# Spatial Variations in Microfiber Transport in a Transnational River Basin
## 1 Introduction  
Textile fibers are made of natural, semisynthetic, and synthetic materials [1]. The first polymer-based fiber (artificial silk) was made in the early 1880s, but synthetic fabrics became popular just in the mid-20 th century [2, 3]. The textile industry is rapidly growing: in 2020 the global synthetic fiber production represented ca. 68 million tons, which is around 62% of all fibers produced annually [4]. The problem with the usage of synthetic textiles originates not only from their increasing consumption and very low recycling ratio (\(\leq\)1% [1]), but also from the pollutant production, including microplastics (MP), during their fabrication [5, 6, 7], and the release of MPs during their usage, washing and drying [8, 9, 10, 11]. The MP contamination of surface waters is probably contemporaneous with the appearance and usage of synthetic products, as, e.g., in China MPs have been found in fluvial sediments since 1962 [12]. Microplastics originating from textiles have longitudinal shapes, they are often colored, and are found in all parts of the Earth. The high abundance of textile-originated MP fibers is connected mainly to wastewater [10, 11], as even after the cleaning process of the wastewater, the effluent water is usually highly polluted by MPs [2, 13]. [PERSON] et al. [2] indicated thatcurrent wastewater treatment technologies retain the majority (80-99.9%) of the MPs during the cleaning process; however, still large number of MPs is emitted into the environment via effluent water (max. 50 item/L), or by the agricultural usage of the wastewater sludge (max. 24 item/g). At the same time, it must be considered, that even though many households are supplied by electricity and piped water, and can thus use washing machines, the number of households connected to sewage pipelines and wastewater treatment plants (WWTP) is much lower, especially in poorer countries. Therefore, MPs, and especially microfibers can enter into the rivers, and they can be transported and/or accumulated in the fluvial environment. Very often, microfibers form 70-100% of all observed plastic items [14; 15; 16]. The significant role of the WWTP effluents downstream of WWTPs is reported by some studies [15; 17]; however, others did not observe a significant spatial change in MP concentrations [16; 18]. However, it must be noted, that most microfibers (\(\geq\)87%) in the freshwater environment are made of natural polymers such as cotton and wool [19; 20]; thus, they are not synthetic. Therefore, these fibers cannot be classified as microplastics without chemical identification of their polymeric identity [21]. The vertical distribution and transport of MPs in rivers is a complex process, as they are affected by a combination of several factors related to the hydrological, hydraulic, morphological, and hydrodynamic conditions of a river, and the physical characteristics (e.g., density, size, and shape) of the MPs themselves [22]. Due to the heterogeneous nature of MPs, they behave differently in river channels [23; 24]. For instance, low-density MPs, such as polypropylene (0.9 g/cm\({}^{3}\)), polyethylene (0.95 g/cm\({}^{3}\)), and polystyrene (1.1 g/cm\({}^{3}\)) are usually floating in the water column; meanwhile, high-density MPs, such as polyvinyl chloride (1.34 g/cm\({}^{3}\)), polyamide (1.42 g/cm\({}^{3}\)) or polyethylene terephthalate (1.42 g/cm\({}^{3}\)) sink into the river bed [22]. According to [PERSON] et al. [25], approximately half of the produced plastic has a density greater than water; thus, approximately half of the MPs may be transported as bed-load, and the other half as suspended load. On the other hand, many studies reported a higher abundance of MP in the riverbed than in the water column [26; 27; 28]; thus, they considered the riverbed as a sink for MPs. Besides, MPs are usually vulnerable to fragmentation, rapid flocculation and biofouling, processed which alter their physical characteristics and consequently their transport rate [22].