Assessing plastic-related environmental risk across EU countries of the Danube basin using a composite risk index

Plastic-related environmental risk in transboundary river systems represents a multidimensional challenge emerging from the interaction between plastic-related pressures, the vulnerability of receiving aquatic environments, and the capacity of circular economy systems to mitigate environmental impacts. This study investigates the structural factors associated with plastic-related environmental risk and the resulting territorial configurations across seven European Union countries of the Danube basin. Using a balanced panel dataset comprising 84 country-year observations for seven European Union countries over the period 2012–2023, the study develops the Danube Plastic-related Pollution Risk Index (DPPRI), a composite indicator that operationalizes plastic-related environmental risk through the integration of plastic-related pressure, organic pollution vulnerability, and circular economy mitigation capacity. The analytical framework combines composite indicator construction, an Ordinary Least Squares benchmark, supervised machine learning, model interpretation, and unsupervised clustering. Random Forest modelling is applied to assess the relative contribution of environmental, economic, and circular-economy indicators to modelled DPPRI variability, while SHAP (SHapley Additive exPlanations) analysis identifies the magnitude and direction of each indicator's contribution to the predicted composite-index values. K-means clustering, supported by an exploratory hierarchical clustering assessment, is further employed to identify structural risk typologies among the 84 country-year observations. The results show that trade-related plastic flows, the operationalized environmental vulnerability dimension, and indicators of circular-economy intensity and decoupling are among the principal factors associated with variability in the composite risk index, while recycling and recovery performance contribute to the mitigation dimension of the framework. The clustering analysis identifies three distinct structural risk profiles among the analysed country-year observations, reflecting different combinations of plastic-related pressure, environmental vulnerability, economic activity, and mitigation capacity. This study contributes an integrated and potentially transferable risk-screening framework for examining structural conditions associated with plastic-related environmental risk in transboundary river systems. Rather than estimating measured macroplastic or microplastic concentrations, the proposed approach integrates structural indicators of anthropogenic pressure, environmental vulnerability, and mitigation capacity to support the prioritization of monitoring needs and coordinated circular-economy and river-basin policy interventions.