Microplastic contamination has appeared as pollution of global concern in the aquatic as well as the terrestrial environment. In India, very few studies are available on microplastic load in freshwater ecosystems despite knowing the fact that rivers play a major role in transporting microplastics into the marine ecosystem. 

Microplastics (diameter < 5 mm) is considered to be ecologically important due to its direct toxicity, bio-accumulative nature, and availability in the food chain. Rivers are known to be the key transit routes for microplastics into the marine environment, but still, there is a lack of large-scale quantitative data on the fate and transport of riverine microplastics, which makes it difficult to properly assess risks and establish regulatory solutions. 

The top 10 rivers that discharge marine plastic annually are located in the Philippines, India, and Malaysia which account for 81% of the annual worldwide marine plastic release (Meijer et al., 2021). If the pace of increase in plastic production continues, an additional 33 billion tonnes of plastic will be generated worldwide by 2050 (Rochman et al., 2013). 

The rivers act as a reservoir of microplastics and other human litter to the ocean which may cause serious environmental and ecological consequences in the long run (Landrigan et al., 2020; van Emmerik et al., 2022). Microplastics in the oceans lead to the "vast garbage patches" formations, consumed by a wide variety of creatures including protozoa and baleen whales, posing a serious threat to marine biodiversity (Meijer et al., 2021). Microplastics can be harmful to aquatic species' growth and development, immune system as well as genetic structures. 

Rivers serve as channels for marine debris to reach the sea, and even though up to 80% of marine debris comes from land, little attention has been paid to this fact. After reviewing the literature, it was found that studies on microplastics load in tributaries of the northern stretch of River Ganga is hardly undertaken by any previous researcher. 

Therefore, this study published in the Journal of Hazardous Materials Advances presents the first comprehensive report on microplastics load in three tributaries (Suswa, Bindal and Rispana) of the River Ganga in Dehradun, India. Microplastic content (particles), shape, size and polymer types in river water and sediment were analysed. Key water quality parameters were also analysed in all three tributaries at different sampling locations.

Sampling site

A total of 22 water and sediment samples were collected from different locations of tributaries (Suswa, Bindal and Rispana) of the River Ganga situated in the Dehradun region of Uttarakhand, India. Doon valley has sporadic Himalayan ranges with outer Himalayan ranges in the south, the Ganga in the east and River Yamuna in the west. 

River Suswa originates from a spring in the Gohro forest ranges. The urban stretch of Suswa is about 36 km and it receives massive urban effluent when passing through the urban areas in Dehradun. River Rispana originates from the foothills of the Mussoorie range of Shivalik Himalaya and the main stretch of this river is about 12.57 km. Rispana acts as one of the main sources of surface water for irrigation and groundwater recharge in Dehradun.

River Bindal flows through the densely populated urban area of Dehradun and carries a huge amount of untreated urban sewage and water from untapped urban drains originating from adjoined urban and forest areas. River Bindal joins the Suswa at Mothrawala before the merger with the River Ganga.

The water sample was collected in pre-acidic washed 500 mL plastic bottles while grab samples of sediments were collected through a customized sediment sampler and then packed in a plastic zip-lock pouch. Samples were stored in the icebox and transported to the laboratory and were stored at 4 °C as described in standard protocol for water analysis (APHA, 2005). Water temperature, pH and electrical conductivity (EC) were recorded on-site using field probes.

This study recorded four types of microplastics i.e., fibers, fragments, pellets and films with an average abundance of 37%, 38%, 18% and 7% in sediment and 36.66%, 39.04%, 17.66% and 6.61% in water, respectively. River water and sediment microplastic load did not show any respective significant variations among different rivers for shape types. 

This research estimates a load of microplastic in water and sediments of disappearing tributaries (Suswa, Rispana and Bindal) of the River Ganga at Dehradun, India. The water and sediment samples were collected from 22 locations and analyzed for microplastic occurrence and characterizations (size, shape, polymer types, and colour abundances) and physicochemical parameters as well. 

The average microplastic load was found to be in the ranges of 7200–16,400 items/kg in sediments and 2800–4200 items/L in water in the studied sites. The highest microplastic load occurred in Suswa followed by, Rispana and Bindal. ATR-FTIR analysis revealed polyethylene (PE) and polypropylene (PP) as the dominant polymer groups of microplastic in the studied sites. 

Fibers/ threads (41%) were the dominant shapes found in water samples while fragments (38%) were in the case of sediment. White and black colour microplastic occurred in the highest amount in both water and sediments. Principle component analysis suggested the mixing of urban runoff and sewage as the major contributors of microplastic s in studied tributaries. 

In summary, illegal dumping of urban solid waste in river catchment areas, human encroachment near the river sides, direct disposal of sewage and industrial effluents, etc. could be a major source of microplastics in such urban rivers.

The analysis suggested the abundance of microplastic polymers like high-density polyethylene (HDPE), polyvinyl chloride (PVC), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), and polyamide (PA) in sediment. Apart from this, polyvinyl chloride (PVC), polyethene terephthalate (PET) and polyurethane (PU) were found in lower amounts in sediment samples. 

Conclusion

Results suggested a high content of microplastics in water and sediments of studied rivers mainly originating from anthropogenic sources like sewage disposals, solid waste dumping, runoff water from urban settlements, industrial activities, etc. Fibers were the major microplastics types present in river sediments and FTIR analysis revealed the presence of polyethylene and polypropylene as the dominant polymer groups of microplastics in the studied sites. 

Colour analysis also revealed that the majority of microplastics originated from polypropylene substances which are commonly used at an industrial scale for plastic bag manufacturing. River water quality parameter analysis suggested high pollution due to the mixing of urban sewage and other runoff water sources in the studied tributaries of the River Ganga. Long-term monitoring of microplastics in the River Ganga and its ecological significance can be investigated in future studies.

The full report can be accessed here