Microplastics in the Ganga-Brahmaputra delta: Sources and Pathways to the Sundarbans Biosphere Reserve - an UNESCO World Heritage Centre

River Ganga, Brahmaputra, and their distributaries form one of the world's largest delta and mangrove forests - the Sundarbans Biosphere Reserve - designated as the World Heritage Centre by the United Nations Educational, Scientific and Cultural Organization (UNESCO). Global warming, climate change, and anthropogenic activities have however made the region fragile due to problems related to sea-level rises, tropical cyclones, salt-water intrusions, and pollutants. Several questions have been raised about the increasing levels of inorganic and organic pollutants in the delta region deposited by the Ganga and the Brahmaputra rivers that drain nearly 1.7 million km² of land with extensive industrial, agricultural, and domestic land activities. Here we present the source, type, and pathways of microplastics (MPs) in water (n = 10) and sediment (n = 17) samples collected from 17 critical locations along the Hooghly River —an eastern distributary of River Ganga in the State of West Bengal—downstream from the megacity of Kolkata up to Sundarbans Biosphere Reserve. The average MPs concentration for the water and sediments were 718 ± 244 items/m³ (n=10, 1 σ) and 428 ± 266 items/kg dw. (n=17, 1 σ), respectively, which is similar to water and sediment samples from other Indian and world rivers. Attenuated Total Reflectance - Fourier Transform Infrared spectroscopy data reveals that the polymer type for the sediment and water samples were predominantly high-density polyethylene (33 %), polyoxymethylene or polyacetal (18 %), polyphenylene sulfide (18 %), polyacrylamide (13 %), polypropylene (7 %), polytetrafluoroethylene (6 %), and polybutadiene (5 %). The MPs present a higher proportion in sediment 0.3 mm-90 µm (49%) and water samples 1-0.3 mm (45%), suggesting a high degradation rate. Our data indicated that River Hooghly transported MPs is one of the factors for ecological risks in the Sundarbans Biosphere Reserve.