Characteristics of microplastics in tributaries of the upper Brahmaputra River along the Himalayan foothills, India

Microplastic (MP) pollution is a global concern, yet its presence in riverine environments has received limited research attention. This study aimed to evaluate MP levels and identify their potential sources in river surface water and nearby soil samples from two rural and remote rivers near the Jaldapara National Park in the foothills of Eastern Himalaya of India. The average MP levels in water and soil samples were 0.14 ± 0.11 pieces m−3 and 633.33 ± 124.72 pieces/kg d.w. respectively. The primary types of microplastics detected were fibres, followed by fragments, and films. MP sizes in water were larger than in soil samples. Blue, black, and red MPs were most abundant. Micro-Raman analysis revealed polyethylene was the dominant polymer type, followed by nylon, and polypropylene. Comparatively, pollution levels in the study area were relatively low when compared to other rivers worldwide. Understanding the sources and characteristics of microplastics are vital in formulating effective mitigation strategies and promoting responsible waste management practices. These findings provide valuable insights for policymakers, environmentalists, and indigenous communities to implement measures that can lead to curbing of plastic use and safeguard vulnerable riverine ecosystems from adverse impacts of MP pollution.


Introduction
The inexpensive, everlasting, versatile nature of plastic made it the most user-friendly consumer product of modern society.However, the lack of efficient and sufficient solid waste management systems resulted in leakage of plastic litter into the environment and found everywhere globally , including in geographically remote areas (Free et al 2014, Obbard et al 2014, Geyer et al 2017, Waller et al 2017, Goswami and Bhadury 2023b).Microplastic (MP) pollution, which refers to the presence of tiny plastic particles (0.1 μm to 5 mm) in various environments, is a worldwide issue affecting both health of aquatic ecosystems, biodiversity and human society.Extensive research has been conducted globally to document the presence of MP particles in different environmental matrices, including terrestrial, freshwater and marine environments (Sruthy and Ramasamy 2017, Goswami et al 2021, 2023a, Lechthaler et al 2021, Dusaucy et al 2021).Scientific observations suggest that rivers are responsible for 80% of the plastic debris released from land into the ocean (Law andThompson 2014, Jambeck et al 2015).There is a growing number of studies on microplastic pollution in rivers worldwide, but they are still limited and require further scientific attention (Blettler et al 2018, Koelmans et al 2015, Koelmans et al 2019).According to a global model, approximately 1.15 to 2.41 million tonnes of plastic waste enter the ocean annually through rivers, mostly from Asian rivers (Lebreton et al 2017).However, there is a significant lack of field-based data on MPs in Asian rivers, in particular from South Asia (Mai et al 2019).
The rapid urbanization, industrialization, and population growth in India have substantially increased waste generation.According to the Central Pollution Control Board (CPCB) of India, nearly 3.1 million metric tonnes of plastic waste was generated in 2020 and almost doubled over the past five years (CPCB 2020).Inefficient waste management practices, such as dumping of mixed solid waste in open landfills, are common in India (Joshi andAhmed 2016, Sharma andJain 2019).Over 85% of plastic waste is mismanaged, leading to its entry into the environment, including surface waterways (Geyer et al 2017).Lebreton et al (2017) predicted that the River Ganga in India is the second most polluted river in the world, releasing approximately 1.05 × 10 5 tons of plastic into the Indian Ocean every year.In a field estimation, Napper et al (2021) reported that the Ganges could release up to 1-3 billion MPs into the Bay of Bengal daily though the information on MP accumulation in Indian riverine networks is reasonably limited hitherto (Amrutha and Warrier 2020, Napper et al 2021, Lechthaler et al 2021, Tsering et al 2021).Incidentally, most research has focused on MP occurrences in marine environments (reviewed by Veerasingam et al 2020).
As a result of global warming, melting of glaciers, changes in weather patterns, torrential rainfall, increase in tropical cyclones and flooding events, there has been a steep rise in the influx of MPs into aquatic environments (Zhang et al 2020).Consequently, it is crucial to study MP pollution in Indian rivers.However, recent studies have primarily focused on perennial rivers and banks of rivers that are heavily populated.(Amrutha and Warrier 2020, Napper et al 2021, Lechthaler et al 2021, Tsering et al 2021).As a result, perennial or seasonal rivers located in geographically remote and rural areas and their tributaries that are often abundant in biodiversity, support freshwater needs and livelihood of indigenous communities, have been neglected.The seasonal rivers have huge significance in terms of biodiversity and people (Arora et al 2024).At the same time, remote rivers face challenges from plastic pollution as waste management facilities are largely rudimentary or absent in these remote areas and often rely on traditional practices such as open incineration or the burial of litter.Given that, it is crucial to investigate MP pollution in these remote rivers due to their ecological significance and potential impact on biological resources.The Jayanti and Kaljani seasonal rivers network, which serves as tributaries of the Torsa River, ultimately converging with the Bramhaputra River, originates from the Himalayan glaciers and runs through Bhutan and India.This riverine network also supports rich biodiversity including in Chilapata Forest near to Jaldapara National Park which is home to charismatic fauna such as the one-horned Rhinoceros (Rhinoceros unicornis) and also a paradise for bird watchers.The region is rich in biodiversity and the rivulets harbor small indigenous fish species (SIS) with international importance.Therefore, our goal was to examine the presence and distribution of microplastics in the surface waters of the Jayanti and Kaljani seasonal rivers network located in the Alipurduar district of northeast West Bengal which originates from the foothills of Eastern Himalaya in India.
Thus, the main objectives of this study were (1) to analyze the concentration, shape, and size of MPs, (2) to investigate the polymer composition of the MPs, and (3) to identify the sources of MP pollution in these ecologically sensitive riverine environments.Therefore, an additional soil sample was collected from a nearby solid waste dumping site in close proximity to atea garden to identify possible pollution sources of MPs.

