Microplastic pollution – a rising threat along an urban lake in the Vellore district of Tamil Nadu, India: abundance and risk exposure

ABS

acrylonitrile-butadiene-styrene

ANOVA

analysis of variance

ATR-FTIR

attenuated total reflection Fourier transform infrared spectroscopy

CF

contamination factor

CL

cellulose

CP

cellulose propionate

EVA

ethylene vinyl acetate

HDPE

high-density polyethylene

LDPE

low-density polyethylene

MP

microplastic

NOAA

National Oceanic Atmospheric Administration

PBT

polybutylene terephthalate

PES

polyethersulfone

PET

polyethylene terephthalate

PLI

pollution load index

PMMA

polymethyl methacrylate

PP

polypropylene

PS

polystyrene

PVA

polyvinyl acetate

PVC

polyvinyl chloride

Plastic is a synthetic material that is featured as a part of our daily lives. It has amazing qualities such as durability, adaptability, lightness, and strength that assist most industries including transportation, building, sports, and cosmetics (Jambeck et al. 2015). Plastics offer an ideal combination of properties, including affordability, excellent oxygen and moisture barrier capabilities, bio-inertness, and light weight, making them a versatile and widely used packaging material (Shaikh et al. 2021). The use of plastic greatly improves people's lives; yet improper plastic trash disposal has seriously harmed the ecosystem (Dusaucy et al. 2021). Over the past six decades, there has been a tremendous increase in the amount of plastic produced annually worldwide, and it is estimated that 4.8–12.7 million metric tons have made their way into the ocean (IUCN 2019). Ineffective solid waste management and anthropogenic activities exacerbate plastic pollution. These plastics persist environmentally for extended periods, often exceeding centuries (Kibria et al. 2023). Plastic litter enters the aquatic ecosystem through surface runoff, wastewater, and atmospheric deposition, accumulating in rivers, lakes, and oceans (Schwarz et al. 2019). Once this plastic enters aquatic environments, it breaks down into smaller pieces due to environmental factors like physical stress (waves), solar UV radiation (sunlight), and biodegradation (microorganisms) (Jansen et al. 2024). Plastic debris is categorized into four size classes: macroplastics (>1 cm), mesoplastics (5 mm–1 cm), microplastics (0.1–5 mm), and nanoplastics (1–1,000 nm), as defined by Hartmann et al. (2017).

Microplastics, abbreviated as MPs, are described as ‘any synthetic or polymer solid particle with regular or irregular shape, size between 0.1 and 5 mm, and insoluble in water’ (Frias & Nash 2019). These minute particles can be grouped into two broad categories as (i) primary MPs, which include microscopic fragments made for industrial purposes, such as personal care products, and microfiber worn from textiles, including garments and nets used for fishing and (ii) secondary MPs, which are pieces of plastic that remain after larger plastic products, such as beverage bottles, are fragmented (Triebskorn et al. 2019). Due to their lack of biodegradability, these fragmented plastics will remain in the environment for a longer time and disintegrate into smaller pieces as a result of biotic and abiotic factors (Horton & Dixon 2018).

MPs permeate the whole marine ecosystem, affecting shallow water (Isobe et al. 2019), surface sediment (Martin et al. 2022), fluvial sediments (Kane et al. 2020), and biota as well as from the equator (Silvestrova & Stepanova 2021) to the poles (Peeken et al. 2018). These tiny particles are ingested as food by a variety of marine species, including zooplankton species (Kvale et al. 2021), turtles (Kühn et al. 2015), and humpback whales (Nelms et al. 2019). When these MPs are ingested, the chemicals that are linked to them are also transported via the food chain and cause various degrees of inflammation (Auta et al. 2017). MP characteristics, such as surface area and polymer density, regulate their buoyancy and interactions with aquatic biota, determining their fate in aquatic environments (Wright et al. 2013).

Freshwater resources are recognized as primary pathways for MP entry into rivers and oceans, facilitating terrestrial plastic waste transport (Lechner 2020). Despite extensive coastal research (Amrutha et al. 2023), freshwater systems encompassing inland wetlands like ponds, rivers, lakes, marshes, and lagoons remain understudied (Gopinath et al. 2020; Srinivasalu et al. 2021). MP pollution in these systems has only recently gained attention (Vaughan et al. 2017). Lakes, characterized by still water unrelated to the sea (Lehner & Döll 2004), are particularly vulnerable to pollutant accumulation due to reduced water flow rates (Yan et al. 2021). India's lake systems are poorly understood in terms of MP distribution and behavior (Sruthy & Ramasamy 2017; Gopinath et al. 2020; Ajay et al. 2021; Srinivasalu et al. 2021; Neelavannan et al. 2022). Therefore, a significant dearth of scientific understanding persists in freshwater ecosystems regarding their source and behavior, emphasizing the need for immediate investigation.

