Assessment Of Surface Water Environmental Quality And Analysis Of Main Pollutant Sources In Ali Area

As the roof of the world,fragile ecological environment and unique biological resources of Tibet show that it has a high ecological value.The surface water quality in Ali area occupies an important position in the ecological research of the whole Tibet and even the whole country,because Ali is one of the ecological barriers in western China,but the research is still lacking.This study focused on the water quality of 22 main sections of surface water in Ali area of Tibet from 2016 to 2020.Twenty-four water quality indicators were measured.On the basis of the above data,the single factor method,the comprehensive pollution index method and the Nemero index method were used to describe the water quality of the above sections,and the similarities and differences of the three types of description methods were compared.The spatial interpolation of monthly pollution index in 2020was carried out by inverse distance weighting method and spline function method with obstacle,and the spatial distribution of water quality and its change in one year were explored.Aiming at the main pollutants of arsenic（As）and fluoride(F_com)in Ali surface water,the monthly changes of spatial distribution in 2020 were described by spatial interpolation.The main anions and cations of water in five sections of Lake Manasarovar,Lake Raksas Tal,Lake Zhari Namco and the upstream and the downstream of Geji County section of Shiquan River were determined.The water quality types and main controlling factors were determined according to above data.Then the sources of As and F_com and the reasons of over standard were analyzed.The results are as follows:（1）The single factor method showed that the surface water quality of all sampling sections in Ali area was excellent in 2016.Later,water pollution occurred in five sections of the upper and lower reaches of Lake Manasarovar,Lake Raksas Tal,Lake Zhari Namco and the upstream and the downstream of Geji section of Shiquan River.Specifically,Lake Manasarovar and Lake Raksas Tal mutated into inferior V water bodies in 2017;Geji County section of Shiquan River also gradually turned into Class IV from 2019,and Lake Zhari Namco turned into inferior Class V;as of 2020,the surface water quality in Ali area has not improved.The comprehensive pollution index method shows that only Lake Raksas Tal was in a mild pollution state in 2020,and other sections were not polluted.The Nemerow index shows that in 2020,the Geji County section of the Shiquan River and Lake Zhari Namco were in a moderate pollution state as a whole.Lake Manasarovar was in a moderate pollution state in the first half year and in a heavy pollution state in the second half.Lake Raksas Tal was in a heavy pollution state throughout the year,and the pollution status was heavy.Among the three methods,the Nemero index method could describe the pollution status of Ali surface water more objectively and comprehensively.（2）In 2020,the spatial difference of surface water quality in Ali area was relatively large,and the pollution mainly appeared in five sections in the above sections.Compared with local polynomial method and ordinary Kriging method,the results of inverse distance weighting method and spline function with obstacles method were more continuous and could better predict the spatial distribution of point pollution and areal pollution.The pollution of Shiquan River in Geji County might be related to the decrease of flow velocity and the increase of temperature.For the three lakes of Manasarovar,Raksas Tal and Zhari Namco,water pollution might be related to the characteristics of internal water system and strong evaporation.（3）The main over-standard factors of surface water in Ali area were As and F_com.The heavy pollution of As was mainly near the Lake Manasarovar and Lake Raksas Tal in the south,and the pollution was light only in May.In November and December,the pollution in the area near Lake Zhari Namco in the southeast gradually increased.The heavy pollution of F_com only occurred near Lake Raksas Tal,and the pollution was the heaviest in the first quarter.The areas of high fluorine and high arsenic were basically the same,and mainly concentrated near Lake Raksas Tal.（4）According to the analysis of the water body types of the five sections with excessive standards,Lake Raksas Tal belongs to Na（K）-Cl type,Lake Manasarovar and Zhari Namco belong to Na-HCO₃ type,while the upper and lower reaches of Shiquan River in Geji County belong to Ca-HCO₃ type.The cations in the three lakes are mainly Na⁺,and the accumulation is mainly due to evaporation and concentration.The cations dominated by Ca²⁺in river water mainly come from the weathering of rocks in the basin.The Cl^-in Lake Raksas Tal is controlled by evaporation and concentration.The HCO3^-in Shiquan River,Lake Manasarovar and Lake Zhari Namco are mainly derived from rock weathering.As the main over-standard pollutants of Ali surface water,As and F_comcome from the recharge of high arsenic and high fluorine geothermal water and the weathering of arsenic and fluorine-containing minerals.The main reasons for the accumulation and high pollution in water are the desorption and release of arsenate and fluoride ions on suspended solids and sediments caused by alkaline reduction conditions,the mineral solubilization caused by co-ion effect and the evaporation and concentration determined by local climate.