Characteristics and Variation of Water Quality During Dry Season in Yudong Reservoir of Zhaotong District in Yunnan Province
Yudong Reservoir of Zhaotong District in Yunnan Province plays an important role of local drinking water supply,irrigation,flood control,electric power generation,and fisheries.It is of great scientific significance to understand the water quality and changes during the dry period to guide hydraulic scheduling and ensure the stability of water quality and quantity in the reservoir.In this paper,by setting 30 monitoring points in different water fields of the reservoir and collecting water samples from the surface,middle and bottom layers of the reservoir during the dry period(January and May),the main physicochemical indicators and nutrient concentrations of water quality were analyzed and the water quality was evaluated by using the integrated index method.The results showed that that water temperature,conductivity and pH were significantly increased from January to May(P＜0.05),but transparency was significantly decreased(P＜0.05).Mean COD in whole reser-voir was decreased from(8.21±3.10)mg/L in January to(7.68±2.26)mg/L in May.Water fields and layers differed,COD presented in difference indeed,but not reaching a significant level(P＞0.05).Mean TN was(2.65±0.35)mg/L in January,in which NO3--N and NH3-N accounted for 90％and 1.5％,respectively,and it was significantly lower in main reservoir area than bays(P＜0.05).However,TN decreased to(0.96±0.12)mg/L in May,in which NO3--N and NH3-N accounted for 74％and 6.7％,respectively,and it was significantly higher in bottom water layers than surface water layers(P＜0.05).Mean TDP and SRP in January were(4.92±1.36)μg/L and(2.25±1.89)μg/L,respectively,and changed with different water fields.However,TDP and SRP in May increased to(11.6±11.0)μg/L and(6.72±9.38)μg/L,respectively,in which different water fields increased by different extents.The integrated indictor showed that the water quality in whole reservoir was in a range of Standard Ⅲ grade(GB 3838-2002),but it raised to Ⅱ grade in May,suggesting the reservoir had a strong self-purifica-tion potential during dry season.The self-purification potential shall be attributed to the growth of diatoms in surface water layers.A high dissolved Si in the water column and the increase in water temperature during dry season can promote the growth of diatoms,which can consume NO3--N and dissolved Si with the reservoir water quality improved.
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