Study area
The study was undertaken in the Alipurduar district (26°29' 30.8040'N and 89°31' 37.5600' E) in West Bengal, India.The district shares an international border with Bhutan.The topography of this district is interspersed by numerous rivers, rivulets, streams, foothills of the Eastern Himalaya (Dooars), tea gardens, and dense forests.The seasonal riverine network of Jayanti and Kaljani, originating from high altitudes of Eastern Himalaya, flows through the region and serves as significant freshwater resource for forests such as the Chilapata Forest near Jaldapara National Park.Besides, the biodiversity of Buxa Tiger Reserve (BTR) is also sustained by these riverine networks.The BTR represents the highly endemic 'Indo Malayan region' and has immense ecological significance.Incidentally, Chilapata Forest act as a corridor for movement of elephants between Jaldapara National Park and Buxa Tiger Reserve (BTR).In many of the peripheral villages surrounding the Chilapata Forest and adjoining tea gardens, solid waste such as plastic packets, paper boxes, plastic bottles, glass bottles, and vegetable waste are typically collected in designated bins.Each household has designated burning places where solid wastes are burned once a week (Bhattacharya et al 2016).Open incineration of such solid waste can lead to further environmental MP pollution (Goswami and Bhadury 2023b).Any remaining unburned materials are buried in the ground.However, the burial of wastes, particularly hazardous ones such as plastic, can negatively impact biodiversity of the area and its inhabitants (Bhattacharya et al 2016).The seasonal riverine network of Jayanti and Kaljani flows through the region and serves as significant freshwater sources.Most of the rainfall occurs from June to September, with occasional pre-monsoon showers in May.

