Weathered Microplastics in Himalayan Watershed Soil Act as Pollutant Carriers

Microplastics have become one of the most pervasive pollutants on Earth, but a new study from India suggests that the particles sitting in soil may be far more dangerous than their pristine laboratory counterparts. Researchers at Panjab University, Chandigarh, working with Queen’s University Belfast, have extracted and analyzed microplastics from the soil of the Sukhna Lake watershed in Chandigarh, a man-made, rain-fed reservoir nestled in the foothills of the Himalayas. Their findings, published in Frontiers in Nanotechnology, reveal that these particles are not merely inert fragments of plastic waste. Instead, they are environmentally aged, chemically oxidized materials coated with minerals and organic matter, and they carry a payload of contaminants that includes lead, one of the most potent neurotoxic heavy metals.

The research addresses a long-standing imbalance in microplastic science. Most studies have focused on marine and aquatic environments, leaving what researchers call the terrestrial gap largely unexplored. Yet soils, particularly in agricultural and urban areas, can harbor up to twenty-four times more microplastic pollution than oceanic basins. This matters enormously for watershed systems, because pollutants trapped in soil do not stay put. When monsoon rains arrive in Chandigarh, seasonal streams known as choes carry sediment, nutrients, and any attached contaminants downhill into the lake. By studying the soil at the source rather than the lake at the point of deposition, the team has captured microplastics at the very beginning of their journey into a vulnerable aquatic ecosystem.

The sampling design was deliberately rigorous. The team selected a single site within the watershed, an area experiencing rapid construction, road works, and pipeline upgrades, and divided it into a systematic grid. From sixteen collection points across the grid, they excavated the top centimeter of soil, the layer where most pollutants accumulate, yielding a total of 113.34 grams of soil. To separate the plastic from the mineral matrix, they used density separation, a technique in which soil is vigorously shaken in a concentrated salt solution. Plastics, being less dense than most soil minerals, float to the surface and can be decanted, washed, and vacuum-filtered. The procedure recovered approximately 5.1 grams of microplastics, an abundance of about 4.5 percent by mass, a figure the researchers describe as relatively high and indicative of both heavy contamination and an effective extraction protocol.

What followed was a forensic characterization campaign using an unusually broad arsenal of analytical instruments. Fourier transform infrared spectroscopy identified the polymer types by matching characteristic absorption peaks to reference spectra. Field emission scanning electron microscopy revealed the particles’ shapes and surface textures. Energy-dispersive X-ray spectroscopy mapped their elemental composition, while X-ray photoelectron spectroscopy probed the chemical state of the outermost atomic layers. Inductively coupled plasma mass spectrometry quantified trace metals, and ultraviolet-visible spectroscopy of solvent extracts provided clues about leachable organic additives. This multi-technique approach matters because discrepancies between analytical methods and sample matrices have long limited the comparability of microplastic data across studies worldwide.

The spectroscopic fingerprints told a clear story about what kinds of plastic end up in this watershed. The dominant polymers were polyolefins, especially polyethylene, which is consistent with global patterns reflecting its ubiquitous use in packaging, mulch films, and disposable products. The team also identified polypropylene, polyethylene terephthalate, and polyvinyl chloride. Each polymer pointed to a plausible local source. Packaging and single-use food containers, often littered in the area, explain the PET and polypropylene. Greenhouses and commercial nurseries nearby account for the high- and low-density polyethylene. The polyvinyl chloride, identified by its distinctive carbon-chlorine stretching bands, likely traces back to the construction activities around the lake, where PVC pipes are used extensively in plumbing and infrastructure.

The electron microscopy images revealed particles in various states of decay. Some were angular and sharply defined, while others showed flat, layered morphologies with fractured edges and surface irregularities that signal erosion and weathering. Fibrous, film-like particles appeared to derive from degraded plastic sheets and bags. One spherical, collapsed structure resembled a microbead or a fragment exposed to high heat, possibly from burning plastic. Another image showed an irregular, porous, aggregated mass, the kind of structure that develops when a particle absorbs environmental contaminants or undergoes advanced weathering. Crucially, all of these degraded features increase the surface area available for chemical interactions, turning each fragment into a more effective sponge for pollutants.

