| Marsh wetlands are widely distributed in the Sanjiang Plain.Eco-environmental safety and health is an important basis for the healthy and sustainable development of wetland ecosystem.Identifying impacts of human activities on wetland ecology and risk factors of environmental decline have become significant components of habitat protection.Since 1950s,Sanjiang Plain has experienced a long period of intensive agricultural reclamation development process,which leads to the shrinking of natural wetland area,resulting in the coexistence of natural wetland and surrounding farmland.Due to the impact of agricultural non-point source pollution,a unique cold area marsh wetland characteristics under the background of agricultural reclamation development has been formed.In this study,taking Qixing River wetland as the target research area,water quality indices of Qixing River wetland under the background of agricultural reclamation development was monitored,exploring law of the water quality evolution,calculating the ecological water requirement of the wetland.According to the current situation of wetland,two kinds of water replenishment methods are put forward,and the water replenishment process is studied.Qixing River wetland is divided into experimental zone,buffer zone and core zone according to its ecological function.The experimental zone,buffer zone,Qixing River channel and surrounding farmland of Qixing River wetland were selected as the research area,and dissolved oxygen,chemical oxygen demand,ammonia nitrogen,total phosphorus,total nitrogen,nitrite and nitrate were selected as the analysis indices.The entropy weight-set pair analysis method is used to evaluate the water quality grade,analyze the spatial change of wetland water quality,and use statistical methods to identify the main controlling factors affecting wetland water quality,so as to clarify the water quality grade of wetland ecosystem in normal period.On this basis,the ecological water requirement of Qixing River wetland was calculated,and two kinds of water replenishment methods,water quantity process and water quality process,were put forward.This study can provide reference for water quality control and water quantity management of wetland.The main research results are as follows:(1)The monitoring results of water quality indices concentration show that,the the water quality indices average concentration of river channel and surrounding farmland were higher than the experimental zone and the buffer zone,and the water quality turned better when transitioning from the experimental zone to the buffer zone.COD,ammonia nitrogen and total phosphorus are the indices exceeding Class III water quality grade concentration.In June 2019,the concentration of each index decreased compared with the same month of the previous year,the river channel pollution was more serious than the other three areas,the overall water quality of the buffer zone was better than that of the experimental zone,and the average concentration of total nitrogen and total phosphorus exceeded the Class III water quality grade.The water quality indices of July 2019showed a certain improvement,which may be caused by the growth of wetland aquatic plants.The concentration of all nitrogen content indices decreased compared with the previous month,while phosphorus did not change significantly,which resulted in the average total phosphorus concentration still above Class III water quality grade..Affected by the heavy rainfall in October,the concentration of water quality indices in Qixing River wetland has been greatly improved,and the differences among regions have also been reduced.Most of the water quality indices reached concentrations between Class I and Class II,except for the sampling points located in farmland.(2)In June 2018,8 of the 15 sampling points were classified as Class III.In each sampling month of 2019,more than half of the sampling points are classified as Class III water quality grade.The sampling points with better water quality grade evaluation mostly appeared in the buffer zone,and the Class V water quality grade gathered in the experimental zone and the surrounding farmland.This performance strongly proves the adverse effect of agricultural non-point source pollution on water quality and the purification effect of natural wetland on water quality.(3)The spatial distribution of June 2018 showed that the high concentration area of dissolved oxygen appeared near the wetland buffer zone,and the pollution indices concentration generally showed a trend of high in the West and low in the East.COD has a high variability,which indicates that it has a high degree of dispersion in the region,and its own unstable nitrogen indices has a similar dispersion performance.A comprehensive analysis of its temporal and spatial evolution characteristics of the water quality indices in the three sampling months of 2019 shows that the high concentration areas of pollution indices mainly appear at the entrance of the wetland.The boundary of the wetland is susceptible to the pollution of the river channels and the surrounding farmland.Irrigation ditches and reservoirs located in low-lying areas are more likely to accumulate pollutants and form high concentration areas.After the heavy rainfall in October,the spatial distribution of the wetland water quality indices was more even,and individual high-concentration points were prominent on the spatial distribution map,but the overall concentration was lower than that in June and July.(4)Based on the discriminant analysis of entropy weight-set pair analysis,it is pointed out that total phosphorus in June 2018 is the main water quality index to control Cass III water quality grade,while COD,ammonia nitrogen and total nitrogen were the key drivers of improving water quality grade and responsible for the Class III grade.COD and total nitrogen fluctuate significantly year-on-year under different water quality grades,which has a profound impact on the determination of water quality grade.PCA/FA showed that among the three principal components in June 2019,ammonia nitrogen and total phosphorus have significant correlation with the first principal component,which are the key indices affecting water quality.The correlation between total nitrogen and the first principal component reached 0.841,which indicated that nitrogen-containing pollutants had the greatest impact on water quality.In October,due to the overall improvement of water quality,agricultural non-point source pollution was reduced,and dissolved oxygen was the biggest contributor to water quality.(5)The minimum ecological water requirement of Qixing Rvier wetland is 310 million m~3,the maximum ecological water requirement is 365 million m~3,and the plant water requirement accounts for 37.2%of the total water requirement,which is the primary water demand index.Based on the requirements of wetland ecological water demand for water quantity,according to the typical agricultural non-point source pollution indices total nitrogen and total phosphorus analyzed in the water quality part as water quality sensitive factors,taking the Class III water quality grade to ensure the stability of wetland ecological environment as the standard,all water requirement months are divided into water quantity shortage and water quality shortage.It can be concluded from the analysis that since April,the temperature rises and the snow melts,which leads to the residual agricultural non-point source pollution in the soil of the previous year.As a result,April is a water quality shortage type month,and the purification of total nitrogen and total phosphorus requires2.74 million m~3 and 3.26 million m~3 of supplementary water respectively.June,July,September and October are the periods of water quantity type shortage,and the replenishment amount in this period can cover the replenishment amount of water shortage of water quality type,so the situation of water quality reaching the standard should be considered on the premise of meeting the water quantity. |
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