Sample collection
Six surface water samples were collected from the Kaljani (5 samples) and Jayanti River (1 sample) in October (2022) representing the post-monsoon season.A hand-held Neuston net (mesh size 150 μm, 25 cm diameter, 50 cm length, Model 438 001, HYDRO-BIOS, Germany) was placed against the current in the mid of each river and filtered for 5 min.The river flow rate was measured by deploying a free-flowing float and recording the time the floating device traveled a specified distance (n = 3).The water volume filtered was calculated by multiplying the flow rate with the time and area of the net.Finally, the trapped particles were back washed with filtered in situ water in a glass bottle and fixed with formalin (2% v/v) until further laboratory analysis.Additionally, to identify possible pollution sources, a soil sample was collected in triplicates (1-2 kg each, up to a depth of 0-5 cm) using a stainless-steel scoop from casual garbage dumping site near an anonymous tea garden that was located in close vicinity (figure 1).The samples were collected in October of 2022 because the region is accessible immediately after the monsoon and also to capture a post-monsoon signature during this period.
Due to specific local conditions, a uniform sampling strategy, such as having three measuring points per river, equal distance between the samples, or sampling simultaneously, could not be implemented.The Kaljani River was selected for more extensive sampling due to its larger water volume and potential as a major water source in the study area.In contrast, the Jayanti River, while relevant, was sampled once to provide a comparative reference point within the context of present study.Further, in most of the areas, Jayanti River was inaccesible as it flows through the core areas of reserve forests.Therefore, the obtained results offer an initial understanding of the MP levels in these remote largely monsoon fed rivers.However, it is important to interpret these results cautiously, considering the potential influence of local and temporal variations.

Microplastic extraction from water and soil samples
Microplastics from the samples were extracted following standard methods (Masura et al 2015) with slight modifications (Goswami et al 2020).In the laboratory, water (whole concentrate) and soil sample (100 g) were dried in a hot-air oven (60 °C).Larger biogenic debris, stones, and pebbles were visually screened and discarded with the help of tweezers.The organic matter was removed from water and soil samples using wet peroxide oxidation [30% H 2 O 2 and ferrous solution (Fe (II); 0.05M), 20 ml each] on a hot plate at 75 °C.Once the organic matter was completely removed, density separation method was employed using zinc chloride (ZnCl 2 , 933.3 g l −1 ; density 1.6 g cm −3 ).Finally, the supernatant was filtered using GF/F (0.7 μm) filter paper under a reduced vacuum to retain the MPs.The filter papers were dried in the oven and stored in sealed Petri dishes for further analysis.The numerical abundance of MPs in water samples is reported as pieces per cubic meter (pieces m −3 ), while for soil samples, it is expressed as pieces per kilogram (pieces/kg).In case of soil, the MP abundance is calculated based on the dry weight of the samples.

Microscopic enumeration and characterization
All the particles retained on the filter papers were observed under a stereo-zoom microscope (Carl Zeiss, Germany, magnification 5 × to 300 ×) to count suspected microplastic particles.Particles were categorized as 'suspected microplastics' based on specific visual identification criteria.These criteria included the absence of cellular or organic structures, fibres with consistent thickness throughout their length and no tapering at the ends, non-segmented fibres without a twisted flat ribbon appearance, homogeneously colored particles, and particles that melted when heated with a hot needle.The shape, color, and size of suspected microplastics' were then recorded and measured using ImageJ software.

Polymer identification by micro-raman spectroscopy
All the putative MP particles on the filter papers were analyzed using a μ-Raman spectrometer (LABRAM HR, Horiba Jobin Yvon, Japan) to confirm chemical compositions.Raman measurements were performed using a grating with 1800 grooves mm −1 .The measurements covered a wavenumber range of 800-3200 cm −1 .The backscattering geometry was consistently utilized, and a spectral resolution of 2 cm −1 was set.The excitation wavelength used was 633 nm.To ensure accuracy, the obtained spectra were cross-checked with a free online database, Open Specy (http://www.openspecy.org/)(Cowger et al 2021), with a Pearson coefficient > 0.8.The smallest and longest particles that were successfully identified were 29 and 3800 μm in length, respectively.