Lakes, vital components of global freshwater resources, support potable water supply, groundwater recharge, irrigation, tourism, and recreation. Despite comprising only 0.013% of global freshwater (Shiklomanov 1993), lakes regulate biogeochemical cycles and maintain biodiversity (Biggs et al. 2017). However, ecological fragility and climate change susceptibility render them vulnerable to contamination accumulation (Dusaucy et al. 2021). As closed basins, lakes retain pollutants, necessitating regular water quality monitoring. Multiple sources contaminate lake water systems, including plastic waste disposal, sewage waste, and pollutants from buildings and farms (Cai et al. 2022; Kallenbach et al. 2022). Consequently, the development and implementation of evidence-based management strategies are imperative to minimize environmental and health hazards associated with MPs (Hussain et al. 2024).

India's extensive network of lakes, crucial for potable water supply and groundwater recharge, necessitates vigilant monitoring of emerging contaminants to safeguard their sustainability. This investigation targets Lake Dharapadavedu, a vital freshwater reservoir in Vellore District, Tamil Nadu, which serves as a rainwater harvesting hub, augmenting water supply for local residents. The primary objective of this study is to elucidate the distribution and characteristics of MPs in the lake's surface waters and shore sediments. By generating baseline data, this research aims to inform evidence-based mitigation strategies for MP pollution, thereby protecting these ecologically sensitive ecosystems and promoting environmental stewardship. The main objectives of this study were (1) to categorize the presence of MPs in terms of their concentration, shape, size, and color; (2) to analyze the different polymeric compositions of MPs; and (3) to evaluate the degree of contamination of the lake through the pollution load index (PLI). Knowledge of MP contamination in Lake Dharapadavedu is vital for protecting biodiversity and ensuring the long-term sustainability of this critical ecosystem.

To determine the existence of MPs in this lake, 15 water and sediment samples were collected from the periphery of the lake during the monsoon season (October 2023). The distance between each sampling point was around 50 m to cover the entire lake. Due to the unavailability of boating facilities at the time of sampling, water samples could not be collected from the lake's center. Water samples were collected through bucket sampling using plankton nets (0.2 m diameter × 1 m length, 37 μm mesh size). At each sampling location, the net was submerged to a depth of 0.5 m and then lifted to collect a 1 L water sample for MP analysis. Surface sediment samples were collected using a Van Veen grab sampler (0.1 m²), and the top 2–4 cm of bed sediments (500–1,000 g) from the grab sampler were collected and packed in aluminum foil (Gopinath et al. 2020). The samples were carefully packed and transported to the laboratory for further examination.

The filter paper kept in the Petri dish was examined under a stereo zoom microscope to quantify the concentration of MPs (Hidalgo-Ruz et al. 2012). The MPs isolated from the filter paper were viewed under a stereo zoom microscope (NIKON) at 400× magnification. MPs were counted and captured separately based on their sizes, shapes, colors, and physical features. The four forms of MPs were classified based on their morphology: fragments, threads, filaments, and pellets. The size of suspected MPs was determined using image analysis software (NIKON). The software enabled precise measurements of MP length, width, and area, allowing for categorization into specific size classes. According to the size (μm) distribution, the MPs were grouped into three categories: <500 μm, 501–1,000 μm, and 1,001–5,000 μm. Black, red, blue, green, white, and translucent were common color categories of MPs, as previously reported by Peng et al. (2017). The abundance of MP particles was reported (items/L) in water and (particles/kg) in sediment. A qualitative examination of the chemical composition of the polymers in the MPs was performed using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). For every particle, 25 scans were conducted to capture the percentage transmission of the spectral region between 4,000 and 400 cm⁻¹ at a resolution of 0.07 cm⁻¹. Prior to scanning, the ATR crystal was always cleaned with chloroform (Gopinath et al. 2020). The chemical nature of the polymers was determined by comparison with reference spectra (Xu et al. 2021). Additionally, to identify the IR spectra, ‘Open Specy’, an open-source platform, was utilized (Cowger et al. 2021).

Strict measures were implemented to prevent the plastic product from contaminating the samples. Water samples were taken with a stainless steel spoon and placed in glass bottles with airtight caps. The sediment was covered with aluminum foil. Contamination control measures were applied during the entire sample processing procedure. Prior to sample processing, each piece of equipment was thoroughly cleaned three times using distilled water. To prevent plastic contamination, field sampling and laboratory analysis employed contamination control measures, including polymer-free attire for researchers and sealed laboratory windows to minimize airflow. To ensure accuracy, MP analysis was conducted through laboratory experiments using two blank samples: airborne particles and distilled water.

