Co-occurrence and Ecological Risk Assessment of Microplastics and Heavy Metals in Hyporheic-Zone Sediments of the Bhairab-Rupsha River, Bangladesh

ABSTRACT This study assesses microplastics (MPs) and heavy metals (HMs) in hyporheic-zone sediments of the Bhairab-Rupsha River in Bangladesh. Nine cores from upstream to downstream were divided into upper, middle, and lower layers. MPs were isolated by density separation with wet peroxide oxidation and identified by Fourier Transform Infrared Spectroscopy (FTIR) . HM concentrations were measured by atomic absorption spectrophotometry following USEPA 3050B acid digestion. Seven ecological risk indices (PLI, PHI, RI, Igeo, EF, CF, ER) quantified sediment contamination. MPs peaked in the upper layer (1,700–5,700 items/kg; mean 3,600 items/kg), declining to 2,744 and 1,177 items/kg in middle and lower layers. Fragments (51%), fibers (26%), and films (18%) dominated, and 106–355 μm particles comprised nearly half of all MPs. FTIR detected seven polymers, led by polyethylene (23%), polystyrene (21%), and polypropylene (18%). The PHI values for MPs predominantly fell within Category III across all sediment layers, identifying polycarbonate and thermoplastic polyurethane as the most hazardous polymers. The PLI values indicated minor pollution across most sites and layers, except for the upper layer at RR6, which exhibited a medium risk (11.40). In contrast, the PERI assessment revealed markedly higher ecological concern, with upper-layer risk scores mostly ranging from 641 to 1405, corresponding to Class IV and Class V risk levels. HMs followed Cr > Ni > Cu > Pb > Cd, with urban outfalls as hotspots. Igeo, EF, CF, and PLI showed moderate-to-high anthropogenic impact, especially for Ni and Pb; ER flagged Cd and Ni as primary threats. These results underscore the need for integrated monitoring, tighter pollution controls, and targeted remediation to safeguard the Bhairab-Rupsha River and downstream Sundarbans mangroves. This multi-layered assessment provides a critical baseline for long-term riverine pollution management and ecosystem health planning in subtropical deltaic environments.