Abstract: Ulansuhai Nur (Wuliangsuhai Lake) is one of the eight freshwater lakes in China and is also the “natural kidney” of the Hetao irrigation area. Receiving 90% of the farmland drainage in the area, Wuliangsuhai inevitably suffers from surface water pollution problems. To understand the water quality of drainage ditches into the Wuliangsuhai, water samples were collected from the tail sections of drainage ditches including the major drainage canal, the eighth, ninth, and tenth drainage canals in the Hetao irrigation area. Laboratory tests were then conducted to investigate the water quality of drainage ditches into the lake on a short-term time scale (2018-2020) from spatial and seasonal perspectives with pH value, chemical oxygen demand (COD), ammonia nitrogen (NH₃-N), total phosphorus (TP), and total nitrogen (TN) as assessment indicators. Pollution was evaluated by using single-factor index method and Nemerow pollution index method combined with cluster analysis. Results demonstrate that: (1) COD and TN were major pollutants in the drainage ditches into the Wuliangsuhai Lake, followed by NH₃-N and TP. (2) The major drainage canal and the eighth drainage canal mainly suffered from severe TN pollution, and the ninth and tenth drainage canal COD pollution followed by TN pollution. (3) From a seasonal perspective, pollution was severe in winter and spring, and moderate in summer and autumn. Throughout the year, the major drainage canal and the tenth drainage canal were in a state of slight pollution, while the eighth and ninth drainage canals were clean. (4) According to the pollution load and cluster analysis, the major drainage canal had the highest annual pollution load, accounting for 81.82%. COD and TN were major factors affecting the pollution load. Therefore, management to the agriculture process in the Hetao irrigation area shall be strengthened. In the meantime, the drainage in winter and spring, in particular, COD and TN concentration in the drainage, should be monitored and treated.

Key words: drainage ditch into lake, water quality indicators, pollution assessment, single-factor exponential method, Nemerow pollution index, Wuliangsuhai Lake