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Figure 1: Schematic representation of the microplastics pollution in Taichung. In rural areas no or very little amounts of microplastics were found. At the first appearance of storm sewers in the transition from rural to urban areas, the number of microplastics increases suddenly. Highest microplastics concentrations were found in the vicinity of storm sewers or at river confluences in the city center.
Microplastics are ubiquitous and affect all environments, including rivers. In recent years the number of studies about microplastics in rivers has strongly increased. But still many questions exist regarding sources, pathways, and the role of land use patterns.
In this study the relationship between microplastics abundance and anthropogenic factors (population density, urbanization, land use types), as well as the potential role of storm sewers as pathways in tributaries of the Wu River in Taichung, central Taiwan, were studied. Two river catchments of the Dali River were studied in greater detail to investigate the influence of land use on microplastics abundance along an urban-rural gradient, and to observe the change of microplastics abundance in the transition from rural to urban areas.
Samples were taken from 41 different locations in urban and rural areas using a manta net with a mesh size of 0.3 mm. Results show abundances ranging from 0 pcs/m3 in unpopulated rural areas up to 230 pcs/m3 in densely populated urban centers, and are positively correlated with population density. Remarkably, a sharp increase in microplastics abundance was observed at the transition from rural to urban areas, which coincides with the appearance of storm sewers.
Land use analysis revealed that microplastics abundance positively correlates with the size of industrial, residential and traffic areas in the catchment areas, and negatively correlates with the size of forest areas. Source areas for microplastics in the studied rivers are likely residential and commercial areas.
Furthermore, the results of this study show that correlations between microplastics abundances and population density or land use patterns along urban-rural gradients are not trivial. Strength of correlations can depend on local factors or how well urban-rural gradients are developed. Absence of correlations need to be considered carefully, as existing correlations might be masked by the above-mentioned factors.
Figure 2: Study area in Taichung (central Taiwan) showing population density, locations of all sampling points, and microplastics abundances. Following locations were sampled twice during different campaigns: C1 and H12, C2 and H5, E1 and T12, E2 and T1, E3 and T3. Please note that the ranges for each month differ and are not directly comparable. However, for each sampling campaign the same trend of higher abundances in urban areas can be observed.