Distribution of microplastic contamination in the major tributaries of the Yellow River on the Loess Plateau.

Microplastic pollution in rivers had gained increased attention worldwide. However, the differences in microplastic characteristics among major tributaries of large rivers and the environmental factors influencing these characteristics remain uncertain. Through field investigation and indoor experiments, the distribution of microplastics and their driving factors were investigated at 96 sampling sites along the three main tributaries (Huangfuchuan, Wuding and Yan River) of the Yellow River in the Loess Plateau. The results revealed that the average microplastic abundance followed this order: Yan River (430.30 items kg-1) > Wuding River (145.09 items kg-1) > Huangfuchuan River (253.33 items kg-1). The abundance was lower than that in most parts of the world. There was a generally increasing trend in average microplastic abundance from upstream to downstream in the three rivers. The most frequently observed microplastic colors observed were black and white, and the most common polymer type were PE and PS in all three rivers. The dominant shape and size in the three rivers were fiber and particles measuring 0.5-5.0 mm, all accounting for more than half of the total microplastic content. The microplastic abundance, shape, and size were primarily influenced by mean annual precipitation and population density. This relationship can be attributed to the fact that increased population density driven by higher demand and consumption of plastic products, while augmented rainfall aggravated the occurrence of floods and provided conditions for plastic degradation and accumulation. This study will provide fundamental data for pollution assessing and ecological protection of the Yellow River, and provide a certain reference for future management and protection on the Loess Plateau.