Weijia Li

Stormflow runoff is an important non-point source of pollution in drinking water reservoirs. Storm runoff is usually very turbid and contains a high concentration of organic matter, therefore affecting water quality when it enters reservoirs. In order to investigate the impact of storm runoff on spatial–temporal variation and stratification of water quality during this rainstorm event, the inflow process of the storm runoff was studied through a combination of field investigation and simulation using the Delft3D-Flow model. Water samples were collected from Biliuhe Reservoir at four different periods: before storm runoff, storm runoff flood peak period, 1 week after storm runoff, and 5 weeks after storm runoff. The results showed that the input of storm runoff resulted in a significant increase in the nitrogen (N) and phosphorus (P) in the reservoir water, especially in the reservoir entrance. The concentrations of total nitrogen (TN) and total phosphorus (TP) gradually decreased after the flood peak period; however, the average concentrations of TN and TP in the entire reservoir remained higher than those before the storm runoff levels for an extended duration. The storm runoff will greatly contribute to the contamination of water quality in a reservoir, and the water quality cannot be quickly restored by self-purification in the short term. During the flood peak period, under the influence of density current, the electrical conductivity (EC) and turbidity increased significantly in the water depth of 10–15 m, so that the reservoir water had obvious stratification between 10 and 15 m. The form of pollutants in storm runoff was mostly in particle phosphorus. Total particulate phosphorus (TPP) concentration was 0.015 ± 0.011 mg/L, accounting for 44.12% of total phosphorus (TP) concentration in storm runoff flood peak period. The process of a rainstorm caused runoff, which carried high levels of turbidity, particulate phosphorus, and organic matter. The storm runoff disrupts the stratification of the reservoir water. In terms of vertical distribution, the turbidity in the reservoir area increased to 73.75 NTU. Therefore, the occurrence of significant turbidity density flow in the reservoir is frequently accompanied by intense rainfall events. Gaining insights into the impact of storm runoff on the vertical distribution of reservoir turbidity can help managers in selecting an appropriate inlet height to mitigate high turbidity outflow.

Significant amounts of nitrogen and phosphorus in sediments will be released into the overlying water during the flood season in the water level fluctuation zone (WLFZ) of reservoirs that undergo periodic drying and flooding. This will result in water quality deterioration of the reservoir. In order to clarify the distribution characteristics and release behavior of nitrogen (N) and phosphorus (P) from sediments in the WLFZ of a reservoir, this study analyzed the sediment distribution characteristics and potential exchange flux sediment–water interface(SWI) through field investigations and sediment core incubation experiments. And the main factors affecting the release of N and P through the incubation experiments in sediments of the WLFZ in the reservoir were determined. Our findings indicated that the sediment in the WLFZ serves as the primary source of NH₄⁺-N and acts as a sink for NO₂-N in the overlying water of sediment. The concentration of NH₄⁺-N in the interstitial water of sediments is the key factor that affects the water quality of Biliuhe Reservoir. Total nitrogen content of surface sediments in the WLFZ of Biliuhe Reservoir ranges from 1052.52 ± 49.39 to 3520.54 ± 30.31 mg/kg. High concentrations of N pollution are the primary increased risk of eutrophication in Biliuhe Reservoir during summer. The sediment N and P release flux of BLH1 located in the main stream is 1.67 ± 1.06 and 12.32 ± 2.42 mg·(m²·d)⁻¹, respectively, which is smaller than that of BLH2 (3.27 ± 2.15 and 15.19 ± 2.36 mg·(m²·d)⁻¹, respectively), BLH3 (4.24 ± 1.74 and 17.02 ± 2.47 mg·(m²·d)⁻¹, respectively) and BLH4 (7.78 ± 2.03 and 20.56 ± 2.38 mg·(m²·d)⁻¹, respectively) located in the tributary. It indicates that the water conveyance project located in BLH1 has an impact on nutrient scouring of sediments in the WLFZ at this site. The main water environment factor affecting the release of N and P in the sediment of the WLFZ is dissolved oxygen (DO). And the Pearson correlation coefficients between TN and TP with DO were −0.838 and −0.777, respectively (p < 0.05). At the same time, the diffusion of nutrients in the sediments can be effectively inhibited by maintaining a certain DO concentration in the overlying water.