Preliminary investigations on the occurrence and risk assessment of microplastics in the Bhoj wetland: a Ramsar site in Bhopal, Central India

Wetlands are fragile ecosystems, which play an important role in maintaining ecosystem services. Moreover, wetlands are also one of the most productive ecosystems similar to coral reefs and forests (USEPA 2023). Therefore, the sustainable management of wetland ecosystems is inevitable. These ecosystems not only harbour and protect floral and faunal biotic lives, but also provide innumerable services to humankind as well. However, in recent times, these wetland systems have been degraded to a significant extent owing to increasing encroachment and pollution-causing activities. Studies have shown considerable presence of heavy metals such as mercury, cadmium, copper, and lead in wetland ecosystems (Li et al. 2022). Apart from metals, various other emerging contaminants such as antibiotic residues (Bernharthorton et al. 2023), pesticides (Goldsborough & Crumpton 1998; Haarstad et al. 2012), personal care products (Haarstad et al. 2012), endocrine disruptors (Xue et al. 2008), and microplastics (Liu et al. 2022a) have also been reported from the wetlands.

Among all the emerging contaminants, research on microplastic contamination in various environmental matrices has gained momentum in the past decade. Microplastics, which are small plastic items lying in the size range of 1 μm–5 mm (Frias & Nash 2019), have been reported from nearly every type of environmental matrix, such as water (Mintenig et al. 2019; Ounjai et al. 2022; Singh & Bhagwat 2022; Singh et al. 2022), soil (Singh et al. 2023a, 2024a, b), sediment (Peng et al. 2018), snow/glaciers (Aves et al. 2022), air (Pandey et al. 2022), biota (Hariharan et al. 2021, 2022), etc. Moreover, these have also been found in the human body (Ibrahim et al. 2021a; Jenner et al. 2022) and its biological fluids (Leslie et al. 2022; Ragusa et al. 2022; Pironti et al. 2023), natural food items (Conti et al. 2020; Dessì et al. 2021) and processed food items (Liebezeit & Liebezeit 2013; Lin et al. 2022). Although the quantum of risk associated with microplastics is yet to be demonstrated, it is speculated that these particles may prove to be harmful in the long run owing to the presence of various chemicals/additives/contaminants in the microplastic particles (Singh et al. 2022; Upadhyay et al. 2022). The presence of microplastics in wetland ecosystems indicates the accumulation and retention of plastic waste materials in the water, sediments, and soil, which breaks down over the course of time resulting in microplastics (Patra & Baitharu 2024). These plastic materials may come either from intentional dumping or unintentional wind-blown deposition from nearby areas (Long et al. 2022; Xiao et al. 2023). Over a period, these microplastics may result in negative consequences for ecological flora and fauna.

The present study was carried out in Bhopal city of the Central Indian state, viz. Madhya Pradesh (M.P.) (Figure 1(a)). Madhya Pradesh has five Ramsar sites, namely, the Bhoj wetland (Bhopal), Sirpur wetland (Indore), Yashwantsagar lake (Indore), Sakhya Sagar lake (Shivpuri), and Tawa reservoir (Narmadapuram) (Ramsar 2024). Among these, the Bhoj wetland is the first Ramsar site in M.P. notified in 2002. Also, it is the biggest Ramsar site in M.P. in terms of area. The Bhoj wetland is situated across 23°14′ N and 77°19′ E (Figure 1(b)). Having ecological, socio-economic, and cultural significance (Ahmed & Wanganeo 2015), this wetland was selected for the present study.

Bulk water samples were collected using the grab sampling method (1-L grab) in February 2023. This sampling method was adopted considering the small size of the lake compared to rivers and oceans where manta trawls/neuston nets are generally used. Furthermore, grab sampling helps to collect samples at a wide variety of locations, such as wastewater discharge points, areas having extensive macrophytes, shallow water bodies, etc. (Hung et al. 2021). This sampling method also helps to capture a higher number of microplastics within the smaller size range (Barrows et al. 2017) which are generally ingested by smaller planktonic species. Water samples were collected in the wide-mouth glass jars of 1 L to avoid any plastic contamination. These jars were rinsed with the lake water thrice at the time of sampling before storing the water in duplicates. Jars were brought to the laboratory for further analysis.