The surface chemistry data were the most striking. X-ray photoelectron spectroscopy showed a spectrum of aging states across the samples. One particle was nearly pure carbon with minimal oxygen, suggesting a weakly aged polyolefin. Another displayed oxygen at nearly fifty atomic percent, almost double its carbon content, along with abundant silicon, aluminum, potassium, magnesium, and nitrogen, evidence of long-term oxidative weathering and persistent association with soil minerals. High-resolution spectra revealed carbon-oxygen and carbonyl bonds formed by photo-oxidation and microbial activity, silicon-oxygen bonds characteristic of clay minerals and quartz, and signals from adsorbed salts, calcium carbonate, and nitrogen-bearing organic residues. The researchers conclude that these particles are not pristine but environmentally conditioned, wrapped in an inorganic-organic eco-corona derived from the surrounding soil.

The heavy metal analysis added a toxic dimension to the story. Inductively coupled plasma mass spectrometry detected magnesium at an average concentration of over 272,000 parts per billion, likely from additives such as magnesium hydroxide or magnesium oxide used in manufacturing. Phosphorus appeared at nearly 69,000 parts per billion, consistent with flame retardants, plasticizers, or stabilizers. Most concerning was lead, detected at 695.96 parts per billion. Lead is added to plastics as a heat stabilizer, ultraviolet blocker, or pigment, and exposure to it carries well-documented neurotoxic effects. Arsenic and cadmium were not detected. Solvent extraction followed by ultraviolet-visible spectroscopy showed absorbance bands suggesting oxidized organic functional groups and leachable additives such as ultraviolet stabilizers, plasticizers, antioxidants, and degradation products, with visible-region bands pointing to chromophoric compounds from dyes, pigments, or adsorbed organic contaminants.

Taken together, the results paint microplastics in this watershed as active vectors for the accumulation and transport of persistent contaminants. The weathering process that roughens and oxidizes their surfaces also enhances their capacity to adsorb heavy metals and organic pollutants, and the mineral coatings anchor them firmly within the soil matrix until rain, wind erosion, surface runoff, or flooding mobilizes them. During the monsoon season, the seasonal choes act as highways, carrying these contaminant-laden particles across the ecosystem and into Sukhna Lake, where previous studies have already measured microplastics at five to seventy-five particles per liter in the water and 1,700 to 2,320 particles per kilogram in the sediment. The bioavailability of the associated pollutants rises accordingly, meaning the particles function simultaneously as physical pollutants and as chemical couriers redistributing toxins across the landscape.

The study is among the first to examine microplastics at their site of origin rather than their site of deposition in this region, and the authors acknowledge that expanding the sampling to forests, agricultural land, and urban zones across the watershed would strengthen the picture of how these particles move through the environment. The broader implications, however, are already clear. As global plastic production has surged from two megatons in 1950 to roughly 380 megatons in 2015, mismanaged plastic has seeded every ecosystem on the planet, and terrestrial soils are now recognized as major reservoirs. The Sukhna Lake findings underscore the need for improved waste management, stricter regulation of plastic use, and continued research into how aged, contaminant-bearing microplastics affect soil structure, plant growth, microbial communities, and the food webs that ultimately depend on them. In the foothills of the Himalayas, the plastic problem is no longer just visible litter. It is a chemically active, pollutant-carrying layer in the soil itself, waiting for the next rain to set it in motion.

Subject of Research: Extraction, identification, and environmental risk assessment of weathered microplastics and associated heavy metals in watershed soil

Article Title: Microplastics in watershed: extraction, identification, and environmental risk assessment from the real sample

Article References: Sahi, A., Sharma, B., Chaudhary, G. R., & Kumar, R. (2026). Microplastics in watershed: extraction, identification, and environmental risk assessment from the real sample. Frontiers in Nanotechnology, 8, Article 1912308. https://doi.org/10.3389/fnano.2026.1912308

Image Credits: AI Generated

DOI: 10.3389/fnano.2026.1912308

Keywords: microplastics, soil pollution, Sukhna Lake watershed, heavy metals, FTIR spectroscopy, XPS, ICP-MS, polyethylene, environmental weathering, pollutant transport, Chandigarh, water contamination