Reservoir Water Quality Change Characteristics And Risk Of Exceeding The Standard Under The Impacts Of Flood

Flood is one of the natural disasters that have the most serious impact on human society.Meanwhile,flood water is also an important component of surface water resources,and the rational utilization of flood water resources under the premise of risk control can alleviate the problem of water shortage.The flood risk comes from water quantity and water quality.As an important control project in the basin,the reservoir is prone to deterioration of water quality and accumulation of pollutants in the flood process,which directly affects the benefit of multiple uses.During flood processes,the water quantity risk is considered first,while the water quality risk is less considered.Therefore,the Biliu River Reservoir,which is the main drinking water source of the Dalian city,was selected to study the flood impacts on the reservoir water quality.The reservoir water quality characteristics and risk of exceeding the standard under flood impacts were studied by using observed hydrological and water quality data,simulation of the SWAT and EFDC model,pollution source apportionment model,and risk analysis model.The water quality risk management measures were put forward further to ensure the safety of the reservoir water quality under flood impacts.The main contents and conclusions are as follows:(1)The water quality characteristics under the flood impacts were studied by using the historical hydrology and water quality data.The data from automatic monitoring stations showed that the turbidity,CODMn,and NH3-N of the river water quality increased obviously during the flood process.For the Biliu River Reservoir,the water quality parameters with obvious increase after the flood included CODMn,NH3-N,TP,F.coli,and Cr6+.The variable fuzzy pattern recognition model was used to evaluate the comprehensive water quality,the results showed a trend of deterioration in water quality,from 2.23 to 2.35(p<0.01).The standardized runoff index and drought to flood abrupt alternation index were used to analyze the antecedent hydrologic condition on reservoir water quality.In all flood events,the water quality deteriorated from 2.22 to 2.47(p<0.05).2.11 to 2.34(p<0.01),and 2.00 to 2.37 under the conditions of antecedent drought,rapid alternation from drought to flood,and joint effects of antecedent drought and rapid alternation from drought to flood,respectively.The accumulation of pollutants during the antecedent drought and the flushing effect during the transition from drought to flood are the main reasons for the water quality decline in the Biliu Rive Reservoir.(2)The SWAT and EFDC models were integrated to study the migration characteristics of pollutants from the watershed to the reservoir and the accumulation effect in the reservoir.In the flood event,the nitrogen and phosphorus load from three main tributaries accounted for 88.44%and 78.89%.with high load areas mainly located in farmland and residential areas along the river and reservoir.There were obvious differences in the spatial distribution of nutrients entering the reservoir,and the fluctuation of water quality gradually decreased from the entrance area to the dam site.Reservoir shape and topography can reduce the flood impact on water quality at the dam site.In the three flood processes,about 1174 t of TN and 114 t of TP were retained in the reservoir,accounting for 27.33%and 78.65%of the nutrient load into the reservoir,respectively.The accumulation effect of TP was greater than that of TN.The water retention time of the reservoir varied during different floods due to the reservoir regulation,the nutrient retention rate increased with the water retention time.The spatial distribution characteristics of nutrients and water retention time are the key factors influencing nutrient accumulation in the flood periods.(3)Based on the study of the reservoir water quality characteristics under the flood impacts,the pollution source characteristics and the risk of exceeding water quality standards in reservoirs during flood seasons were analyzed by the PCA/FA-APCS-MLR model and BNs model.The results showed that TN,TP,and F.coli had the risk of exceeding the standards during the flood seasons of the Biliu River Reservoir.The agricultural non-point source was the main pollution source of the reservoir during flood seasons,which contributed 53.20%,48.41%,68.06%,and 72.69%to TN,TP,F.coli,and turbidity.The risk of water quality parameters exceeding the surface water standard under different hydrological conditions was assessed.The results showed that the probability of exceeding the standard of TN,TP,F.coli,and Cr6+ increased from 91.00%,4.30%,3.51%,and 1.85%to 95.83%,25.98%,12.53%,and 7.34%,and the probability of turbidity greater than 30 NTU increased from 4.80%to 18.23%.The general water quality risk of the Biliu River Reservoir increased with the increase of rainfall intensity.Accordingly,the corresponding control countermeasures were put forward from the perspective of the basin,reservoir,and comprehensive management.It was proposed to ensure the safety of reservoir water supply by controlling the pollutants loss from the watershed during floods,reducing the retention and accumulation of pollutants entering the reservoir,building a decision support system for water quality risk management,improving cross-administrative division watershed water environment management regulations,and utilization of flood resources based on water quantity and quality characteristics during flood periods.