Physical And Chemical Characteristics Of The Water Environment In Downstream Wetlands Of Lhasa River And Pollution Risk Assessment

Wetland means an area where the surface is too wet or water often accumulates,grow wetland biological species,wetland ecological protection is an important task in our country,water is the soul of the wetland,the change of wetland water quality affects the wetland ecological process and water ecosystem directly,and the water quality state of the wetland has a very important influence on its ecological function.The unique high altitude,hypoxia,cold and dry natural conditions of the Qinghai-Tibet Plateau wetland make the plateau wetland ecosystem very fragile.The structure and function of the wetland are relatively simple,the environmental capacity is very limited,and the ability to adjust and recover from external interference is poor.Under the influence of natural factors and human activities,plateau wetlands begin to suffer from wetland degradation,shrinking,species diversity change and other problems,which affect local water quality security,agricultural and animal husbandry conditions and sustainable development ability.Lhasa River Basin wetland is a typical representative of Qinghai-Tibet Plateau wetland,and plays an important role in urbanization development,agriculture and animal husbandry development and tourism development in the basin,especially in the downstream basin.In order to understand the water environment index distribution characteristics and nutrient status of the lower Lhasa River wetland in each water period,and to provide reference for the protection and utilization of plateau wetland,this paper collected 118 sample sites from 5 representative wetlands in the Lower Lhasa River basin in 2021,which are three water periods:dry season（May）,wet season（September）and normal season（December）.The water environment indexes such as p H（p H）,water temperature（T℃）,electrical conductivity（EC）,salinity（Salt）,total dissolved solids（TDS）,turbidity（TD）,chemical oxygen demand（COD）,total nitrogen（TN）,total phosphorus（TP）,ammonia nitrogen（NH₃-N）in the sampled water were detected.Using mathematical statistics analysis,WQI model water quality evaluation,pollution index method,principal component analysis,logarithmic power function universal eutrophication index evaluation method,water purification capacity evaluation method and other methods to analyze the spatial and temporal distribution characteristics and nutritional status of water quality indexes in the lower reaches of Lhasa River Basin.The results show that:（1）The water temperature in the lower reaches of Lhasa River Basin ranged from 12.71℃to 23.22℃in dry season,and from 13.78℃to23.68℃in wet season.In normal water period,the water temperature ranges from 5.36℃to 15.98℃,and the p H value of water body ranges from 7.47 to 8.59.The water sample of wetland is generally weakly alkaline.The water sample of wetland is relatively clear,but the salinity is high,the salinity is high under the influence of Lhasa base value,and the water quality indexes such as ammonia nitrogen are relatively good.Chemical oxygen demand,total phosphorus,total nitrogen concentration is higher.In terms of time distribution,most of the water quality indexes showed obvious seasonal changes,while in terms of spatial distribution,there was little difference between different seasons of lake water quality,and there was a significant correlation between most indexes.（2）By evaluating the water quality index of wetlands in the lower reaches of the Lhasa River basin,the WQI water quality index is generally at the"good"level,and the WQI index is the highest in wet season.The WQI water quality index of the wetlands with sufficient water and the wetlands with broad area and developed river system is better.The average value of the comprehensive pollution index of each monitoring site is 0.16--0.35,with an average value of 0.26,indicating clean water quality.The pollution index of COD value and TN value fluctuates widely,P value is greater than 0.40,and the pollution sharing ratio is more than 70%,which is one of the main pollution factors of wetland water.The results of principal component method showed that the wetland water body in the study area was generally in line with the levelⅡstandard of surface water environmental quality,but there were some differences in the results of each component,indicating that the water period had a certain effect on the water quality index of each wetland,and the spatial heterogeneity of each index of water environment was high.It was found that the principal component scores of Zhaxigang wetland and Baga Snow wetland were better,while the scores of Lalu wetland and Chabalang wetland were worse,which might be related to the surrounding land use.（3）The average EI value of each sampling point of the five typical wetlands in the lower reaches of Lhasa River in each water period in 2021ranged from 22.41 to 64.24,with an average value of 38.40,indicating moderate nutritive state on the whole and normal water nutrition degree.EI values showed significant time differentiation,and water stage had a great impact on wetland eutrophication.It was found that the spatial distribution of eutrophication was combined with zonal differentiation and spot differentiation,indicating that transportation,farming and animal husbandry activities and residents’life had a certain impact on wetland water quality.（4）All wetlands have the effect of purifying water quality.Zhaxigang Wetland has the best removal effect on COD and NH3-N,Jiama wetland has the highest removal effect on TP,and Chabalang Wetland has the best removal efficiency on TN.The removal effects of pollutants in different water periods were as follows:dry period>normal period>wet period.The area,water amount and vegetation growth of wetland had the greatest influence on the purification capacity of wetland.