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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.

Citizen science is increasingly prominent in the field of freshwater monitoring. Although there is a large body of literature related to surface water monitoring, less experience exists regarding citizen-based monitoring of groundwater resources. This calls for a better understanding of the actual experiences of citizen science in the field of groundwater monitoring, including specific project designs and results. Based on a systematic review of 33 citizen-based groundwater monitoring activities, we analyze (1) the design of citizen science projects, including their general project characteristics, institutional characteristics, citizen characteristics, and forms of interactions, as well as (2) their results, including their data outputs, outcomes on citizens, and impacts on problem-solving. Results show that projects mainly have positive results on data quantity and quality and partly influence the citizens themselves and their contributions to groundwater-related problem-solving. Information on project design characteristics is scarce and mostly hints at the relevance of specific process mechanisms such as training and feedback. Based on these results, we suggest groundwater-related projects to consider involving citizen scientists in monitoring activities in order to benefit research and citizens alike. Such involvement needs, however, careful design including relevant training to unfold its full potential for both sides. Researchers are advised here to rigorously report on both design and results to further improve citizen science practice in the future.

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