

“…Additional challenges are caused by the use of different measurement methods, resulting in variation in the units, environmental compartments included, or calculated variables. Available data are, therefore, often not directly comparable (van Emmerik et al, 2023). Recent efforts have started to harmonize river plastic monitoring methods and strategies.…”

“…We applied the method to the Mekong river around Phnom Penh, Cambodia, where it forms a complex river network with the Tonlé Sap and Bassac rivers (Haberstroh et al, 2021a). The available data were collected through net sampling from boats in the wet season of 2019, and through visual counting from bridges in the dry season of 2022 (Haberstroh et al, 2021a;van Emmerik et al, 2023). We use the available raw data to estimate the total amount of floating plastic items and their mass transport.…”