Arsenic Pollution Record And The Mechanism Of Migration And Transformation In Lake Yangzong

Arsenic was a persistent pollutant with active chemical properties and strong bioaccumulation which got widely concerned because of its changeable speciation and environmental toxicological effect.Arsenic pollution in lakes was one of the most serious environmental problems.Moreover,the sources of pollutant and the toxic mechanism were the hot spots of ecological research.However,the migration and transformation mechanism of arsenic was still in the initial stage.Meanwhile,the occurrence system and release mechanism of arsenic were controversial,and the influence of geochemical environment and biogeochemical processes remained unclear.As the most important recording carrier of environmental information,lake sediment was the reservoir of pollutants and the primary node of endogenous material cycle.Thus,based on the research of sedimentary,rebuilding the history of pollution and investigating the origin of pollutants were valuable for understanding the migration and transformation of arsenic under the complex environmental background.Lake Yangzong,a typical arsenic polluted lake in China,was selected as the research target in this study.Thirty-six surface and bottom water samples,three water column samples,two sediment short core and thirty-six surface sediment samples were collected for geochemical and microbial molecular analysis including As speciation,Fe,S,ionic species,TOC,granularity and 16sr DNA sequencing information.Based on²¹⁰Pb-¹³⁷Cs dating,a reliable chronology was established,and the temporal and spatial variation of arsenic was analyzed.The material circulation process,the migration and transformation mechanism of arsenic under the different sedimentary stages and environmental background were discussed.The main conclusions were as follows:As a typical deep-water plateau lake,Lake Yangzong was the main water source for tens of thousands of residents in the catchment.The function and ecological environment of the lake was seriously affected arsenic pollution.Moreover,some new environmental problems had emerged after chemical remediation.Now,the ecological and environmental effects of arsenic pollution and emergency measures were also remained unclear,and the mechanism of arsenic migration and endogenous release was complex which was lack of sufficient understanding and scientific support.In order to solve the problems of water safety and ecological environment caused by arsenic pollution in Lake Yangzong.The sediment and water in lake were selected as the research target.The geochemical and biogenic proxy including water quality parameters,grain size,carbonate,organic matter,total arsenic,ions,diagenetic elements,heavy metal elements,arsenite and arsenate were analyzed.Based on the²¹⁰pb-¹³⁷cs dating result,the reliable chronological record was established.The distribution,variation law and influencing factors of arsenic were analyzed and the pollution history was reconstructed.Meanwhile,the causes of secondary pollution and arsenic retention were discussed and the ecological impact of arsenic pollution was also evaluated.Furthermore,the impact of emergency measures for environment on the natural evolution of the lake was discussed and the migration of arsenic at the water interface was proposed.Besides,the biogeochemical processes were also summarized.These results provided a scientific basis and data base for solving the water security in lakes,and improve raised the awareness of scientific mechanism in arsenic migration.The results were as follows1)The spatial distribution of arsenic in Lake Yangzong were clarified.The content of total arsenic in surface water was lower than that in bottom.The content of total arsenic in summer was significantly lower than that in winter.The arsenic concentration in surface layer increased from 28.03μg/L to 48.85μg/L in bottom layer at summer time,and the arsenic reduction rate was 88%.Similarly,the arsenic concentration in surface layer increased from 48.63μg/L to 55.35μg/L in lower layer at winter,and the arsenic reduction rate was only 16%.The spatial distribution of arsenic was mainly affected by lake hydrochemistry and endogenous circulation.2)The spatial variation of arsenic in surface sediments were determined.The content of total arsenic in the surface sediments ranged from 45.69 mg/kg to334.67 mg/kg.The content of total arsenic was abnormally high at South Bank and ranged from 297 mg/kg to 334.67 mg/kg.The average content in the south of the lake was higher than that in the middle and north,which was ranged from 137.41-223.97mg/kg,89.75-113.42 mg/kg and 45.69-77.62 mg/kg,respectively.For every single region,the content of arsenic in surface sediment at lakeside was higher than that at lakebasin.The spatial variation of arsenic content in surface sediment was affected by lacustrine topography and geochemical background.3)The vertical variation of arsenic in sediment record was clarified.The content of total arsenic sediment records had increased and declined with fluctuation.The content of arsenic increased from 35.44 mg/kg to 281.17 mg/kg,and fluctuating decreased to 82.35 mg/kg.The PCA principal component analysis showed that the arsenic deposition process could be divided into four stages,i.e regional-natural stage,local-natural stage,local-anthropogenic stage and unnormal stage.The vertical variation of arsenic was mainly affected by anthropogenic impact and climate change.4)The effect of microbial community on arsenic migration was revealed.There was great response relationship between microorganisms and environmental factors into the sediments of Lake Yangzong.Microorganisms had impact on the migration and transformation of arsenic.There were significant differences in the microbial communities during different sedimentary stage.Proteobacteria was the most dominant bacteria in the sediments with an average relative abundance of 22.2%.The relative abundance of Firmicutes,Acidbacteria,Spirochaetes,Verrucomicrobia,Planctomycetes and Bacteroidetes were ranged at 1.5-4.2%,2.4-4.2%,1.1-4.6%,3.4-8.2%,1.1-7.7%and 2.1-5.6%,respectively.The OUTs number of 16S DNA ranged from 2714 to 10130.The interface at lake basin showed the higher microbial biomass.