River-to-sea plastic pollution: A pilot toolbox for the Caspian Sea

Abstract

This study presents the first integrated assessment of plastic pollution at the Kura River delta, where the river enters the hydrologically enclosed Caspian Sea. We applied a modular toolbox comprising four complementary components: high-resolution hydrodynamic modeling to predict debris convergence zones, UAV-based mapping to survey shoreline conditions, automated object-based image analysis for debris detection and classification, and standardized field monitoring by trained community participants for ground-truthing and source identification. Using this framework, we identified debris accumulation hotspots and developed a replicable approach for assessing plastic pollution in semi-enclosed systems. Surveys along a 1.2km shoreline recorded over 600 items, dominated by beverage containers by count and marine-sourced plastics by weight. Transboundary inputs were confirmed through labeled debris from five countries. The initiative also expanded institutional collaboration, trained over 150 participants, and established a local monitoring network. These findings highlight the value of integrating technical diagnostics with community engagement to enable scalable, site-adapted responses to plastic pollution.

Keywords: Caspian Sea; Citizen science; Community engagement; Digital techniques; Marine litter; Plastic pollution; Riverine input; Transboundary pollution.