Evaluation of eutrophication of water body in Huailai section of Yongding River

In order to understand the pollution characteristics of the Huailai section of the Yongding River, which is the key water pollution prevention and control area of the Haihe river system, the water quality monitoring was performed for a period of 1 years from November 2016 to October 2017. Six sampling points with 6 monitoring indicators were selected to expound the temporal and spatial variation characteristics of nitrogen and phosphorus in the river. The N/P ratio and the universal index formula using logarithmic power function were used to analyze the temporal and spatial variations of nutrient structure of nitrogen and phosphorus in water, assess the degree of eutrophication, and analyze the eutrophication risk. The results show that the water pollution in the Huailai section of the Yongding River is comparatively serious. TN is 13.25 times over the Class Ⅲ concentration limit of the surface water environment function area. The ON and N {\mathrm{O}}_3^{-} -N are the main forms of TN. The mean value of TP is 0.64 mg/L, and each index has the characteristics of agricultural non-point source pollution. The temporal and spatial distribution of nitrogen and phosphorus in the water body is obvious. On the time scale, the concentration of nitrogen and phosphorus in the water body shows seasonal variation, and on the spatial scale, the distribution of nitrogen and phosphorus in the water body is similar. The average N/P ratio of the river section is 32.78, and the phytoplankton is overall in the state of phosphorus limitation. During the average water period, phosphorus is the limiting nutrient element, and the N/P ratio in the dry and flood periods is suitable for the growth and reproduction of algae. The temporal and spatial variation of water body in the Huailai section of the Yongding River is mostly in the ‘eutrophication’ stage. The maximum EI value is 228.11 in dry season and the lowest is 213.06 in flood season, while that in average water period is 218.30. The fluvial process of the river section is good, and the flow pattern is continuous. The flow velocity of the main stream is 0.43~1.45 m/s, which is much larger than that for the algal bloom. Therefore, the risk of cyanobacteria blooms in the main stream is low.