Mechanism Of Tidal Influence On Water Quality In Tidal Reach Of Yellow River Delta

The tidal section of the Yellow River Delta is affected by both runoff and tides,resulting in complex changes in water quality.It is of great significance to study the influence of tides on river water quality for clarifying the causes of water pollution,formulating regional water environment treatment plan,and promoting ecological protection and high-quality development of the Yellow River Delta.This paper takes the rivers of Yellow River Delta as the research object,analyzes the temporal and spatial variation characteristics of river water quality in the study area by using the cross section monitoring data,explores the response of water quality indexes of tidal reaches to tides by correlation analysis,wavelet analysis and other methods.The coupling model of hydrodynamic and water quality was established with Mike11 software and the influence range of tides was identified,and the process of migration and diffusion of river water pollutants was visualized with GIS technology.The water quality of tidal reach was studied by setting different tidal level.Through the experimental analysis,the influence of sediment on the overburden water quality under the combined action of tide and runoff is clarified,the salinity limit of the current COD measurement method is determined,and the influence mechanism of tide on the water quality of the tidal reach of the Yellow River Delta is preliminatively discussed,which provides a theoretical basis and scientific basis for the comprehensive treatment of regional water environment.（1）From 2013 to 2019,the concentrations of permanganoate index(COD_Mn),five-day biochemical oxygen demand（BOD₅）,chemical oxygen demand（COD）,ammonia nitrogen（NH3-N）and total phosphorus（TP）of the nine rivers all showed a decreasing trend,but the concentration of fluoride（F^-）showed an increasing trend.Attention should be paid to the overall spatial distribution of water quality pollutants,which is high in the south and low in the north or high in the southeast and low in the northwest.The single factor evaluation results show that among the 9 monitoring sections,the water environment quality of the Yellow River Lijin Hydrology Station is the best,reaching the Class II water quality standard,and the Yihong river is the worst,reaching Class V water quality.COD is the primary pollutant of the river in the study area,and F^-is the characteristic pollutant.The comprehensive factor evaluation results show that the overall pollution degree of the river in the study area is:dry period>normal period>wet period.（2）The tides in Dongying Port have a stable periodic pattern,with a strong cohesive 24-hour continuous main oscillation cycle throughout the year.According to the results of continuous wavelet transform,the water quality index lacks continuous main oscillation period.From the time scale of the whole year,there is a resonance period of 16-32h between the tide level and each water quality index,and there is a certain continuity.In the wet period,the trend of periodic synchronization of water quality with the change of tide level is more obvious.p H and DO had the strongest response to tide level,COD_Mn and NH₃-N followed,TP and TN had the weakest response.In a small time scale,there is a period of low correlation between tide level and water quality cycle in most months,that is,the absence of resonance phenomenon.Because there are many factors affecting the change of water quality,the positive and negative relationship and time lag effect between tide level and water quality have no obvious regularity in most periods.（3）The simulation result of the hydrodynamic water quality model of the spillway river established by Mike11 software shows that the limit of tidal influence is about 10km upstream of Minggang Gate.Set different scenarios to simulate the impact of tidal level changes on water quality:when the lower boundary tidal level decreases or increases by 50%,COD is most sensitive to tidal level changes during the process from the tidal zone boundary to the estuary,with the earliest concentration changes,followed by NH3-N,and the latest TP response.When the lower tidal level is set to a unified value of 3m and 5m,the changes in the tidal zone boundary and tidal current boundary are similar to the trend of a 50%increase in tidal level.In the estuarine area,when the tide levels are 3 meters and 5 meters,the decrease in COD and NH3-N concentrations is not significantly different from their original tide levels.The 5 meter tide level does not show a significant dilution ability higher than the 3 meter tide level,indicating that the impact of tides on the changes in COD and NH3-N concentrations is close to the threshold.The TP concentration in the estuarine area shows a significant decrease with increasing tide levels.The tidal level had a significant effect on the water quality in the tidal reach.（4）Temperature has an obvious effect on the release of nitrogen,phosphorus and organic matter in the sediment,but there is no obvious regularity with the increase of salinity.Salinity has a significant effect on the release of nitrogen and organic matter in the sediment,and the release of phosphorus is inhibited first and then promoted,and the peak value appears at 0.9%salinity.The disturbance has a significant effect on the release of pollutants in the sediment.After 10 h of disturbance,the change of NH₃-N tends to be flat,TP still fluctuates,and COD shows a sharp decline.The degree of TP release in sediments is as follows:salinity>disturbance time>temperature.Salinity results in positive error of COD measurement value,and the error increases with the increase of salinity.When the salinity is less than 0.7%,the error can be ignored,and≤0.7%is taken as the salinity limit that affects the COD measurement result,which solves the problem of large COD concentration changes in the tidal river section that has plagued the environmental management department for a long time.In summary,the influence of tides on the water quality of tidal reach can be divided into long time scale and short time scale,direct effect and indirect effect.In the long time scale,the water quality resonates with the tide in the period of 16-32h and has continuity in the whole year scale.In the short time scale,the tide causes the water quality changes through the tidal level（hydrodynamic）,salinity,temperature,disturbance,etc.Tidal wave is a direct factor affecting water quality change,and the tidal sweep range is the influence range,which decreases with the weakening of tidal current capacity.Under the influence of tides,sediment suspension and resettling occur alternately,which on the one hand directly affects the turbidity of water,and on the other hand,the amount of pollutants released by sediment increases.Salinity,temperature and disturbance intensity all have influence on the release of pollutants.In addition,the influence of salinity on the COD measurement results of tidal reach results in a misjudgment of COD concentration to some extent.Indirect influencing factors are very complex,not only affected by tides,especially salinity,but also by river ecosystems,and this area is the direction of future research.