Quality control and quality assessment
Several measures were implemented to mitigate the risk of environmental contamination during microplastic collection and analytical procedures.Firstly, all personnel involved in sample collection and handling wore cotton attire to minimize potential contamination from clothing fibres.Furthermore, all tubing, samplers, and accessories that came into contact with the samples were pre-rinsed with deionized water to remove any residual contaminants.In the laboratory, additional precautions were undertaken to minimize contamination.All liquids used in the analysis were filtered through 0.45-μm membrane filters (Whatman, United States of America), and containers were rinsed with deionized water before reuse.Equipment used during the analysis was covered with pre-rinsed aluminum foil, and laboratory coats made of cotton and nitrile gloves were always worn.The sample processing and identification steps were conducted in a closed-door room to prevent external contamination.Procedural blanks were simultaneously analyzed in hexaplicate to monitor background MP contamination.These blanks served as control samples to detect any potential contamination that may have occurred during the analysis.Field blanks (n = 6) were also maintained during sampling and analyzed in parallel with other samples.
To validate the recovery and extraction efficiency of the method, random positive controls (n = 8) were selected using garden soil samples.Known polymers such as low-density polyethylene (LDPE), acrylic, polyvinyl chloride (PVC), polypropylene (PP), and polystyrene (PS) were cut into 2 × 2 mm 2 squares and spiked into each positive control at a concentration of 20 pieces per 10 g of sample.During microscopic observation, encountering near-perfect squares of the specified dimensions indicated the presence of spiked microplastics in the soil.The mean count of these spiked MPs from the positive controls was used to estimate the recovery percentage, which was then applied to the final calculation of microplastic count from each sample.

Statistical analysis
Mean differences of particle shape, color, and polymer types were tested for non-parametric Kruskal-Wallis test.Results were considered statistically significant if p < 0.05.

Results
Quality control and quality check Stringent measures were implemented during sample handling and analysis to prevent contamination from airborne MPs.The environmental and procedural blanks indicated minimal contamination, with an average of not detected (n.d.) and 0.33 ± 0.51 MP fibres, respectively, which were acrylonitrile butadiene fibre, possibly originating from nitrile gloves (figure S1 and table S1 in supplementary material; SM).Therefore, if detected in water or soil samples, they were not considered for further analysis.Positive control samples demonstrated 100% recovery for all the polymer types (table S2; SM).

Microplastic abundance in water and soil samples
In the present study, MP particles of different shapes, sizes, and types were detected in various sites represented by sampling points.Combining all water and soil samples, 33 particles were confirmed as the polymer in μ-Raman analysis (table S3 in SM).These confirmed particles were only considered for further analysis.Microplastics were detected in 4 out of 6 water samples, except at S4 (Jayanti River) and S6 (Kaljani River).Since no MP was detected in these two sites, they were not considered in further calculation or discussion.On an average 16.67 ± 0.08 m 3 water was filtered at each site (table 1).The mean MP concentration in the surface water was recorded as 0.14 ± 0.11 pieces m −3 and varied slightly among different sampling sites (table 1).The highest concentration of MP in surface water was recorded in S1: 0.30 pieces m −3 .In the sole soil sample, average MP levels were 633.33 ± 124.72 pieces/kg d.w.

