A new study by researchers from the Indian Institute of Technology (IIT) Kharagpur National Institute of Technology (NIT) Srinagar and Durban University of Technology, South Africa has revealed how rainfall dramatically alters the movement, concentration, and composition of microplastics in Dal Lake, Srinagar. The findings highlight rainfall dual role as both a disperser and intensifier of microplastic pollution in freshwater systems.
Microplastics plastic fragments smaller than 5 millimetres come from sources such as cosmetic microbeads, textile fibres, and the breakdown of larger plastic waste. Persistent and non-biodegradable, they have now infiltrated every part of the planet, from deep ocean sediments to mountain streams. Yet, how these particles behave in freshwater systems under different weather conditions has remained poorly understood until now.
Tracking Microplastics Through Rain and Dry Spells
The researchers conducted two rounds of sampling in Dal Lake: one before the monsoon rains (mid-July 2023) and another after continuous rainfall (August 2023). They collected samples from multiple pollution pathways stormwater drains, wastewater treatment plant (WWTP) outlets, littering zones, and laundry discharge points and compared them with cleaner reference sites in nearby Nigeen Lake.
Their results revealed a striking pattern the concentration of microplastics nearly doubled after rainfall rising from 5.9 particles per litre during dry spells to 10 particles per litre afterward. The surge was linked to rainfall washing accumulated plastic debris from streets, drains, and informal settlements into the lake, while also resuspending microplastics previously settled in sediments.
Stormwater and Wastewater: The Main Culprits
Wastewater treatment plants were found to be constant sources of pollution, releasing microplastics even in dry weather. During continuous rainfall, however, concentrations at WWTP outlets rose from 7.9 to 14.03 particles per litre suggesting that heavy rain reduces treatment efficiency and allows more particles to escape. Stormwater outfalls showed an even more dramatic jump from 2.5 to 9.2 particles per litre as runoff flushed large amounts of urban litter into the lake.
Fibres, Fragments, and Toxic Polymers
The study also identified pathway-specific trends. Fibres from synthetic textiles dominated near WWTPs and laundry areas, while fragments typically from degraded packaging and plastic goods were prevalent near stormwater and littering sites. Alarmingly, black rubber particles from tyre wear were detected exclusively at stormwater outlets, pointing to road runoff as a growing source of pollution.
Rainfall didn’t just increase the amount of microplastics; it changed their chemistry. The researchers found new polymers, such as polyurethane (PU) acrylonitrile butadiene styrene (ABS) and polyvinyl chloride (PVC) near stormwater sites after rainfall. These high-hazard plastics, common in industrial and road waste, intensified the Pollution Risk Index (PRI) shifting stormwater pathways from low to considerable risk levels.
The study concludes that rainfall acts as a powerful driver of microplastic mobility and diversity redistributing particles and reshaping their chemical profiles. This has critical implications for urban freshwater systems, especially in climate-sensitive regions like the Himalayas, where changing rainfall patterns could worsen pollution risks.
To combat this, the researchers urge upgrades to wastewater treatment facilities better stormwater management and tighter controls on high-risk polymers. Such interventions, they argue, are essential for protecting not only Dal Lake fragile ecosystem but also public health in the surrounding communities.