Rivers worldover are choking on plastic and transferring more than two million tonnes per year of microplastics (MPs) into the marine environment informs this paper titled 'Assessment of pollution and risks associated with microplastics in the riverine sediments of the Western Ghats: a heritage site in southern India' published in the journal Environmental Science and Pollution Research. 

River sediments are likely to be a sink for MPs rather than acting as carriers into the oceans. Highly dense MPs can quickly sink through the water column and mix with the sediments while less dense MPs can get deposited after forming a biofilm on their surface, increasing their density. 

What are microplastics?

Plastic debris in the aquatic environment can be divided into four size classes ranging from mega-particles (>100 mm), macro-particles (>20 mm) and meso-particles (5–20 mm) to micro-particles (<5 mm). Most of the plastic present in the aquatic environment is represented by microplastic particles.

Microplastics (MPs) are tiny particles of plastic debris. The chemical properties of plastic materials help them persist in nature for a long time and disintegrate into MPs due to biotic and abiotic processes.

The microplastic particles can be divided into two sub-classes, primary and secondary microplastics. Primary microplastics are directly released in the aquatic environment from industrial spillage or discharge in sewage water.  Secondary microplastics are formed when larger pieces of plastic debris break down due to ultraviolet light, heat, biological processes and physical action by wind and waves.

How do they affect aquatic life?

Microplastic particles can impact plants and animals in the water negatively due to accumulation of undesirable accumulation of micro-organisms, plants, algae, and animals on the surface of the plastic structures (biofouling) leading to disturbances in the aquatic ecosystems and through ingestion by organisms living in the water. Biofouling can also lead to introduction of invasive species in the water leading to disturbances in the ecology of the water body. The ingestion of microplastic (e.g. by bivalves, crustaceans, echinoderms, bryozoans and vertebrates) causes stress responses such as inflammation or oxidative stress at tissue and cellular levels in animals.

Microplastics can enter food chains due to ingestion of plastic particles by filter feeders that form the bottom of the food chain. This leads to accumulation of microplastic particles in digestive tracts of animals leading to reduction in feeding affecting nutrition and energy reserves among animals living in the waters. Microplastic particles can also expose plants and animals living in the water to environmental contaminants attached to their surfaces. Consumption of marine/freshwater organisms can also lead to the transfer of MPs to the humans affecting health. 

Rivers, carriers of microplastic

Rivers play an important role in transporting plastic debris of different sizes into the marine system. Hydrodynamic conditions affect the transport of MPs in sediments in the river channel, along with the physical properties of the plastic materials and sediments and determine their retention period in the sediments.

MPs can affect the porosity, permeability and bulk density of sediments and soils. The deposited MPs can also re-enter the riverbed due to turbulence at the water–sediment boundary or disturbance in sediments due to movement of organisms living in the water. 

The study

Most of the studies on microplastics have looked at its impact on coasts while research on the impact of MPs on terrestrial water bodies is scanty. There is little knowledge on the behaviour of MPs with sedimentary and organic properties in river systems. 

This study explored the distribution of microplastics in the sediment samples of the River Sharavathi, a pristine waterway originating in the Western Ghats, one of the world’s heritage sites. 

The Western Ghats (WG) include a 1600 km long linear mountain chain with dense forests and an aerial coverage of more than 140,000 sq. km. The rivers originating in the highland regions of the Western Ghats are an essential source of freshwater. 

The WG are also one of the 35 biodiversity hotspots recognised by UNESCO and according to the IUCN’s Red Data List, more than 300 globally threatened species are found in the WG. Recent studies also show that the ecosystem of the WG is undergoing a drastic change due to rampant anthropogenic activities in the region. 

The Sharavathi river basin is known for its rich biodiversity, encompassing various species of fora and fauna, including endemic and endangered species.

The study found that:

The concentration of MPs ranged from 2.5 to 57.5 pieces/ kg and 0 to 15 pieces/kg during the pre-monsoon and post-monsoon seasons in the river. The lower concentratons of MPs during the post-monsoon season had to do with the extremely high rainfall in the river basin, which transported the sedimentary MPs to the coast/Arabian Sea.

Smaller MPs (0.3–1 mm) were more abundant than the larger MPs (1–5 mm), mainly due to the breakdown of sedimentary plastics by physical processes. 

Fragments, films, foams, and fibres were the main categories of MPs, and the main polymers were polyethylene, polyethylene terephthalate, and polypropylene.

Smaller particles (0.3–1 mm) were more abundant than the more signifcant fractions (1–5 mm) in the sediments, mainly due to the breakdown of plastic materials deposited in the sediments by various physical actions. During both seasons, fragments predominated in the sediment samples. Transparent and white-coloured MPs were the most abundant in the sediments.

Risk assessment indices such as the Pollution Load Index (PLI), the Polymer Hazard Index (PHI), and the Potential Ecological Risk Index (PERI) were calculated to detect the levels of plastic pollution. Even though the PLI values were low, the risk of microplastic pollution was found to be under the high to hazardous risk category based on the PHI values. The PERI value ranged from 160 to 440 and 40 to 2240 during the pre-monsoon and post-monsoon seasons, respectively showing high ecological risk. 

Thus, even though the MP abundance was low, microplastic pollution in the study area posed a significant threat to the ecosystem, as shown by the risk factors (PHI and PERI). Besides, the risk contributed by 0.3–1-mm-sized microplastics is more significant during the pre-monsoon, which decreased during the post-monsoon season, which could be due to the flushing out of microplastics during the extremely strong southwest monsoonal experienced in the region.

Microplastic distribution in the sediments of the river catchment was influenced by population distribution and their activities (land use and land cover), geomorphological features, hydrodynamic parameters, and processes such as precipitation, surface runoff, and sedimentation.

Knowing about the spatial distribution of microplastics will be greatly helpful in identifying hotspots of microplastic pollution in the river catchment and can be used by policymakers and environmental managers to take initiatives to minimise pollution.