Vertical Profiles of Microplastics in the Hyporheic Zone Sediment: A Case Study in the Yangtze River, Nanjing Section

Microplastics are ubiquitous in the river environment, although their abundance in sediment profiles has received little attention. The river hyporheic zone (HZ) sediment is the area influenced by surface and groundwater flow dynamics, and pollutants are more likely to be transported vertically in this area, thus entering the groundwater. Understanding the microplastic abundance and composition in the HZ sediment is crucial for microplastic pollution management. Hence, this study investigated the vertical distribution and characteristics of microplastic in the HZ sediment of the Yangtze River (Nanjing section). The results show that the abundance of microplastics in the HZ sediment ranged from 207 ± 95 to 1817 ± 467 items/kg dry wet in a vertical profile. With the increase of sediment depth, the abundance of microplastics decreased obviously in most sites, whereas the proportions of pellet shapes and smaller sizes of microplastics increased only at S1. No significant variation was found in the microplastic colors between different depth layers. Polypropylene and polyethylene were the dominant polymer types in all sediment samples. These results provided insights into the understanding of the microplastic fates in a river HZ region.