Water pollution characteristics and influencing factors of closed lake in a semiarid area: a case study of Daihai Lake, China

In semiarid areas, closed lakes are essential for maintaining local ecosystem functions. However, many closed lakes are facing continuous deterioration of water quality. To scientifically and comprehensively evaluate the water quality of Daihai Lake (a closed lake in a semiarid region of China) and analyze its spatiotemporal variations and influencing factors, this study analyzed the following nine water quality parameters at five monitoring stations from 2008 to 2017 using the water quality index (WQI) and multivariate statistical techniques: permanganate index (COD_Mn), biochemical oxygen demand (BOD₅), total nitrogen (TN), total phosphorus (TP), fluoride (F⁻), water temperature (WT), pH, ammonia nitrogen (NH₄⁺-N), and dissolved oxygen (DO). The results showed that the average WQI at the five monitoring stations varies between 45.53 and 46.98, and the water quality of Daihai Lake has improved (2013–2017) after an initial period of pollution (2008–2013), but it remains poor. The main contaminants were identified as COD_Mn, BOD₅, TN, TP, and F⁻, which showed average concentrations of 13.12, 6.19, 0.98, 0.13, and 2.82 mg/L, respectively. The water quality showed small spatial variability, but distinct variations between the dry and wet seasons. Discriminant analysis showed that 97% of the water quality data can be classified into the correct seasonal with six parameters (WT, pH, COD_Mn, TN, and NH₄⁺-N). Anthropogenic activities were the main factors driving the variation in water quality. Climatic conditions promoted the release of internal pollutants and aggravated the impact of external pollutants during the dry and wet seasons, respectively. The minimum WQI determined using only key water quality parameters (DO, BOD₅, NH₄⁺-N, TP, and TN) could effectively reflect the actual water quality. These results can provide references for lake ecosystem protection and water resources management.