Variation Characteristics And Risk Assessment Of Water Quality In The Xiaoqing River Estuary

In recent years,with the rapid economic and social development along the Xiaoqing River basin,industrial pollution and agricultural non-point source pollution have become increasingly serious,water pollution is a prominent problem.At the same time,the water quality in estuary is affected by the supporting effect of the tide level,which aggravates the complexity of water pollution problems.Research has shown that runoff from Xiaoqing River,which carries a lot of organic matter and nutrients,has caused serious damage to the water environment in the Laizhou Bay.As the most important land-based source of marine pollution,the rivers entering the sea have been widely concerned and become the key area of pollution control.Therefore,it is of great significance to identify the characteristics of water quality changes in the Xiaoqing River estuary.In this paper,the estuary of the Xiaoqing River from Shicun to Yangjiaogou was selected as the research section.Based on the measured data,the water quality characteristics of the estuary were analyzed,the water quality was evaluated and the influencing factors of water quality were explored.A MIKE21 based hydrodynamic—water quality coupling model was established to verify the applicability of the model in the estuary,the transport and transformation of different pollutants in estuary were simulated and predicted.Finally,based on the results of the previous analysis,the integrated risk assessment system of water quality was constructed and the risk assessment was carried out,the purpose of this paper is to provide reference for water environment treatment of estuary.The main conclusions are as follows:(1)The analysis results of the water quality indicators showed that the Total Nitrogen concentration in the estuary exceeded the water quality standard of Class IV of the environmental quality standards for surface water(GB3838-2002),the grade of water quality is poor V,which indicates that nitrogen pollution is serious;the concentration of Chemical Oxygen demand in 2019 was relatively high,and concentration in 2021 met the Class IV water quality standard.Dissolved oxygen,Total Phosphorus,Ammonia Nitrogen and other indicators are low concentration,pollution is relatively better.According to the result of water quality assessment,the Total Nitrogen is the main index,which the contribution rate of pollutants is about 70%.On the whole,the water quality of the estuary has improved,with all indicators showing a positive trend.Among them,the concentration of Total Nitrogen is still seriously above the prescribed level,which should be the focus on.Nitrate is the main form of Nitrogen in the estuary,and the concentration is about 70%.The main source of Nitrate in Shicun is agricultural non-point source pollution,and the concentration of Nitrate in Houxinzhuang was affected by domestic sewage and manure pollution.According to the correlation analysis of water quality factors,the upstream water quality,hydrometeorology,human activities and other factors have different degrees of impact on the water quality into the sea.Based on the analysis of runoff data and pollutant concentration,it is concluded that the transport flux during the wet season is the main contributor of annual flux into the sea,the runoff is the main influencing factor during the wet season and the flux in flat and dry season was affected by the concentration of pollutants and runoff.(2)The MIKE21 hydrodynamic model was used to construct the hydrodynamic model of the estuary,and the simulated results were compared with the measured water level data of the Wangdaozha;Based on the hydrodynamic module,the coupled hydrodynamic-water quality model was established through the Transport Module,the applicability and reliability of the hydrodynamic-water quality coupling model in the estuary were verified by using the measured water quality data of Wangdaodazha and Houxinzhuang,improving the applicability of the hydrodynamic-water quality coupling model in the estuary,The results can reflect the change of water quality truly and accurately.The results showed that when the upstream water quality is Class IV,the Chemical Oxygen Demand and Total Phosphorus can only meet the Class IV water quality standard,while the Total Nitrogen concentrations exceeded the standards Therefore,it is necessary to strengthen the pre-treatment of Total Nitrogen and the treatment of Nitrogen pollution while controlling the quality of upstream water within the scope of Class IV.After entering the study reach,the concentration of high-concentration water mass caused by sudden pollution in the upper reaches of the river increased gradually with the diffusion of the water body,and the degree of influence on the concentration value decreased gradually.The migration velocity of high-concentration water mass in wet season was slightly faster than that in dry season,but the concentration in flat season is higher.According to the simulation results of pollution load reduction,point source pollution load reduction has a greater impact on Chemical Oxygen Demand,and non-point source pollution load reduction has a greater impact on Total Nitrogen,Wangdaozha is more affected by the reduction of non-point source pollution load,and Houxinzhuang is more affected by the reduction of point source pollution load.(3)Taking the Pressure—State—Response model as the frame,considering the upstream water quality,point source pollution,non-point source pollution,hydrometeorology,water quality,social economy and water pollution control,et al,the comprehensive assessment system of water quality risk in the estuary was established by selecting 15 indexes,using the improved analytic hierarchy process and entropy method to calculate the weight,and the integrated risk assessment of the water quality from 2016 to 2021 was conducted.The result show that the water quality risk assessment value from 2016 to 2021 showed an upward trend and then a downward trend,with the risk values from 2016 to 2018 showing an upward trend and reaching a maximum in 2018 and the risk values from 2019 to 2021 gradually decrease,with the minimum value appearing in the 2021,the higher weights of the point source,nonpoint source pollution and water pollution control criteria layers,which have a greater impact on the water quality risk of the estuary.