Seasonal and spatial distribution of microplastics in the Upper Bhima River basin, Western Ghats, India
Abstract
Microplastics (MP) are recognized as emerging global pollutant with significant ecological consequences for freshwater ecosystems. Information regarding MP pollution in rivers originating from the Western Ghats is insufficient and underreported, including from one of the largest rain-fed riverine systems, the Bhima River. The present study was undertaken across seasons to understand the seasonal variations of MP in the upper stretch of the Bhima River. Seventeen sampling stations were surveyed across pre-monsoon (May 2023) and monsoon (July 2023) periods, and MP were extracted following established protocols. Visual identification of MP was conducted through stereomicroscopy, and chemical composition was validated using ATR-FTIR spectroscopy, with micro-Raman spectroscopy applied to a subset of particles for spectral confirmation. MP were detected at all stations and during both seasons. Abundance ranged from approximately ~ 1,000 to 8,000 MP/m3 (~ 1 to 8 MP/L) in May and ~ 333.3 to 6,000 MP/m3 (~ 0.33 to 6 MP/L) in July. Fibres dominated (> 80 percent) across the river continuum, while fragments represented a smaller fraction. Transparent and black particles were most prevalent, followed by blue and red categories. Size analysis indicated that MP in the 500–1000 µm category constituted the largest proportion of the samples, but no particles below 200 µm were observed due to optical resolution limitations under the stereomicroscope. Spatially, the Bhama River tributary exhibited the highest MP loads, particularly at urban-influenced stations within Pune. Polymer analysis revealed four dominant plastic types: polyethylene terephthalate (PET; 59%), polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC; 6%). PET prevalence, coupled with fibre dominance may suggest significant inputs from textile effluents and domestic wastewater in the basin. The findings demonstrate that the Upper Bhima River basin is experiencing widespread microplastic pollution, influenced by hydrological seasonality and urban discharge. The study provides an initial baseline for monitoring MP dynamics in a critically important freshwater system and underscores the necessity for targeted solid waste management strategies to mitigate upstream sources and downstream transport of MP.