Microplastic characteristics
Three types of microplastic were detected in water and soil samples: fibre, fragment, and film.Some of the representative microscopic images of detected MPs are shown in figures 2(a)-(c).In the river water, fibre was the most dominant particle type contributing 50%-100%, average of 65% ± 21% (figure 3(a)).Next to fibre, fragment was the second predominant type, varied from n.d. to 50% (average 28.8% ± 19%).Occurrences of MP films were rare in water samples and were detected only at site 3 (Kaljani River).Relative contribution of fibre was significantly higher than MP films (Kruskal-Wallis test, p = 0.027).In the soil sample, particles were detected in the order of fragment (avg.48% ± 13.2%) > fibre (47.8% ± 10.1%) > Film (5.9% ± 4.2%).
The size distribution of different MPs in surface water and soil samples is illustrated in figure 3(b).In case of surface water, MPs sizes ranged from 360 to 3800 μm (average 1056 ± 1021 μm), with almost 65% of particles above 500 μm in size.The length of the fibres in water samples was much longer, with nearly 66% above 1000 μm, while fragment lengths varied between 100-500 μm.The only MP film detected in the water sample was around 360 μm.In case of soil samples, size of MP was relatively smaller and varied between 29 and 2182 (average 530.4 ± 607.20 μm).Nearly 29% and 43% of fibres in the soil sample were below 100 and 500 μm, respectively.Similarly, almost 82% of fragments in soil samples were below 500 μm.The length of the single detected film in soil sample was 93 μm.
Regarding particle color, in water samples, blue, black, and red MP particles were relatively abundant, accounting for nearly 71% cumulatively, followed by amber, yellow, and green (figure 3(c)).Fibres were mostly of blue and red color (44% each), whereas fragments were black and transparent.However, blue and transparent particles were the most prevalent in soil sample, 63% cumulatively, followed by black, red, green, and brown.In soil, fibres were predominantly of blue color (57%), followed by red (29%) and transparent (14%).Fragments were mostly transparent (36%), followed by blue, black, and green (18% each).These differences were statistically insignificant (p > 0.05).

Polymer composition
All the putative microplastic particles were checked for polymer compositions in μ-Raman spectroscopy (SI figure S2).Four different polymer types were detected in this study, viz.nylon, polyethylene (PE), polypropylene (PP), and isotactic polypropylene (iPP) (figure S2).Among these, PE was found to be the most dominant polymer in both water and soil samples, accounting for 57% and 58% of the detected microplastics, respectively (figure 3(d)).Nylon was detected in up to 36% of water samples and up to 26% of soil samples.PP was detected in lower proportions, with a maximum of 7% in water samples and 5% in soil samples.On the other hand, iPP, was only detected in the soil sample, representing 11% of the microplastic particles.These differences were statistically insignificant (p > 0.05).In water and soil samples, fibres comprised only nylon and PE, whereas fragments comprised PE, PP, and iPP.

