Vertical distribution and environmental controls of microplastics in the hyporheic zone of the Beiluo River, China: Roles of water exchange and microbial responses

The transport and fate of microplastics in deep hyporheic sediments remain poorly understood. This study investigated microplastic distribution and driving mechanisms in the Beiluo River, China. Five sampling sites and sediment samples from six depth intervals (0–60 cm) were analyzed using microplastic identification, a one-dimensional heat advection–diffusion model, 16S rRNA sequencing, and metagenomic analysis. Microplastic abundance decreased from 312 items·kg −1 in surface sediments (0–10 cm) to 140 items·kg −1 in deep sediments (50–60 cm), with 0.05–0.5 mm particles accounting for more than 70% of total microplastics. Downwelling dominated water exchange, and exchange intensity showed a significant correlation with 0.05–0.5 mm microplastic abundance ( r = 0.91, p < 0.01), indicating the key role of hydraulic processes in microplastic transport. Firmicutes and Actinobacteria showed significant positive and negative correlations with small-sized microplastics ( r = 0.67 and − 0.72, respectively; p < 0.05). Functional gene analysis detected alkB1_2 and β-oxidation pathway genes in all samples, indicating microbial potential for polyethylene and polypropylene degradation. This study addresses the knowledge gap regarding microplastic migration mechanisms in deep hyporheic zones by integrating hydrological and microbial perspectives. A conceptual framework integrating hydraulic transport and microbial responses is proposed to describe microplastic fate and potential transformation pathways.