## 4 Discussion  
Hong Kong has been recognized as a hotspot of microplastic pollution because of the ubiquitous presence of microplastics in the environment. Microplastics have been detected in beach sediments and coastal surface waters with abundances of 5595 n m\({}^{-2}\) and 3.973 n m\({}^{-3}\), respectively [30; 31]. In addition, microplastic ingestion has been reported in various organisms in Hong Kong, such as captive and wild flathead grey mullet [32], crabs and bivalves [33]. To the best of our knowledge, this is the first report on microplastic pollution in Hong Kong's tap water. Since the first media report released by Orb media [34], several research studies published in the following years have reported the presence of microplastics in tap water, mainly groundwater-sourced, at different concentrations. The abundance of microplastics was found to vary from 0.0007 [35] to 628 n L\({}^{-1}\) (the highest microplastic concentration observed among three sampled water treatment plants) [23] for tap water, despite the varied sampling methods and analytical protocols adopted. With an overall concentration of 2.181 \(\pm\) 0.165 n L\({}^{-1}\), the microplastic contamination level of Hong Kong's tap water was not only comparatively lower than the global average, approximately a 2.5-fold concentration difference [25], but also about 2 orders of magnitude lower than that in Czech Republic (Table 1) [23]. In addition, a markedly higher percentage of samples with lower numbers of microplastics was observed (78.2%). These findings are in accordance with those reported by [PERSON] et al. [23] and [PERSON], [PERSON] and [PERSON] [25]. The prevalence of microplastics in tap water means that tap water could be an unneglectable source of microplastics to humans. Despite the difficulty in determining the exact origin of microplastics in this study, the presence of microplastics in treated water is expected to be linked to the types of source water bodies and various anthropogenic activities in the nearby surroundings, atmospheric input and mechanical abrasion of plastic equipment during water treatment and distribution. In comparison with previous studies of microplastics in groundwater-sourced treated water, higher microplastic abundances tended to be reported for samples sourced from surface waters (Table 1), as surface waters, compared to groundwaters, are susceptible to airborne particulate contamination and less prone to natural filtration of rocks and soils. In Hong Kong, raw water has long been sourced from the East River and local impounding reservoirs. As one of the highly developed areas in China, the East River basin has been undergoing rapid economic development and urbanization. With an increasing population (over 28 million inhabitants in 2010) [36] and expanding production and consumption of plastics, the East River was recently reported to have microplastic pollution with an average abundance of 0.40 n L\({}^{-1}\) in surface waters [37], which could represent a potential input pathway of microplastics to raw water. Additionally, there is a high likelihood of microplastics reaching the local reservoirs via atmospheric wet deposition (i.e., precipitation), as a growing body of evidence on the presence of microplastics in urban precipitation samples has indicated. For instance, a previous study in Dongguan, southern China, reported an average daily microplastic concentration of 36 n m\({}^{-2}\) in dry and wet atmospheric deposition [38]. Another recent study in the Hamburg Metropolitan Region of Germany also showed a consistent result, with an even higher daily concentration that reached 275 n m\({}^{-2}\)[39]. Therefore, it could be possible for airborne microplastics to enter the reservoirs through downward wet deposition fluxes and surface runoff from the catchments.