| Arsenic as a toxic metalloid element is widely distributed in various environmental media,which poses a huge threat to human health and ecological environment.High arsenic groundwater in this area is related to the corresponding rivers,and most of the rivers originate from the Tibetan Plateau in China.Among them,heavy metal pollution caused by unreasonable mining and construction of cascade hydropower stations in Lancang-Mekong River has attracted the attention of downstream countries.At present,most of the studies focus on the geochemical behavior and influence of arsenic in water bodies of Southeast Asia and India,while the research on the migration and transformation of arsenic in the source area of the Tibet Plateau is relatively limited.In this study,Angqu as the largest tributary of Lancang River was investigated.The main work is to collect samples of water and sediment in the basin of Angqu,Endaqu,Mangdaqu and high arsenic spring.Based on the field investigation,indoor sample analysis and simulation experiment,the distribution,transportation and transformation of arsenic in the study site are investigated.The keyfindings are:(1)Under the impact of particular geological structure of the Tibet Plateau,the average arsenic concentration in Endaqu and Mangdaqu water samples(dry season 83.7?g/L,wet season 45.6?g/L)and spring samples(dry season 679.2?g/L,wet season 250.5?g/L)are much higher than normal As value.Due to the large runoff and the adsorption of arsenic by sediment,the arsenic content in the main stream of Angqu is at a low level,14.1 ?g/L in the dry season and 13.8 ?g/L in the wet season,but it was still higher than 10 ?g/L(the arsenic concentration limited standard for drinking water in(GB5749-2006).(2)The arsenic in the tributary is mainly exists in dissolved arsenic form,while in the main stream is mainly exists in dissolved arsenic form in the dry season and exists in insoluble arsenic form in wet season.During the wet season,arsenic and Fe/Mn oxide particles combine to form suspended particulate arsenic and migrates with the water flow,resulting in enhanced arsenic transportation ability.Dissolved arsenic mainly exists in As(V)form,the ratio of As(V)in dry season and wet season was 81.7% and 82% respectively.(3)River sediments of study area are able to adsorb large amounts of arsenic and thus fix the As from hot springs on sediements.The sediment in the Endaqu,Mandaqu and hot spring are abnormally enriched with arsenic.The average arsenic content of sediments in Endaqu(39.2mg/kg,n= 9),Mandaqu(38.5mg/kg,n=17)and hot spring(486.7mg/kg,n=2)is much higher than that in the main stream of Endaqu(12.7mg/kg,n=11).The distribution of arsenic in the sediment and the corresponding water is consistent,and there is a significant positive correlation.Arsenic in the sediment samples mainly exists in residual,the average percentage was 70.4%,while the highly mobile arsenic(exchangeable,bound to Carbonates,and bound to iron and manganese oxides)only 12.1%,so the mobility of arsenic in the sediment is weak.(4)The simulation experiment indicates that the sediment has a strong adsorption on arsenic,which is positively related to the content of iron and manganese and pH.The desorption capacity is related to pH,dissolved oxygen and sediment particle size,and the water environment of Angqu basin is not conducive to the desorption of arsenic,so the release level of arsenic from sediment is relatively limited. |
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