In the laboratory, water samples were poured directly through the filtration sieves ranging in size from 5 mm to 90 μm. Plastic items larger than 5 mm were discarded, while other plastic items obtained over the sieves were carefully collected using metal forceps and kept in a glass Petri dish. The process of wet peroxide oxidation was omitted as no visible organic impurities in the collected water samples were seen. The sieved water was then exposed to a density separation process using a concentrated saline solution of NaCl (1.2 g/mL) (Singh et al. 2023a) for the floatation of smaller plastic items. The obtained microplastic items were subjected to stereomicroscopic analysis (Make: Carl Zeiss, Model: Stemi 305, Magnification: 8X–40X, Zoom ratio 5:1) for evaluating the physical characteristics, such as size, shape, and colour. For chemical characterization, Fourier transform infrared (FTIR) spectroscopy having attenuated total reflectance (ATR) assembly (Make: Perkin Elmer, Model: Spectrum Two) was used. Owing to the availability of only ATR–FTIR spectroscopy, the present study has the limitation of detecting and quantifying microplastic items >300 μm (Liu et al. 2024). During the experimental procedures, necessary precautions were taken to avoid contamination of the sample. All the work surfaces were thoroughly wiped before performing the experiments. Samples were collected and covered in glass Petri dishes and further wrapped in aluminium foil.

Here, C_mp refers to the degree of microplastic pollution over a period; Cⁱ refers to the concentration of microplastics in the collected sample, mg/L; C⁰ refers to the concentration of microplastics in the unpolluted sample, mg/L; T_c refers to the toxicity coefficient; n refers to the number of microplastic items; P_n refers to the concentration of a specific polymer in the microplastic sample; S_n refers to the hazard score of plastic polymers; E_mp refers to the potential ecological risk factor due to microplastics.

Furthermore, the available literature also does not provide any congruent expression for assessing the hazard level based on the values of the ecological risk index and it varies among the researchers for different environmental matrices, as shown in Table 1 (Xu et al. 2018; Du et al. 2020; Kabir et al. 2021; Pan et al. 2021; Ranjani et al. 2021; Liu et al. 2022b; Sparks et al. 2023). Therefore, the assessment of hazard levels owing to microplastics' induced risk becomes challenging.

The microplastic items obtained were also evaluated for ecological and human health risk assessment. The ecological risk assessment was made for each of the upper lake and lower lake samples, while human health risk assessment was done only for the upper lake samples because the water of the lower lake is not utilized for drinking purposes.

As far as human health risk is concerned, owing to the presence of microplastics in the upper lake samples, CDI_ingestion was calculated as per the procedure shown in Equation (5). With a microplastics concentration of 0.0009 mg/L, corresponding to 24 ± 1.41 microplastic items found in the upper lake samples, CDI_ingestion was calculated for carcinogens and non-carcinogens. In order to further evaluate the carcinogenic and non-carcinogenic risk, slope factors (SFs) and/or oral reference dose (RfD) for PVC, PET, PP, and PE microplastic items were obtained since only these four types of chemical compositions were found in the upper lake samples. As per the available literature, PET does not have any known carcinogenic effects (Blagoeva et al. 1990). Notably, commonly occurring PE and PP are also non-carcinogens (IARC 1979; Final report 2007; Moalli et al. 2014). Being a non-carcinogen, the oral reference dose (RfD) of PET is reported as 0.5 mg/kg-d (Ball et al. 2012). Oral RfD for PE and PP is not reported in the available literature. Therefore, the calculation of non-carcinogenic risk was done corresponding to PET, as shown in Equation (6). Obtaining this value as <1 suggests an acceptable level of non-carcinogenic adverse risk (Saleh et al. 2019; Mohammadi et al. 2019). PVC, on the other hand, is a carcinogen with an oral slope factor of 0.72 kg-d/mg (USEPA 2007). Using this value of the oral slope factor, the carcinogenic risk was calculated in accordance with Equation (7). Since this value was also found to be less than the permissible limit of carcinogens, viz. 10⁻⁶ to 10⁻⁴ (Mohammadi et al. 2019; Alsafran et al. 2021), there does not seem any potential carcinogenic risk owing to the presence of microplastics in the upper lake samples. Nevertheless, (upper) lake water gets the opportunity to pass through a water treatment plant before being supplied for drinking purposes; therefore, some amount of reduction in microplastics' concentration is expected (Singh et al. 2023b; Singh 2024), thus further reducing the human health risk. However, it is important to mention here that the inference about risk assessment has been drawn from the analysis of only 20 water samples collected from the Bhoj wetland and, therefore, results should be interpreted with caution.

