Impact Of Continuous Rainstorm Runoff Underflow On Water Quality Of Stratification Drinking Water Reservoir During Flood Season

The rainstorm runoff extent under the global warming process exerted a strong impact on water quality in drinking water reservoir.In order to investigate the impact of continuous rainstorm runoff underflow on water quality of stratification drinking water reservoir during flood season,this paper takes the Jinpen Reservoir as the research object,and conducts high-frequency in-situ monitoring of the main reservoir area and the upstream river in 2021,the water quality changes in the main reservoir area was taken as the entry point,the formation process of thermal stratification and the corresponding water quality response pattern of the Jinpen Reservoir,spatial and temporal evolution characteristics of continuous rainstorm runoff underflow during flood season and corresponding water quality response pattern,lasting impacts of continuous rainstorm runoff underflow on water quality in the subsequent main reservoir area after the flood season were taken as the focus of the study,and systematic analysis of the historical maximum storm runoff event of the Jinpen Reservoir were completed.The main results and conclusions of the paper are as follows:（1）Heat transfer between the two phases of air-water is the main factor influencing the process of thermal stratification changes in the Jinpen Reservoir.There was a trend towards increased MOM and bottom anoxia to form and intensify under the dual oxygen depletion of the water column and sediment,and the aerobic environment of the water column in natural state was capable of effectively inhibiting the release of endogenous pollution.The retention of upstream water during reservoir storage was the main cause of the elevating pollution loads in the main reservoir area.The limited scale of rainfall runoff events was not sufficient to significantly change the water temperature structure of the main reservoir area and upgrade the pollutant loads.During the dynamic stratification period,the concentrations of COD_Mn and TP in the whole layer of the main reservoir basically met the water quality standard of Class II water bodies of the Environmental Quality Standard for Surface Water（GB3838-2002）.（2）The rainstorm runoff underflow during the flood season was mostly flowed into the main reservoir area as the middle underflow,resulted in the turbidity of the underflow layer in the main reservoir area to exceed 100 NTU for a long period.The underflow input weakened the thermal stratification of the water column and mitigated the trend of increasing MOM and bottom anoxia,with exogenous pollution input caused a rapid reduction in subsequent DO concentrations.The impact of rainstorm runoff underflows contributed to elevating pollution loads of particulate organic pollutants and phosphorus nutrients,characteristics of pollutant loss and differences in particulate pollutant carriers may result in multiple peaks in inflow turbidity from a single continuous rainfall event,and the peak of organic pollutant and phosphorus nutrient concentrations distributed in different water layers.（3）The largest historical storm runoff event in the Jinpen Reservoir reached a peak flow of 2063m3/s,which flowed into the main reservoir area as bottom underflow,and the whole layer of water in the main reservoir area was close to a completely mixed state through water displacement,and the sand-carrying flood carried high concentrations of reductive pollutants caused rapid reduction of DO concentration along the bottom underflow layer.Particulate pollutants were reduced simultaneously with suspended particles settlement,and the temperature heterogeneous formed by the subsequent storm runoff underflow which flowed into the main reservoir area as the bottom underflow and resulted in water body replacement again,induced the water layer below 570m in elevation in the main reservoir area to reach a low temperature complete mixing state.（4）The low-temperature complete mixing limited the oxygen consumption rate and accelerated the rapid recovery of DO concentration,and shortened the time required for the transition to the fully mixing state,and resulted in extending the natural mixing period over 2 months in the Jinpen Reservoir.The rainstorm runoff impacts caused continuing exceedance of particulate pollutants in the main reservoir area.COD_Mn and TP concentrations decreased simultaneously with suspended particles settlement,the variation among the concentration of different pollutants was significant,the retention of highly turbid water exerted a greater water safety risk for excessive TP concentrations.