A Tri-Medium Perspective on Microplastic Pollution in a Tropical Riverine Ecosystem: Insights from Water, Sediments and Bivalve Clam Shells in Pamba River

Microplastic pollution in tropical riverine ecosystems poses multifaceted environmental challenges across water, sediment, and biotic compartments. This study aims to understand the extent and potential ecological impact of microplastic contamination in the Pamba River, Kerala. Specifically, it investigates microplastic distribution in water, sediments, and Villorita cyprinoides clam tissues, while also examining correlations between them. Water, sediment, and bivalve samples were systematically collected and analysed following standard environmental protocols. Sediment texture was assessed using mechanical sieving and pipette analysis. Bivalve morphometrics were recorded to evaluate size-related microplastic uptake. Microplastics were extracted via digestion, density separation, and filtration, then characterized using stereomicroscopy and Raman spectroscopy. Risk assessments included Pollution Load Index, Polymer Hazard Index, and Estimated Dietary Intake. Sediment granulometry exhibits downstream fining, shifting from 100% sand upstream (Seethathodu–Mallappally) to mixed sediments with 35% clay and 10% silt in Kuttanad (Nedumudy, Kainakary). Correspondingly, microplastic concentrations escalate downstream, with water samples ranging from 19 MPs/L (Seethathodu) to 149 MPs/L (Nedumudy), sediments from 11 MPs/kg (Seethathodu) to 211 MPs/kg (Kainakary), and clam shells averaging 2.95 MPs/shell (total 59 MPs in 20 specimens). Microplastic morphotypes are fiber-dominant (water: 503, sediment: 514, clam shells: 34), followed by films and fragments, with beads minimally detected. Size distribution reveals particles < 1000 µm prevalent in water (527) and clams (48), whereas sediments favor > 3000 µm particles (583). Coloured microplastics outnumber colourless counterparts significantly (sediments: 843 vs. 184). Polymer analysis identifies polyethylene (PE) as dominant (water: 215, sediment: 288, clam: 17), succeeded by polypropylene (PP), with denser polymers PET and PS absent from clam tissues, indicating selective bioaccumulation mediated by polymer density and morphology. Pollution Load Index (PLI) peaks in sediments at Kainakary (4.4), Nedumudy (4.2), and Thakazhy (3.7), while Polymer Hazard Index (PHI) reaches 13,811 in sediments, highlighting sediment as a critical pollution reservoir. Estimated Dietary Intake (EDI) suggests annual human microplastic ingestion of 0.128 MPs/g/kg body weight via clam consumption (13 kg/year). These findings underscore the complex interplay of hydrology, sedimentology, and biotic interactions shaping microplastic fate and risks in tropical riverine systems, informing targeted mitigation strategies.