The abundance of MPs in lakeshore sediment and surface water was quantified as items per kilogram (items/kg) and items per liter (items/L), respectively. Statistical analysis employed a one-way ANOVA to compare the abundance of MPs across sampling stations, supplemented by Tukey's multiple pairwise comparison test to identify significant differences. All statistical computations were performed using Microsoft Excel software version 21.

A comparative analysis revealed that Dharapadavedu Lake's average MP abundance is significantly lower than that of Kodaikanal Lake, India (24.42 ± 3.22 items/L; Laju et al. 2022), Veeranam Lake, India (water: 13–54 items/km², sediment: 92–604 items/kg; Srinivasalu et al. 2021), Rawal Lake, Pakistan (1.42 particles/L; Irfan et al. 2020), Al-Asfar Lake (2.7 items/L), and Al-Hubail Lake (3.7 items/L) in Saudi Arabia (Picó et al. 2020). Conversely, Poyang Lake (5–34 items/L; Yuan et al. 2019) and Taihu Lake (3.4–25.8 items/L; Su et al. 2016) in China exhibited substantially higher MP levels. Significantly, Dharapadavedu Lake's MP abundance ranks among the lowest reported in freshwater lakes globally. However, inter-study comparisons are limited due to variations in sampling and analytical protocols, emphasizing the need for standardized methods in MP research. Despite Dharapadavedu Lake's relatively low MP abundance compared to global lake systems, its lentic nature and lack of outlet points necessitate concern. Particularly, MP presence was detectable throughout the lake, with elevated concentrations at stations KS2, KS3, and KS4, indicating potential MP sources. Anthropogenic activities and inadequate waste management practices are identified as primary contributors to plastic pollution within the lake (Gosavi & Phuge 2023). Furthermore, the proximity of sampling stations KS2, KS3, and KS4 to the lake's entry point, where high human activity and unregulated access facilitate the influx of plastic waste, correlates with elevated MP abundance. Plastic debris, including single-use bottles, food packaging, and other items discarded by lake users, contributes to this increased MP presence. Consequently, the retention and accumulation of MPs in Dharapadavedu Lake are inevitable, posing a significant threat to this ecosystem's integrity and potentially leading to long-term environmental degradation.

In MP-based contamination research, particle size is a critical factor. Using an optical zoom stereo microscope (400× magnification), we measured MP sizes and categorized them into three ranges: 1–500 μm, 500–1,000 μm, and 1,000–5,000 μm. The size distribution analysis revealed that 44.6% of MPs fell within the 500–1,000 μm range, with 33.9% measuring less than 500 μm and 21.5% ranging from 1,000 to 5,000 μm. Figures 5 and 6 display a zoom stereo microscope at 400× magnification images of MPs isolated from Dharapadavedu Lake, showcasing fibers, fragments, and films in both water (Figure 5) and sediment (Figure 6) samples. Scale bars represent 500 and 1 μm. The size of MPs significantly influences their behavior and ecological impact. Smaller, less dense MPs can persist in the water column for extended periods, posing significant risks to aquatic organisms through ingestion (Dris et al. 2016; Mak et al. 2019). Local climate conditions, including thermal degradation, transportation, distance from source, and residence time in the aquatic environment, determine MP sizes (Naidu 2019). Notably, the increased abundance of tiny plastic pieces (<500 μm) threatens the survival of various aquatic species that consume them (Mak et al. 2019). Understanding MP size distribution is essential for assessing ecological risks and developing effective mitigation strategies.

MP colors are useful in tracing the origin and source of plastic materials (Nelms et al. 2020). In aquatic ecosystems, MPs can be mistakenly ingested as food, contaminating lake biota and posing health risks. The ingestion of various colored MPs significantly impacts the growth rate of microorganisms like Scenedesmus and the feeding habits of Daphnia magna (Chen et al. 2020). This study identified multiple MP colors (red, black, green, yellow, and white) (Figure 4). Particularly, red and blue MPs (21.43%) predominated in water and sediments, followed by white (17.85%), black and green (14.29%), and yellow (10.71%). The detection of red and blue MPs suggests diverse origins, including packaging materials and personal care products (Li et al. 2020). The presence of varied-colored MPs across all samples indicates human activities as a primary contributor to plastic pollution. The breakdown of daily use plastic products, such as clothing, containers, and packaging, primarily drives the production of colored MPs (Wang et al. 2021). Consistent with previous findings (Pan et al. 2021), the significant proportion of white transparent MPs in this study indicates prolonged persistence in the lake, followed by environmental weathering-induced fading. The diverse range of MP colors suggests multiple pollution sources, aligning with previous research (Li et al. 2020; Wang et al. 2021) on MP color diversity and source variability.

