Abstract
<title>Abstract</title> <p> Based on measured data of nine water quality indicators from 15 monitoring sites in the Binan River small watershed of Bishan District in 2023, a PMF model was applied to trace pollution sources, aiming to investigate the water quality characteristics and pollution origins across different hydrological periods in a hilly small watershed. The results indicate that: (1) Nitrogen pollution is the primary contaminant in the Binan River, with total nitrogen (TN) being particularly severe. (2) Seasonal variation shows that water quality deteriorates during the dry season compared to the wet season within the year. (3) Source apportionment reveals that agricultural runoff, domestic sewage, in-stream internal sources, and natural origins are the main pollution contributors. Among these, agricultural non-point sources account for a high proportion of NH <sub>3</sub> -N, TP, and TN—reaching 85.48%-95.20% in the dry season and 65.35%-78.22% in the wet season. Therefore, seasonal differences and source characteristics of regional pollution should be taken into account when formulating water environmental protection and management strategies for small watersheds in hilly areas. The findings provide a scientific basis and practical guidance for precise watershed management strategies and systematic pollution control measures. </p>