Distribution Characteristics And Ecological Risk Assessment Of Heavy Metals In Wetland Soils In The Lower Reaches Of Lhasa River Basin

As a representative of the alpine wetland on the Qinghai-Tibet Plateau,the Lhasa River Basin is in a special position in the sensitive area of global change and the typical vulnerable area of the ecosystem,and plays a pivotal role in the regional ecosystem.With the aggravation of the impact of climate change and human activities,wetland degradation is becoming increasingly serious and the vulnerability of wetlands is a significant problem,in order to protect the wetland ecosystem,the study on the distribution characteristics and ecological risk assessment of heavy metals in the wetland soil of the lower reaches of the Lhasa River Basin can know the soil environmental quality of wetlands in the lower reaches of Lhasa River Basin,and provide scientific basis for wetland management and heavy metal control in the lower reaches of Lhasa River Basin.Sixtynine soil samples were collected from four representative wetlands in the lower reaches of the Lhasa River Basin（Ganqu,Bagaxue,Lalu,Chabalang）,and their physical and chemical properties and heavy metal content were determined.The distribution characteristics,ecological risk and sources of heavy metals in wetland soils of the lower reaches of Lhasa River basin were studied using Nemero pollution index method,geoaccumulation index method,potential ecological risk index method,mathematical statistical analysis and multivariate statistical analysis.The results showed that:（1）The wetland soils in the lower reaches of Lhasa River Basin is mainly neutral,and the average p H values of the soils were as follows:Chabalang（6.85）（29）Ganqu（6.73）（29）Lalu（6.50）（29）Bagaxue（6.39）.The differences of conductivity,total salt and cation exchange capacity at various points are large,because there are high outliers.The REDOX potential showed that the four wetlands were dominated by strong reductive environment.The soil nutrient indexes（organic matter,total nitrogen,total phosphorus and total potassium）were basically between the first and third grades,indicating that the wetland soils in the lower reaches of Lhasa River Basin were fertile.（2）Except for the non-normal distribution of Zn,Pb and Cd in the soil of Lalu wetland with large coefficient of variation（86.49%,92.38% and136.23%,respectively）,the 8 elements in the soil of Ganqu wetland,Bagaxue wetland,Chabalang wetland and Cu,Cr,Ni,As,Hg in Lalu wetland showed normal distribution with small coefficient of variation,indicating that the distribution of elements in the lower reaches of The Lhasa River Basin wetland soil was relatively uniform.Compared with the Soil Environmental Quality Standard-Soil Risk Control Standard for Agricultural Land（GB15618-2018 6.5≤ p H ≤ 7.5）issued by the Ministry of Ecology and Environment of China in 2018,the average contents of the eight elements did not exceed the screening value of standard,but the concentration of Cu,Zn,Pb,Cd and As exceeded the screening value at 1,1,1,2 and 7 samples,respectively.Sample points of Bagaxue wetland and Lalu wetland.（3）Based on the evaluation of single factor index method and Nemero comprehensive pollution index,the heavy metal pollution status of the wetlands in the lower reaches of the Lhasa River Basin is as follows: the comprehensive pollution degree of heavy metal in the Ganqu wetland is slight pollution,the Bagaxue wetland is moderate pollution,the Lalu wetland is severe pollution,and the Chabalang wetland is slight pollution;geo-accumulation index method in the evaluation results show that the heavy metals in Ganqu,Chabalang and Lalu wetland soil for most of the sample point is unpolluted state,Cr,Ni,Zn,Pb,As and Hg in Bagaxue wetland soil were mainly presenting the state pollution-free,Cu and Cd were non-pollution to moderate pollution;the evaluation results of the potential ecological risk index method showed that the comprehensive potential ecological risks by 8 heavy metals exposed in the soil were all low,and Cd contributed the highest risk in the four wetlands,it was the main risk factor.（4）According to correlation analysis and principal component analysis,heavy metal content in wetland soils of the lower reaches of Lhasa River Basin is affected by the source and soil physical and chemical properties.Bagaxue wetland and Chabalang wetland distribution along the road to the traffic,the Lalu wetland located in city,surrounded by traffic road,traffic transportation source for the three important sources of heavy metals in wetland soil,the farmland within and surrounding Chabalang wetlands and Bagaxue also make the agricultural activities become the important source of heavy metal,atmospheric deposition after longdistance transport is also an important source of heavy metals in the soil of the remote Tibetan Plateau.Ganqu wetland because of the close distance to Leqingla lead-zinc deposit located in Gangdese metallogenic belt,caused the high content of heavy metal elements in soil.In addition,the correlation analysis showed that the heavy metals in each wetland soil were highly correlated with different parameters of soil physical and chemical properties,indicating that the heavy metals entering wetland soil were also greatly affected by the unique physical and chemical properties of wetland soil.