The present study showed the presence of microplastics in the Bhoj wetland comprising the upper lake and lower lake (Figure 1(b)). Approximately 2.4 items/L in the upper lake, while ∼ 6.6 items/L in the lower lake reveal that plastic disposal and/or accumulation is taking place in these water bodies. Similar kinds of results were reported in the studies conducted in the Lalu wetland, Tibet (0.06–3.05 items/L) (Liu et al. 2022a), Setiu wetland, Malaysia (0.36 items/L) (Ibrahim et al. 2021b), and Hashilan wetland, Iran (2–6 items/L) (Abbasi 2021). However, some other studies have reported relatively higher concentrations of microplastics in wetlands such as Kucukcekmece Lagoon, Turkey (33 items/L) (Cullu et al. 2021), and Kallar Kahar wetland, Pakistan (88 items/L) (Dilshad et al. 2022). Among Indian studies, the Sundarban wetland (Ramsar site) reported the mean microplastic abundance as 0.086 ± 0.033 items/L (Chatterjee et al. 2024). Another study was conducted in Ashtamudi lake, a Ramsar wetland on the southwest coast of India. It was found that among the water, sediment, fish, and shellfish samples collected, 16.7% of the microplastics were found in water samples of the wetland with PP, PE, nylon, and polystyrene in abundance (Devi et al. 2024). A study in Vembanad-Kol wetland, which is also a Ramsar site in Southwest India, reported the abundance of microplastics as 144.25 ± 59.62 items/L. However, this study was conducted after an episode of a flash flood, which might have increased the abundance of microplastics (Nisari & Sujatha 2024).

As far as contamination of the lower lake is concerned, it receives a significant amount of wastewater/sewage from the city (World Lake Database; Nissa et al. 2022). 28 drains carrying over 50 million L/day are directed towards the lower lake (Kodarkar & Mukerjee 2009) which may be an important source of plastic/microplastic pollution (Singh et al. 2023b, 2024c). Although there have been considerable improvements in the city's sewerage system since these data were reported (CPCB 2016), the breakdown of the previously accumulated plastic items may serve as an important source of microplastic pollution even today. The presence of PA and PES items in the lower lake samples indicates that sewage is one of the important sources of pollution because PA and PES items generally originate through domestic washing activities (Hernandez et al. 2017; Singh et al. 2024c). Furthermore, improper solid waste disposal owing to lack of public awareness is also one of the causes of microplastic pollution in the studied area. The possibility of atmospheric transport of microplastics also cannot be neglected as it plays an important role in dispersing small- and light-weight microplastic items in far-off areas (Figure 6) (Zhang et al. 2021).

The present study has evaluated the ecological and human health risks which may be posed by microplastics found in the Bhoj wetland. Since the Bhoj wetland is a notified Ramsar site, it holds ample importance in terms of ecology apart from having various other significant roles. Ecological risk assessment was carried out by considering the presence of microplastic items at any particular time in the given area and the hazard score of the respective polymer. As per the analysis, the hazard level was identified as ‘very low’ or ‘low’ for both the upper lake and lower lake (Figure 5). This interpretation is based on the classification given by (Table 1) Kabir et al. (2021). Implications of even ‘very low’ risk may prove to be harmful for various components of the wetland in the long run owing to the process of bioaccumulation in the organisms (Miller et al. 2020). Moreover, accidental ingestion of microplastic items by the planktonic species may cause negative impacts on their physiological and reproductive processes, as reported by various studies (Jin et al. 2018; Wang et al. 2019).

Human health risk was also evaluated owing to the presence of microplastics since the upper lake water is utilized for drinking purposes in some parts of Bhopal city. As the obtained varieties of microplastic items were found to have non-carcinogenic and carcinogenic nature, both types of risks were calculated. The value of non-carcinogenic risk was found to be <1, which implies no adverse impacts on human health. Similarly, the value of carcinogenic risk was also found to be less than the notified risk range of 10⁻⁶ − 10⁻⁴, which suggests that there is no possibility of carcinogenic risk as per the current concentration of microplastics in the upper lake. Nevertheless, it is imperative to mention that although the four types of microplastic items (viz. PVC, PE, PP, and PET) found in the upper lake did not result in any quantifiable risk, research carried out in human cell lines exposed to PE, PP, and/or PET microplastics demonstrated negative impacts on digestive, respiratory, and circulatory systems as represented by decreased cell viability, oxidative stress, and genomic instability (Cobanoglu et al. 2021; Gautam et al. 2022; Zhang et al. 2022).

