Sediment and surface water microplastic contamination along the Ganges River, from the Himalayan foothills to the tidal reach downstream of Kolkata, India.

Twenty rivers have been estimated previously to account for almost 70% of global plastic emissions to the marine environment. Of these, the Ganges River was second only to the Yangtze River. Yet, scale and complexity of the Ganges River network make assessing microplastic contamination along the watercourse, including the attribution of suspected pollution sources challenging. Here we analyse post-monsoon spatial variation of microplastics along a 2500km length of the Ganges. River sediments and surface water were sampled during Nov/Dec 2019 at 81 locations that covered a variety of hydrological conditions, morphological attributes and land-use categories (e.g., downstream of large urban areas, rural areas with agriculture, major tributaries). Sediment samples were collected from the riverbank (waterline) and stored in glass jars while 100L water samples were filtered on-site through a 63µm sieve. Plastic particles between 63 and 5000µm were extracted by means of density separation followed by organic matter digestion with Fenton reagent. Extracts were subsequently stained with Nile red to improve detection of ambiguous particles. Physical and chemical properties such as shape, size, and polymer type where characterised using Fluorescence microscopy, Confocal Raman, and FT-IR imaging to understand differences along the river continuum. Our findings were analysed in relation to other key water quality parameters, various land-use descriptors and hydrogeomorphological characteristics, which allowed us to better understand the main drivers of microplastic transport and fate. Our survey results reveal an increasing downstream trend in sediment microplastic concentrations. The existence of distinct microplastic pollution hotspots in areas of high population density as well as areas of high cultural and spiritual relevance are detected. Interestingly, longitudinal concentration profiles in sediment and surface water samples do not match, hinting at differences in source attributions as well as transport, accumulation and long-term storage mechanisms, which is especially important in light of the variations seen in microplastic shape and size distribution patterns. Our results can be used as a springboard for future research assessing local microplastic contamination and aid in the refinement of medium and large-scale models estimating microplastic export.