Microplastics, defined as plastics with a size of less than 5 mm, are frequently encountered in soil, water, the atmosphere, and living organisms. Comprising carbon and hydrogen atoms bound together in organic polymer chains [1], these polymers encompass polyethylene, polypropylene, polyvinyl chloride, and polyester, with polypropylene and polyethylene being the most predominant [2]. Microplastics are categorized as primary or secondary. Primary microplastics, known as microbeads, are microscopic (< 1 mm) and are manufactured for products like facial scrubs, exfoliators, cleansers, soaps, detergents, and plastic fibers used in synthetic textiles (e.g., nylon) [3,4]. Secondary microplastics result from the photo-degradation of larger plastics and are more prevalent than primary microplastics [5]. Photo-degradation of secondary microplastics is commonly facilitated by solar ultraviolet radiation, wind, currents, and other natural factors [6]. Microplastics exhibit diverse shapes or forms, including fiber, film, foam, spheres, and pellets [7].

Being non-biodegradable, microplastics persist and eventually transform into nanoplastics [8]. This property also enables them to traverse long distances in the environment, posing both health and environmental hazards. Although there is limited information on the health effects of microplastic exposures, studies suggest they can induce cellular and physiological changes in the body [9, 10]. Additionally, microplastics have been linked to obesity, cardiovascular disease, reproductive disorders, and breast cancer [9]. Laboratory studies have demonstrated significant cell damage and even cell death due to microplastic exposure [11]. Human exposure to microplastics is also believed to result in lower metabolic activities, DNA damage, oxidative stress, and inflammation [12]. The impacts of microplastics can be transmitted through various levels of the food chain, adversely affecting organisms [13]. The effects vary depending on the species of the organism and the type and concentration of microplastics [13].

Water, serving as a major "sink" for pollutants, represents a primary source of chronic exposure to microplastics. Exposure can occur through drinking, bathing, during the primary production of food, cleaning and sanitation of food processing plants, as an ingredient or component of food ingredients, and through various processing operations [14]. To minimize microplastic exposure and health risks, periodic monitoring of water sources is imperative. In Nigeria, groundwater (boreholes and wells) is the predominant source of drinking and domestic water due to its cost-effectiveness, availability, and easy maintenance. Additionally, there is a deficiency and inefficiency in piped water, mirroring the situation in developing nations. Given the threat posed by microplastic pollution through water, constant assessment of the pollution status of groundwater sources in the country is essential. Several studies, including those by Yalwajiet al. [15], Aliyu et al. [16], Attah et al. [17], and Ibetoet al. [18], have been conducted on microplastic pollution of water in Nigeria. However, literature searches revealed no studies in Kebbi State or its neighboring states. The majority of Kebbi residents depend on borehole water owing to the reasons mentioned earlier, yet they dump plastic materials indiscriminately within the metropolis, potentially entering groundwater sources, compromising their quality. Thus, evaluation of microplastic pollution status of groundwater in the city becomes necessary to raise public awareness. Therefore, this study aims to ascertain the abundance, polymer types, and health risks of microplastics in borehole water in Birnin Kebbi metropolis.

The present study aimed to assess the abundance, distribution, and health risks associated with microplastics in borehole water in Birnin Kebbi, Nigeria. Microplastics were identified in all water samples from the boreholes, ranging from 86 to 97 particles/L, which is lower than the range of 206 to 1691 particles/L found in borehole water in Lagos by Oni and Sanni [28]. It is also lower than the 153 particles/L detected by Aliyu et al. [16] in untreated water in Kaduna, Nigeria, but higher than the 10 to 34 particles/L reported in borehole water in Northwest Mexico by Alvarado-Zambrano et al. [29]. Similarly, it exceeds the 5.00 to 10.5 particles/L found in water in China by Chen et al. [30]. Oni and Sanni [28] proposed that locations with elevated microplastic levels might be attributed to high population density and intense industrial activities, while areas with lower microplastic abundance could result from lower population densities and reduced industrial activities. Similarly, Jessieleenaet al. [31] suggested that municipal wastewater, dependent on population and anthropogenic activities, is a major contributor to microplastics in aquatic environments. In contrast to other cities in Nigeria mentioned above, Birnin Kebbi is less populated and industrialized, potentially explaining its lower pollution levels. Advanced cities mentioned above adhere to strict environmental protection laws, preventing indiscriminate waste dumping, including plastic materials, which may contribute to their less polluted groundwater despite industrialization and higher population density compared to Birnin Kebbi.