The concentration of microplastics in the Bhoj wetland in present-day conditions does not seem to pose any considerable ecological and human health risks; however, this issue needs to be addressed judiciously to avoid any serious outcomes in future. As plastic/microplastic items are considerably stable in nature, even their small presence in the environment may result in numerous nanoplastics in the long run. Furthermore, since this was a preliminary study, the presence of microplastics was identified only in the surface water, while deep water, sediments, and biotic species were not studied. It is a known fact that in stagnant or slow-moving water bodies, such as lakes, the majority of the microplastic items remain suspended in the sediments and benthic region (Gopinath et al. 2020; Scherer et al. 2020). Mechanisms of advection and dispersion act as some of the driving mechanisms to transport microplastics into the sediment bed (Molazadeh et al. 2023) where these have the possibility to remain suspended for a very long time. Apart from these, the aggregation of microplastic items with the organic aggregates present in water may further facilitate the movement of microplastics into the sediments (Leiser et al. 2021). Deposition of microplastics in the sediments may have serious repercussions for the benthic organisms, which ultimately have the potential to affect the entire food web. Therefore, the management of microplastics is crucial even if their presence is minimal in the present-day scenario.

Various discussions are taking place at the global and local levels regarding the management aspects of microplastics. As a basic premise, it is known that the majority of the microplastics originate from the breakdown of large-sized plastics (69–81%) (viz. secondary microplastics), while the contribution of primary sources (15–31%) in the overall pool of microplastics is rather limited (EU 2018). Hence, emphasis has been laid on minimizing the secondary sources, especially the single-use plastics that are highly likely to be thrown out immediately after use for a short span of time. Many countries worldwide, therefore, have banned/restricted the production, sale, and use of single-use plastics (UN 2018) which might prove a significant step towards minimizing microplastic pollution (Singh & Biswas 2023). Nevertheless, steps towards prohibiting primary microplastics in various commercial products have also been addressed by bringing up certain rules and regulations (McDevitt et al. 2017; US FDA 2017). The promotion of bioplastics and compostable plastics is another way of reducing plastic waste; however, their long-term sustainability needs to be evaluated further (Singh & Biswas 2023). At the local level, the most effective means of addressing microplastics is effective solid waste management. This needs extensive public awareness and inculcation of behavioural change practices among individuals. Developing a conducive environment for waste collection, such as an increased number of waste collection bins at tourist spots, regular clean-up and maintenance, strict waste disposal guidelines for local businesses, etc. also promotes appropriate waste management. Moreover, regular encouragement, an enabling environment, availability of alternative materials to plastics, etc. are some other steps for promoting the adoption of sustainable behavioural practices.

The present study has been carried out to investigate the abundance of microplastics in the Bhoj wetland (Ramsar site) situated in Bhopal, Central India. A total of 20 water samples were collected; 10 each from the upper lake and lower lake. 24 ± 1.41 microplastic items (in 10 samples) were found in the upper lake, while 66 ± 1.41 items were found in the lower lake (in 10 samples). Chemically, these microplastic items were characterized as PE, PP, PVC, PA, and PES. Ecological risk assessment analysis indicated that the presence of microplastics poses a ‘very low’ or ‘low’ risk to the environment. Moreover, human health risk was also found within the acceptable level of carcinogenic and non-carcinogenic adverse risks. This study further emphasizes the need to evaluate the presence of microplastics in deep water, sediments, and biota to have an overall assessment of the contamination in the wetland. Considering the potential of microplastics for being converted into nanoplastics in the long run, effective management of plastic waste is also a crucial aspect to look into.

The authors are thankful to Mr Surendra Singh Mehra for the support provided during water sampling in the Bhoj wetland.

The authors are thankful to the Indian Council of Medical Research (ICMR), New Delhi for the financial support (project grant number ICMR-NIREH/BPL/IMP-PJ-44/2021–22/469: Principal Investigator – Surya Singh, ICMR – NIREH, Bhopal).

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

The authors declare there is no conflict.