Variation Characteristics And Influencing Factors Of Water Quality In Sanhekou Reservoir During Initial Impoundment

Water source reservoirs are the main source of drinking water for urban residents.Although the quality of upstream water is generally good,the water quality of newly built reservoirs often deteriorates due to changes in water depth,hydrodynamic conditions,and dissolved oxygen during the storage process.This article takes the newly built Sanhekou Reservoir as the research object.Through continuous in-situ monitoring and laboratory analysis,the water quality changes and influencing factors in the reservoir area during the initial storage period are studied,and the effects of endogenous and exogenous pollution on the water quality in the reservoir area are explored.The main content of this article includes:(1)The changes in water quality during the initial storage period of Sanhekou Reservoir and their influencing factors;(2)Distribution characteristics of endogenous pollution in Sanhekou Reservoir;(3)The impact of different runoff inflow on the water quality of Sanhekou Reservoir.The main achievements and conclusions are as follows:(1)The phenomenon of thermal stratification in the initial stage of Sanhekou Reservoir impoundment is significant.The on-site monitoring results show that the formation period of the stratification structure in Sanhekou Reservoir is from April to July,and from July to August it is a stable period.The structure gradually disappears from September to October.There are differences in water storage conditions,rainfall runoff frequency,flow rate,and temperature changes between 2021 and 2022.Therefore,the evolution pattern of thermal stratification structure in the main reservoir area is significantly different between the two years.From June to August 2021,two thermocline structures were formed within the depth range of 0-10 m on the surface and5-15 m above the bottom during the initial water storage period.At the same time,a large area of anoxic water bodies appeared below the thermocline,with the maximum thickness of the anoxic layer at the bottom reaching 26m;From June to August 2022,an isothermal layer structure appeared within the water depth range of 0-5m on the surface,and no second thermocline structure appeared at the bottom of the reservoir.(2)The total nitrogen concentration of Sanhekou Reservoir fluctuates greatly in the initial impoundment period.During the initial impoundment period from June to September 2021,the total nitrogen concentration can only meet the Class Ⅴ standard,and from October to August 2022,the total nitrogen concentration can meet the Class Ⅲwater quality standard in most cases.The total phosphorus concentration showed a decreasing trend with the increase of water level from June to August 2021,and the total phosphorus concentration could meet the Class II water quality standard during this period.With the increase of total phosphorus concentration in September,it can only meet the Class III water quality standard.From October 2021 to August 2022,total phosphorus concentration in water can meet the Class II water quality standard in most cases.The permanganate index decreases gradually with the progress of water storage,and can meet the water quality requirements of Class II by 2022.Iron and manganese concentrations have remained at low levels.The water quality of Sanhekou reservoir is mainly affected by the upstream inflow at the initial stage of impilling,and the stratified structure also has some influence on the water quality.During the water storage process from June to September 2021,most of the pollutants carried by runoff were trapped in the reservoir area,and the concentration of pollutants in the reservoir water increased rapidly.In addition,the water density difference caused by thermal stratification structure will hinder the vertical transfer of pollutants and dissolved oxygen,so there will be pollutant accumulation in the thermocline.(3)By setting up static simulated release experiments of sediments and water-level zones,the pollutant release characteristics of sediments and soil in water-level zones at different sites were verified under different conditions.For the total nitrogen,the soil and sediment in the fluctuation zone reach the maximum release at 14℃(dissolved oxygen is about 6mg/L).For total phosphorus,the maximum amount of soil and sediment release was reached at 14 ℃(dissolved oxygen is about 6mg/L).Under anaerobic conditions,the release of pollutants in soil and sediment was promoted.The comprehensive pollution index evaluation results showed that the sediment of Sanhekou Reservoir was moderately polluted,and the soil in the subsidence zone was mildly polluted.(4)The variation law of water quality in reservoir area after large flow runoff and small flow runoff are compared.The results show that the effects of runoff on the stratification structure and water quality of reservoir area are different under different flow.When the runoff of large volume enters the reservoir(runoff in early September and early October 2021),the reservoir water body is affected by the runoff in a large range and mainly concentrated in the middle and lower layers.Therefore,the bottom thermocline structure is damaged after the runoff enters the reservoir.At the same time,the concentrations of nitrogen,phosphorus and organic matter in storm-water runoff increased rapidly and reached the annual maximum.The maximum concentration of total nitrogen and total phosphorus was 1.92mg/L and 0.056mg/L in the period of large runoff.During the low-flow runoff period(the storage stage from June to August 2021),the inflow water enters the middle and lower layers of the reservoir,affecting the distribution structure of water temperature at the bottom of the reservoir and making the thermocline structure at the bottom of the reservoir persist.In this stage,the inflow flow is small(22m3 /s ～140m3/s),and the pollution load carried by it is also small,so the concentration of nitrogen,phosphorus and organic matter changes in the process of storage.