Microplastics (MPs) are a major threat to aquatic ecosystems because of their very small size, higher surface area, capacity to adsorb harmful pollutants, inert nature, and ubiquitous presence in the environment. Primary MPs are bigger in size and found in personal care products or commercial abrasives, while secondary MPs are formed after disintegration of bigger plastic materials due to physical, chemical or biological forces.
MPs can enter aquatic ecosystems through air, wastewater effluents, improper plastic waste disposal, or stormwater run-off, and drains are one of the important sinks for terrestrial MPs and can accumulate and distribute microplastics to various aquatic systems.
Rainfall, flow rate, type of catchment (like residential, agricultural, and industrial), and treatment facilities affect the properties and types of MPs released through drains. Treatment processes lead to surface oxidation of MPs and their interaction with the organic matter in the water enhances their capacity to absorb toxic pollutants like pharmaceuticals, persistent organic pollutants, heavy metals etc which cause harmful effects among aquatic animals.
This study titled 'Urban drainage channels as a pathway for microplastics in riverine systems: A case study of Delhi, India' published in Water, Science and Technology analyses the microplastic distribution in the Najafgarh drain, one of the most important and largest stormwater drains in Delhi, India. Delhi generates about 3,45,000 tons of plastic waste annually and is one of the top plastic waste generator in India. Waste from the city is not treated efficiently and a large amount of wastewater discharge from the Najafgarh drain drains into the Yamuna.
Najafgarh drain is a 57 km long stormwater channel which begins at Dhansa in southwestern Delhi and ends at Wazirabad and drains all of its discharge along its entire flow into the Yamuna. The drain meanders through the city as an open surface water body and acts as a major sink for wastewater discharges from nearly 78 small and large secondary drains and a dump site for a variety of solid wastes including plastics. The wastewater contains effluents from domestic, industrial, and agricultural locations of Delhi. Samples were collected along different points across the drain.
The study found that:
The Najafgarh drain showed high plastic contamination
The Najafgarh drain was contaminated with MP particles in the range of 5,400 and 2,400 MPs/m3 during pre- and post-monsoon and included fragments, films, pellets, foam, and fibres. Fragments were the dominant during both seasons followed by pellets . Fibres were prominent during the pre-monsoon season while foam during the post-monsoon season only. Films occurred in a higher proportion during the post-monsoon season as compared to that in the pre-monsoon season. Most of the MPs were found to be white followed by blue during both seasons. The diversity of colours was higher in the pre-monsoon season.
Secondary drains carried microplastics in different concentrations
Out of all the secondary drains, the supplementary drain (S8) had the highest MPs during both seasons. This drain joins the Najafgarh drain just before its outfall in the Yamuna River.
Eleven types of polymers were detected, namely, high-density polyethylene (HDPE, 27 percent), low-density polyethylene (LDPE, 15 percent), blended polypropylene and polyethylene (PP-PE, 15 percent) polyethylene terephthalate (PET, 11 percent), polypropylene (PP, 10 percent), polyvinyl chloride (PVC, 3 percent), acrylonitrile butadiene styrene (ABS, 3 percent), ethylene-vinyl acetate (EVA, 3 percent), polystyrene (PS, 2 percent), nylon (NY, 2 percent), vinylidene chloride acrylonitrile (2 percent), while the remaining MPs (7 percent) could not be identified successfully .
Keshopur STP outlet was detected with 300 MPs/m3 during the pre-monsoon and 500 MPs/m3 during the post-monsoon season. The Badshahpur and Basai drain originates from the Gurugram district in Haryana and carries wastewater load from industrial, residential, and agricultural areas and discharges it into the Najafgarh Jheel. MPs found in this drain decreased due to dilution from pre-monsoon (1,400 MPs/m3 ) to post-monsoon (500 MPs/m3 ) season, possibly due to wastewater dilution or differential MP releases from the source.
The outlet from Goyla Dairy was found to be the second-highest MP contributor to the Najafgarh drain in the pre-monsoon season (3,400 MPs/m3 ). Wastewater in this outlet included cattle feeding material, cow dung and brittle plastic fragments. The continuous discharge of partially treated or raw wastewater from this outlet in the Najafgarh drain led to contamination of the drain due to organic load and MPs.
The Palam drain contributed the third-highest MP abundance (1,500 MPs/m3 ) to Najafgarh drain out of all drains during the pre-monsoon season. The agricultural and residential area that dominated Mungeshpur drain was found as a new contributor of MP in the Najafgarh drain and a huge mass of plastic macro litter was found accumulated on the surface water of this drain and adjoining parts of the Najafgarh drain due to blockage and it contributed to a large fraction of plastic waste found in the Najafgarh drain.
MPs released from the Keshopur sewage treatment plant were diverse in nature during both the seasons (pre-monsoon: 300 MPs/m3 and post-monsoon: 500 MPs/m3 ) while other drains like Basaidarapur drain, Kanhaiya Nagar drain, and the Supplementary drain (S8) were majorly carrying wastewater from residential and industrial locations. The individual contribution of MPs through these drains was 1,100, 200, and 4,300 MPs/m3 in the pre monsoon season and 500, 1,200, and 6,700 MPs/m3 during the post monsoon season.
STPs prove to be inefficient and inadequate to remove pollutants from the wastewater
MP abundance in the river was found to be 8-fold and 13-fold during pre- and post-monsoon season, respectively. While common effluent treatment plants (CETPs) are located for effluent treatment in the vicinity of industrial areas in Delhi, their working capacity was found to be insufficient to treat the huge loads of effluent generated.
A relatively higher MP abundance was found in the Yamuna river after the Najafgarh drain outfall in comparison to its upstream location. Considering an inflow of 56.29 m3 /s of Najafgarh drain wastewater into the Yamuna and the abundance of 2,900 MPs/m3 (for the month of May) detected at the outfall point of the drain in the present study, it was estimated that a discharge of approximately 14.1 billion MPs/day went into the Yamuna River through the Najafgarh drain. The study argues for the need for regular monitoring and setting up efficient wastewater treatment facilities in the Najafgarh drain basin so that the loadings of MPs before its discharge in the Yamuna River can be restricted.