Figure 3 illustrates the percentage distribution of individual microplastic shapes in the water samples. Across all locations, fibers were the predominant shape, followed by fragments, foams, and filaments. This aligns with findings by Olarinmoyeet al. [32] and Yahaya et al. [2] in Lagos and Badagry Lagoons, respectively, where fibers dominated water samples. Inconsistently, Ebere et al. [33] and Oni and Sanni [28] reported fragment dominance in water in Imo State and Lagos State, respectively. Aliyu et al. [16] found a prevalence of fragments in water in Kaduna. Microplastic fibers, often used in clothing and detergents, may originate from clothing and laundry, especially wastewater discharged from manual washing and washing machines [34]. Hard plastics such as bottles, kegs, buckets, and outer packaging, visible in the studied area, may be sources of fragments, while plastic bags like poly bags and food packaging could contribute to foams [35].

Figure 6 depicts the percentage distribution of polymer types in the water samples, revealing polyethylene as the most abundant polymer in all locations, followed by polyamide, except in Rafin Atiku and Kalgo Market, where ethylene vinyl acetate exceeded polyamide. In Lagos, Fred-Ahmadu et al. [36] identified polyethylene and polypropylene as the most abundant polymers. Oni and Sanni [28] reported polypropylene and polyethylene dominance, consistent with Aliyu et al. [16] in Kaduna. Notably, polyamide's prevalence in drinking water in Birnin Kebbi is a unique finding in Nigeria. Polyethylene's ubiquity is attributed to its use in daily materials such as plastic bags, films, containers, and bottles. Polypropylene, used in textiles and durable materials, and polyamide, primarily for nylon production, serve various applications [37].

Table 3 displays the risk levels of various polymers in borehole water samples, calculated using polymer risk index, a mathematical model. Polystyrene, ethylene vinyl acetate, and polyethylene exhibited risk level III across all locations, while polypropylene showed risk level II, and polyamide showed risk level IV. This indicates a prevailing moderate risk (level III), with polyamide posing the most significant risks. These findings suggest potential health hazards for consumers. Studies link polypropylene exposure to metastatic features in human breast cancer and colonic apoptosis, while polyethylene exposure promotes inflammation, organ damage, and chemical alteration of plastics already in the body. Polyamide ingestion induces lipid metabolism and intestinal disorders, while polystyrene ingestion inhibits reproduction and causes DNA damage. Overall, microplastic exposure induces various toxic effects, including oxidative stress, metabolic disorders, immune responses, neurotoxicity, and reproductive and developmental toxicity [38].

Based on the results, it can be concluded that borehole water contains a significant amount of microplastics. The three predominant shapes observed among these microplastics are fibers, fragments, filaments, and foams, with fibers being the most prevalent. The two most common polymer types found in the borehole water at all sample points were polyethylene and polyamide, while polystyrene was the least frequently identified. The predominant risk score for these polymers is III, suggesting that the microplastics in the boreholes may pose some health and environmental risks. Notably, polyamide presents the highest risk, registering a risk score of IV. While we recommend further studies to substantiate our claims, consumers of groundwater in the affected areas should consider treating the water before consumption. Discouraging the indiscriminate dumping of plastic waste into the environment is crucial, and prioritizing the recovery and recycling of plastic will help reduce the environmental impact of plastic pollution.

During the preparation of this work the authors used ChatGPT in order to correct grammatical errors and make sentences flow. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.