Mitigating Riverine Microplastic Flux into Coastal Wetlands via Engineered Bioretention Cells: A Field-Validated Retention Efficiency Model for Subtropical Estuarine Systems

Abstract

Microplastic transport through riverine corridors into coastal wetland ecosystems represents a critical and insufficiently quantified pathway of anthropogenic polymer contamination. This study presents a field-validated retention efficiency model developed for subtropical estuarine systems, integrating bioretention cell infrastructure with real-time particle-size fractionation monitoring across three instrumented catchments in southeastern Brazil. Sampling campaigns conducted over 24 months captured seasonal hydrological variability, enabling robust parameterization of a modified SWAT+ framework augmented with microplastic fate-and-transport subroutines. Engineered bioretention cells demonstrated mean retention efficiencies of 78.4% for fibrous polymers (≤500 µm) and 64.1% for pelletized resin fragments under peak discharge conditions. Substrate depth, hydraulic residence time, and vegetation density emerged as the dominant design variables governing removal performance. The proposed model offers a scalable, cost-effective engineering design framework applicable to deltaic and estuarine management programs globally.