Spatial–temporal Distribution of Microplastics in Lowland Rivers Flowing Through Two Cities (ne Poland)

Urbanised areas are known to cause plastic pollution, which consequently ends up in water sources. In this study, we measured microplastic concentrations in two lowland rivers flowing through two different cities. In both cases, the lowest downstream sites were located below local wastewater treatment plants. Samples were processed according to ricin oil separation and visual counting methods (considering abundance as well as shape, size, and colour groups). Microplastic concentrations in the surface water within the two studied rivers were found to be similar despite the differences in the cities they flow through. Mean microplastic abundance in the rivers was 10.83 MP/L in the Biała River and 10.29 MP/L in the Czarna Hańcza River. Regarding microplastic shapes, films were the dominant type (Biała River 47.8%, Czarna Hańcza River 51.9%), but higher proportions of less abundant plastic types such as fragments (Biała River 20.6%, Czarna Hańcza River 18.6%), foams (Biała River 12.1%, Czarna Hańcza River 14.2%), and fibres (Biała River 12.9%, Czarna Hańcza River 11.1%) were also found. The least common type of plastic was pellet (Biała River 6.6%, Czarna Hańcza River 4.2%), which was increasing significantly on stations situated downstream, behind the wastewater treatment plants. The results of our study show that city river systems are influenced by plastic waste, especially after precipitation and the snowmelt season. For small and medium urban rivers, hydrology does not play as important a role as it does for large rivers.