Spatiotemporal Distribution and Sources of Pollutants in the Middle Reaches of the Yellow River

Ecological protection and high-quality development of the Yellow River Basin in China have received considerable attention; however, water-quality changes and pollution sources remain unclear. On the basis of water-quality monitoring data from 14 consecutive years, in this study, the water-quality index (WQI) of the study area from the Hekou–Longmen section of the middle reaches of the Yellow River was evaluated, and the spatiotemporal trends in water quality and pollution sources in the study area were clarified. The results revealed that the average annual WQI value increased from 72.06 to 85.88, with the poorest water quality (74.71) during wet periods and the best (78.82) during normal periods. Water quality showed spatial differences: chemical oxygen (CODCr) was lowest at Site 1, whereas ammonia nitrogen (NH3-N) was highest at Site 5, and both differed significantly from those at the other sites. Total phosphorus (TP) was lowest at Site 2 and differed significantly from that at Sites 1, 5, and 6. Permanganate index (CODMn) and biochemical oxygen demand after 5 days (BOD5) tended to increase in the upstream section of the study area but decreased or stabilized in the downstream section. Pollution was primarily attributed to inputs from the Wuding and Sanchuan Rivers. This study provides a theoretical basis and technical support for ecological protection and pollution control in the middle reaches of the Yellow River Basin and has important practical significance.