Figure 7(a) shows the ATR-FTIR spectrum absorption bands (cm⁻¹) of CH stretching at 2,933 cm⁻¹; CH₂ bend = C–H bending at 1,456 cm⁻¹; CH₂ bending at 1,367 cm⁻¹; and C–C stretching at 1,088 cm⁻¹ in comparison with the reference spectrum. The major peaks were observed in the graph and the FTIR spectrum showed a significant band corresponding to the nylon (polyamide) polymer.

Figure 7(b) shows the ATR-FTIR spectrum absorption bands (cm⁻¹) displaying CH bend stretching at 2,914 cm⁻¹; CH₂ bend = C–H bending at 2,845 cm⁻¹; CH₂ bending at 1,461 cm⁻¹; and C–C stretching at 1,006 cm⁻¹ in comparison with the reference spectrum. The major peaks were observed in the graph and the FTIR spectrum showed a significant band corresponding to the HDPE polymer.

Figure 7(c) shows the ATR-FTIR spectrum absorption bands (cm⁻¹) corresponding to CH stretching at 2,914 cm⁻¹; CH₂ bend = C–H bending at 2,847 cm⁻¹; CH₂ bending at 1,470 cm⁻¹; and C–C stretching at 1,036 cm⁻¹ in comparison with the reference spectrum. The major peaks were observed in the graph and the FTIR spectrum showed a significant band corresponding to the LDPE polymer.

Figure 7(d) shows the ATR-FTIR spectrum absorption bands (cm⁻¹) corresponding to CH stretching at 2,950 cm⁻¹; CH₂ bending = C–H bend at 2,916 cm⁻¹; C–H stretching at 2,838 cm⁻¹; and CH₂ bending at 1,375 cm⁻¹ in comparison with the reference spectrum. The major peaks were observed in the graph and the FTIR spectrum showed a significant band corresponding to the PP polymer. Chemical composition analysis revealed potential MP sources, as summarized in Table 2.

The current study gives information on MP contaminants in both water and sediments of the entire Dharapadavedu Lake region. There are certain limitations that need to be considered due to the sampling having been done during the monsoon season. The seasonal variation in MPs cannot be anticipated. Due to the inaccessibility of the lake, sampling in the center of the lake was not conducted. Despite growing concern about MP pollution in lakes, there is a paucity of information on smaller size water and sediment fractions together with their material composition in freshwater lakes. Furthermore, the dynamics of MP transport and fate within lake ecosystems remain unclear. To combat MP pollution in freshwater lakes, future study should focus on effective removal techniques, source prevention solutions, investigating MP transport mechanisms and assessing human health impacts of MP exposure. Evaluating existing methods and developing innovative approaches will inform evidence-based policies and practices to mitigate MP pollution and protect lake ecosystems. Addressing these knowledge gaps, future studies are essential for understanding MP impacts and developing targeted mitigation strategies to protect lake environments. Moreover, the human health impacts of MP exposure in lake environments remain largely unknown, highlighting an urgent need for research to inform mitigation strategies. Closing these knowledge gaps demands a collaborative, multidisciplinary response from scientists, policymakers, and stakeholders to address the growing environmental threat of MP pollution.

With a focus on freshwater, we combined field observations, laboratory analysis, and PLI modeling to provide a holistic understanding of MP distribution, abundance, and composition. This study provides critical insights into MP pollution in Dharapadavedu Lake, focusing on abundance, morphology, size distribution, color, and polymer type. Our research yields the following conclusions: mean concentrations of 2.46 ± 1.06 items/kg in sediment and 1.26 ± 0.88 items/L in surface water. MP morphotype analysis revealed that fragments and fibers predominated, followed by films and pellets, with a primary size range of 500–1,000 μm and colors primarily red, white, and green. The primary polymer compositions identified were polyamide, LDPE, HDPE, and PP. PLI values indicate moderate to high levels of MP pollution risk with station specific differences, likely influenced by anthropogenic activities. To mitigate MP pollution, a comprehensive approach integrating efficient waste management, plastic waste reduction, and community awareness programs is essential. Remediation efforts must target multiple sources, considering MPs' persistence and accumulation in the lake's ecosystem. This pioneering research addresses a critical knowledge gap, providing valuable insights for conservation efforts in the previously unexplored Dharapadavedu Lake ecosystem.

The authors thank the Vellore Institute of Technology, Vellore for providing research facilities for carrying out this research article.

The authors declare that no grant was received for conducting this study.

D.R.: Investigation, methodology, sample collection, writing – original draft. S.L.: Methodology, sample collection, writing – original draft. M.S.: Conceptualization, methodology, supervision, and writing – review & editing. H.M.A.: Writing – review & editing, methodology, supervision. All authors then read and approved the final document.

All procedures followed were in accordance with ethical standards.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.