Plastic pollution of freshwater ecosystems in India is increasing - particularly that of rivers informs this paper titled 'Microplastics in freshwater ecosystems of India: Current trends and future perspectives' published in ACS OMEGA.
Rivers directly or indirectly support a billion people in the Indian subcontinent and India has one of the world’s largest river networks, which includes 12 major, 7 medium, and many minor rivers and watercourses, with an estimated total length of around ∼16 × 104 kms.
In addition to rivers, India also has a variety of freshwater environments, such as lakes, ponds, canals, estuaries, floodplains, coastal water bodies, and marine systems. India’s economic development is heavily reliant on its freshwater aquatic environment, which supports agriculture, aquaculture, navigation, electricity generation, and various industrial and commercial operations.
And freshwater ecosystems in India are being heavily contaminated due to the discharge of untreated wastewater from industries, urban areas, agricultural fields as well as from solid wastes from urban areas and industries. Plastic waste forms a major part of this waste.
Plastics are mainly categorised based on their size into nanoplastic (<1 μm), microplastics (MPs; <0.5μm to >0.5m), mesoplastic (<5 to >25 mm), and macroplastics (>25 mm). MPs are also classified as primary and secondary. Primary MPs include plastic microbeads from cosmetics and other products while secondary MPs include plastic fragments resulting from the breakdown of larger plastic debris that degrades through various processes such as physical, photodegradation, chemical, and biodegradation.
Microplastics (MPs) can directly or indirectly affect human health by acting as carriers of physical stressors or environmental toxins.
Microplastics in India
MPs research has mostly focused on coastal environments, coastal sediments, seawater, biota, sea salt, lakes and fish while that on freshwater ecosystems continues to be scanty.
Microplastics in rivers
India has the second-highest level of MPs pollution in its river systems after China and there are significant differences in the concentration of MPs in water and sediment samples obtained from freshwater environments.
For example, sediment samples from the Kaveri (1−699 items/kg) and Netravathi rivers (9−253 items/kg) in the south have high concentrations of MPs compared to the lower Ganga River (17−36 items/kg), Indus (60−340 items/kg),18 and Brahmaputra (20−240 items/kg).
However, in terms of river water samples, the Netravathi has a lower MP concentration (288 items/ m3 ) than the Ganga (466 items/m3 ), Adyar (330 items/ m3 ) and Kosasthaiyar (670 items/m3 ) rivers.
Rivers in the north originate in the Himalayas and pass through high-population-density urban areas, while southern Indian rivers do not flow continuously throughout the year and retain a significant number of MPs within sediments during low flow periods. River flow and MPs concentrations are negatively correlated, with high flow diluting the concentration of MPs. For example, studies show that MPs abundance in the Ganga River is lower during the rainy season than in the dry season due to flushing out of contaminants due to the rains. The density, buoyancy, and adsorption capacities of MPs can also influence their transport, migration, and distribution in surface water.
Microplastics in freshwater lakes
MPs are higher in the sediments of northern Indian lakes such as Anchar (233−1533 items/ kg), Pangong (160−1000 items/kg dw), Tsomoriri (960−3800 items/kg dw), Tsokar (160−1000 items/kg dw), and Renuka Lake (180 ± 143 items/kg dw) as compared to lakes in south India such as Red Hills Lake (27 items/kg), Kodaikanal (28.31 items/kg), and Veeranam lake (309 items/kg).
High-altitude Himalayan lakes have high levels of MPs and include Anchar (1,500 m.a.m.s.l.), Pangong (4,250 m.a.m.s.l.), Tsomoriri (4,522 m.a.m.s.l.), and Tsokar (4,572 m.a.m.s.l.). Anchar lake is located near a city and MPs in the river come from textile, packing, and automotive sectors. While Pangong and Tsomoriri lakes are in remote areas, MPs in these lakes are derived from rain and anthropogenic activities such as vehicles, tourism, tents, clothes, drinking water bottles, food packing, and plastic litter.
The concentration of MPs in Kodaikanal Lake has been found to be lower (24.42 items/L) than in Renuka (21 items/L) and Red Hills Lake (5.9 items/L; Figure 2B). In Manipal Lake, the concentration of MPs is higher during the monsoon season due to the surface runoff from surrounding areas and resuspension of MPs. Human activities have been identified as one of the most important factors contributing to the presence of MPs in freshwater bodies. The amount of MPs in sediment cores around Kodaikanal Lake decrease with increasing depth, with the maximum concentration found in the top layers.
Types of microplastics
MPs include fibres, films, fragments and foams, which are more common than pellets: Fibres and fragments are the dominant shapes found in both north and south India. For example, in Anchar Lake in the NW Himalayas 91 percent of MPs are fibres while in Manipal Lake they are 95 percent during monsoon and 96 percent postmonsoon and 91 percent in the Ganga River.
MPs such as pellets can be traced back to the cosmetic and industrial products while fibres are derived from fishing gear, synthetic clothing, and wastewater. Films are often derived from agricultural films and plastic bags, and foams can be attributed to packing materials and thermocol buoys.
The color of MPs may affect how they are ingested by organisms: For example, brightly colored MPs may be more attractive to some organisms, which could increase their exposure to toxic chemicals associated with plastic particles. Gobies (rayfinned fish) are visual predators and are inclined to consume MPs that have colors similar to their prey. The color of plastics themselves will significantly affect how much sunlight is absorbed because different colors of plastics absorb light with varying wavelengths and energies.
Black or dark-colored MPs may absorb more heat from the sun than light-colored ones, which can accelerate the ice melt. The most common colors noticed in MPs of the freshwater environment are white. White MPs make up the majority (65 percent) of MPs in the sediment and water of Red Hills Lake and 51 percent of MPs detected in Anchar Lake bottom sediment.
Multiple colors of MPs are found in Veeranam Lake with red (20 percent), black (22 percent), blue (13 percent), green (5 percent), and yellow (1 percent). Coloured MPs may have come from various sources including cloth wastes, fishing nets, ropes, and agricultural mulching applications. The white or transparent color MPs originate from carry bags and packaging materials.
Two of the most commonly found polymers in freshwater environments are Polypropylene (PP) and polyethylene (PE), which have a density lower than that of water thus aiding their wide dispersion and taking up by aquatic organisms. Over time, as biofouling occurs, the density of these polymers increases, causing them to sink and become deposited in sediment.
Lack of information on microplastics in freshwater
- At present, there is no clear understanding of the factors that contribute to the variability of polymer types in freshwater environments, and further research is needed to determine if there are specific groups of polymers that are more prevalent in MPs in different locations and over different distances traveled.
- While some information on microplastic distribution in freshwater resources in the north and south India is available, the entire central part of India is a white spot indicating that there is very little information available on the type and extent of microplastic pollution of freshwater resources in the area.
- Central parts of India are heavily populated and has many large river systems such as the Mahanadi, Godavari, and Krishna. Such data would not only help quantify the extent of MPs contamination in Indian river systems but also help to better understand the land-to-ocean MPs fluxes and the role these large Indian rivers play to control MPs in the marine ecosystem.
- While some studies indicate that MPs are present in some of the most pristine lakes in the Himalayas (e.g., Pangong and Tsomoriri Lake, information on these is very limited and more studies are required in the Himalayan region to constrain the impact of MPs on the Himalayan ecosystem already vulnerable to climate change.
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