Discussion
In this study, microplastic assessment in surface water of two geographically remote rivers in close proximity to the Chilapata Forest Reserve of the Dooars region of Eastern Himalayan foothills was attempted.The region represents part of the endemic 'Indo Malayan' region with huge ecological significance.Further, an additional soil sample was collected to predict possible sources of MP pollution in these environments.MPs were estimated based on their chemical compositions to identify their origin in these remote and ecologically susceptible environments.The average MP concentration detected in this study was 0.14 ± 0.11 pieces m −3 compared to the global dataset published in scientific literature (table S4 in SM).Comparing the MP pollution in rivers worldwide remains challenging due to the absence of standardized sampling, analytical protocols, and uniform units (Hidalgo-Ruz et al 2012, McCormick et al 2014).Thus, for global comparisons with our results, only those studies were selected that provided measurements in terms of the number of items per volume of water.Out of the 54 documents analyzed regarding MPs in river waters worldwide, 94% of them (51) reported higher mean or range values for MP abundance in water compared to the findings of the present study.Although MP levels detected in studied rivers were somewhat comparable to that reported in the Citarum River, Indonesia (0.0574 ± 0.025 piece m −3 , Sembiring et al 2020), lower stretch of Ganga River, India (0.34-0.68 piece m −3 , Singh et al 2021), and Beijiang River, China (0.28 ± 0.06 piece m −3 , Tan et al 2019).However, despite being one of the remotest locations, Napper et al (2020) reported 1 ± 0.3 piece/l MP in stream water near the Mt.Everest.Hence, when comparing the MP pollution levels reported in other rivers, MP concentrations detected in the Jayanti and Kaljani riverine networks can be considered relatively low.Various factors, including local topography, climate, hydrology, hydrodynamics, land use, population density, proximity to industrial areas, and socio-economic conditions in the surrounding regions, influence the presence of MPs in surface waterways.In the present study, these factors differed significantly from those in other studies, leading to lower pollution levels in these rural riverine networks.It is worth noting that particles that were confirmed as polymers after μ-Raman spectroscopy were only included.Several previous studies have used FTIR-ATR (see table S4 in SM) to assess polymer composition, which has technical limitations for smaller particles (<300 μm).Consequently, researchers have resorted to analyzing a small fraction (as low as 5%, e.g.Lechthaler et al 2021) of 'suspected particles' and extrapolating to the total counts, which could inevitably result in an overestimation of total MP count.Soil represent a significant repository for MPs in the environment (Bläsing and Amelung 2018, He et al 2018).Alterations in the physico-chemical characteristics of soil may adversely affect organisms, making MP pollution in soil a significant concern (Nizzetto et al 2016, Ding et al 2020).The MP level in the soil sample analyzed in the present study was 633.33 ± 124.72 pieces/kg d.w. which is nearly three-fold higher than the concentration (up to 205.06 pieces/kg) reported by Amrutha and Warrier (2020) from the catchment area of Netravathi River, India.Such high MPs in the soil can be attributed to the improper disposal of plastic waste and littering (Hurley andNizzetto 2018, Wang et al 2019).As mentioned earlier, the soil sample was collected from an open-littered site near a tea garden, which might have resulted in high concentrations of MP in soil.Given the rate of precipitation during monsoon season in the region, high concentrations of MP in soil will eventually end up through the riverine networks magnifying in higher trophic levels (e.g.fish).
Anthropogenic influences play a significant role in shaping the MP footprints in various environments.Studies have suggested that shape, characteristics and polymer composition of microplastic can serve as valuable indicators for identifying their sources (Auta et al 2017(Auta et al , et al 2020)).Several studies, including the present study, have consistently reported a high proportion of fibres in riverine microplastic samples (table S4 in SM).These studies have indicated fibre compositions ranging from 37.9% (Konechnaya et al 2021) to over 51.6% (Amrutha and Warrier 2020) and 65% in the current study, with some studies even reporting as high as 100% (Dris et al 2015).Fibres, being lightweight, are highly susceptible to being transported over long distances by wind and water currents.This characteristic makes them easily dispersed and distributed in various environments.The sources of fibre pollution are diverse and can include multiple products and activities.Common contributors to fibre pollution include laundry waste, fishing lines and nets, plastic packaging, plastic sacks, textile products, suspended atmospheric particles, and industrial plastic waste (Dris et al 2018, Yang et al 2021).These sources collectively contribute to the widespread occurrence of fibres across various ecosystems.It is essential to recognize the challenges in visually identifying microplastic fibres, which can result in misclassification, such as confusing natural fibres with synthetic cellulose fibres (Wesch et al 2017, Ryan et al 2020, Scopetani et al 2020).
Additionally, the presence of fibres in samples could be attributed to cross-contamination, including airborne contamination during sampling (Scopetani et al 2020) or laboratory analysis (Wesch et al 2017).Microplastic fragments and films are primarily generated through the fragmentation of larger plastic debris discarded by tourists and local residents (Eerkes-Medrano et al 2015).This is further supported by field observations, as it was seen that mismanaged littering practices were followed in the study region.These findings highlight the role of human activities, particularly improper waste disposal, in contributing to the generation and accumulation of MPs across studied environments.
The size and color of MPs are linked to their ingestion by aquatic organismal groups, especially when they resemble natural prey (Wright et al 2013).This can result in organisms mistakenly ingesting MPs, as they mistake them for food sources (Goswami et al 2020).The MPs observed in the water samples were larger than in the soil sample.This is probably due to screening out particles below 150 μm through the Neuston net during collections.This result corroborated the earlier reports (Di andWang 2018, Alam et al 2019) and was within the ranges observed in other studies (table S4 in SM).In MP colors, blue and red were most common among fibres, while transparent and dark-colored particles such as black were predominant in fragments.Blue plastics are widely used in the production of synthetic clothing on a global scale and in manufacturing plastic materials utilized in fisheries (Gago et al 2018).Transparent microplastics primarily originate from plastic carry bags, packaging materials, and fishing lines (Cole et al 2014).The colored microplastics identified in this study were sourced mainly from packaging materials, clothes, fishing nets, and ropes (SI figure S3).The dark-colored fragments could also originate from agricultural practices, particularly from plastic mulching.Fibres in various colors can be attributed to the breakdown of textile waste (Amrutha and Warrier 2020).
Polyethylene was the most abundant polymer type detected in water and soil samples, followed by nylon fibres and PP.Further, iPP, a thermoplastic used for non-stick coatings, was rarely detected in soil samples.The prevalence of PE could be attributed to their positive buoyancy (density <1 g ml −1 ) in water (Klein et al 2015, Koelmans et al 2019).PE and PP are extensively used in packaging industries, and nylon is widely used in textile and fisheries sectors (Campanale et al 2020).All these polymers are ubiquitously reported in studied riverine environment (table S1 in SM).Once in the environment, these polymers can undergo disintegration and shred into smaller-sized MPs.This disintegration can occur due to various factors such as from strong ultraviolet radiation (Song et al 2018, Rodrigues et al 2018), abrasion and aging (Zhang et al 2020), and the influence of intense climatic and hydrodynamic variations in high altitude rivers (Yang et al 2021).The presence of plastic or packaging industries is minimal in the study area, indicating that their contribution to microplastic discharges can be ruled out.In recent years, tourism sector in the Alipurduar and surrounding districts is experiencing rapid growth, especially between September and June, leading to increased use of single-use plastic.Additionally, unregulated solid waste disposal practices have also contributed to the accumulation of microplastics in the river.
Furthermore, geographically remote regions of these waterways may have received microplastics through atmospheric deposition, possibly transported over long distances (Napper et al 2020, Neelavannan et al 2022) or from transboundary regions.The higher tourist footfall also contributes to increased consumption and dumping of plastic surrounding Chilapata Forest Reserve.Therefore, it is suspected that, if adequate measures are not taken, the number of microplastics in nearby river water will increase, which may lead to disturbance in overall ecosystem health.Further long-term studies are needed to understand the temporal trends of microplastic pollution and their impact on these environments.

