Investigation On Pollution Of Alkylmercury In Urban Transit Rivers

Alkylmercury(methylmercury,ethylmercury),as one of the forms of organic mercury,is more prominent in physiological toxicity,bioaccumulation and environmental content than other organic mercury,and triggers mercury pollution incidents such as Minamata disease in Japan.It is the most widely observed form of organic mercury at home and abroad.Studies have confirmed that natural environmental waters that are greatly affected by human activities are conducive to mercury activation,methylation and bioaccumulation.Early studies on alkylmercury in China have focused on the distribution of higher levels of methylmercury in soil and reservoir sediments and the mechanism of mercury methylation process.The study of trace or ultra-trace level alkylmercury in natural environment water body is lack of detection methods,and there is lack of content background and pollution evaluation methods for reference.In recent years,the detection methods of alkylmercury have been matured,and the new standard for analysis of alkylmercury,"Determination of water-based alkylmercury,purge and trap/gas chromatography cold atomic fluorescence spectrometry"(HJ977-2018)was published on March 1,2019.It provides technical guarantee for study the level and distribution characteristics of alkylmercury in natural waters such as lakes and rivers.Therefore,this study selects the urban transit river Nanfei River as the research object,based on the literature research and method suitability verification experiments,by deploying monitoring points and collecting overlying water and sediment samples in the main river basin.The conventional physical and chemical indicators of water samples and the concentrations of total methylmercury(TMe Hg)and dissolved methylmercury(DMe Hg)were analyzed.The basic properties of the sediment samples and the content of methyl mercury(Me Hg)were determined.Based on the measured data,the overall content and distribution characteristics of alkylmercury in Nanfei River are reflected.Based on this,the relevant factors affecting the distribution of methylmercury are discussed,and the existing evaluation methods are used to pollute the alkylmercury in the basin.In order to provide data support and methodological reference for water quality investigation and pollution control of Nanfei River.The main conclusions are as follows:(1)The average concentration of total methylmercury in the overlying water of the Nanfei River is 0.122±0.074 ng/L,which is lower than that of other natural waters in China;ethylmercury is not detected.The average concentration of dissolved methylmercury is 0.076±0.054 ng/L,which is the main source of total methylmercury in the overlying water,and has a significant correlation with the total methylmercury concentration.The concentration levels of total methylmercury,dissolved methylmercury and particulate methylmercury in different monitoring sections are quite different,and the spatial representation is:midstream section>downstream section>upstream section.The total methylmercury concentration level in time showed:wet season>temperate season>dry season,peaked in August,and there were obvious seasonal changes.The change of water temperature affected the distribution of methylmercury in different seasons to some extent;The p H and dissolved oxygen have significant negative correlation with the concentration of methylmercury,indicating that methylmercury is more easily produced in acidic or low dissolved oxygen water;the distribution of nitrogen and phosphorus in water is not significantly correlated with methylmercury.(2)The methylmercury in the surface sediments of the Nanfei River ranges from0.28 to 1.61 ng/g,with an average of 0.97±0.43 ng/g.The methylmercury content is at a medium level compared with other domestic water sediments or soils.Ethyl mercury is only detected in small amounts in some sections.The concentration of dissolved methylmercury in the sediment gap water ranged from 0.71 to 3.10 ng/L.The concentration of dissolved methylmercury in the wet season and the temperate season was higher than that in the dry season,showing a seasonal distribution similar to that of the overlying water,but the difference of the concentration is smaller than that of the overlying water.The average solid-liquid partition coefficient of methylmercury in dry season is 7.5×10~2L/kg,which is higher than the wet season and the temperate season,reflecting the higher concentration of methylmercury in the solid phase and the migration ability to the interstitial water is weak.The correlation between sediment p H,organic matter and methylmercury content was not significant.(3)Using the single factor pollution index method to evaluate the degree of contamination of alkylmercury in the overlying water and sediments of the Nanfei River,it can be seen that the methylmercury in the overlying water meets the pollution index requirement of Class IV water in the recommended standard for methylmercury;ethylmercury was not detected at all times,so the water was not contaminated with ethylmercury.The pollution index of methylmercury in sediments in Luzhou Luqiao section during the flood season is less than 5,which is mildly polluted.The pollution index of the remaining sections in all periods was between 5and 50,and the average pollution index was 19.4,which was moderately polluted.Using the risk entropy method to estimate the risk of methylmercury in the Nanfei River,it is known that some sections of the Nanfei River have lower ecological risks,and the other sections have no significant ecological risks.