Conclusion
This study presents findings on MP pollution in two rivers remotely and in close proximity to Chilapata Forest Reserve in the foothills of Eastern Himalaya in India and also in close proximity to Bhutan with known endemicity.The average concentrations of MPs in water and soil were determined to be 0.14 ± 0.11 pieces m −3 and 633.33 ± 124.72 pieces/kg dry weight, respectively.Consequently, these rivers serve as conduits for transporting MPs into the Brahmaputra River, one of the largest perennial rivers flowing through the east of India.Compared to global levels, MP pollution in these riverine networks around BTR was relatively low.Blue and red microfibers were commonly found in the river water, while microplastics in soil were predominantly smaller in size (< 500 μm) than those in river water.The identified polymer types included polyethylene (PE), nylon, polypropylene (PP), and isotactic polypropylene (iPP), with PE being the primary contributor.The prominent MPs in these environments were secondary sources such as laundry waste, fishing gear, mismanaged packaging waste, and discarded single-use plastics from tourists.This study serves as valuable baseline evidence of MP pollution in geographically remote rivers originating from high altitudes.The information on MP provides an understanding of the potential sources that ultimately contributes to the MP sink pool, the coastal Bay of Bengal of the Indian Ocean.Given the ecological importance and diverse biodiversity of the region, further studies are necessary to establish long-term monitoring and modeling of MPs in these riverine systems.This would help towards assessment of ecological impacts and develop effective strategies for long-term sustainable management.

Figure 1 .
Figure 1.Study area map showing water and soil sample collection points in Alipurduar district, West Bengal, India.Red circles represent water sample locations and blue triangle represents location of soil sample.

Table 1 .
Details of water and soil samples and spatial variations of microplastic concentration detected in the present study.