Migration And Transformation Characteristics Of Mercury In The Estuary Of Ruxi River,a Typical Tributary Of The Three Gorges Reservoir

Heavy metals,such as Hg,Pb,Cd,Cr and metal-like As,are non-essential elements for life activities.They are easy to accumulate in the natural environment,it is a kind of pollutant that is more harmful to the life activities of organisms and human beings.The Three Gorges Reservoir is currently the largest water conservancy project in the world.Its completion and operation have changed the original hydrological and hydrodynamic characteristics of the main stream of the reservoir area.Under the influence of the backwater of the main stream,the large and small tributaries of the reservoir area form the backwater areas of the reservoir bay,making the tributaries an important area for the deposition of pollutants such as sediment and heavy metals.Therefore,it is extremely important to explore the distribution of heavy metals and their ecological risks in the sediments of tributaries in the reservoir area.On the other hand,Hg in heavy metals,especially its organic product--methylmercury(Me Hg),has extremely high food chain enrichment and biological toxicity,and the aquatic environment,especially the estuary system,is a conducive to mercury methylation.System,the methylation/demethylation of mercury in the tributary estuary of the reservoir area also deserves special attention.Therefore,our study selected the Ruxi River,a typical tributary of the Three Gorges Reservoir area,as the research object,and systematically investigated 8 heavy metals(Cd,Zn,Pb,Mn,Cu,Hg,Cr)in the water body and sediments of Ruxi River.And Ni,analyzed its sources and ecological risks;at the same time,considering the particularity of the biogeochemical properties of the Hg environment,in spring(recession period),summer(falling dry period),autumn(storage period),in the winter(submerged period),the distribution characteristics of various forms of mercury in the estuary water and sediments were investigated,the migration and transformation process of mercury and its main influencing factors were investigated,and the bistable isotope(¹⁹⁹Hg Cl₂ and Me²⁰¹Hg)The tracer method explored the main influencing factors of the methylation/demethylation of mercury in the sediments of the Ruxi River estuary.The main findings are as follows:(1)Except for Ni,the contents of the eight heavy metals in the surface waters of Ruxi River are all lower than the standard limit for surface water environmental quality class I.The contents of the eight heavy metals in the sediments all exceed the background value of the sediments in the Yangtze River system.The overall spatial performance is the section affected by the life of Ruxichang Town>backwater area>natural river section,and the enrichment degree is:Cd>Hg>Zn>Ni>Cr>Cu>Mn>Pb,and the content of heavy metals in the sediments and soils along the river in the backwater area is much lower than that in the sediments,indicating that the sediments and soils along the river are not the heavy metals in the sediments in the backwater area of the Ruxi River The main source of pollution.In terms of vertical distribution,the Hg pollution in the river section affected by the life of Ruxichang Town is heavier,which is6.1 times the background value,and the estuary is 2.14 times the background value.At the same time,the distribution coefficients of Hg and Cd in the water-sediment are relatively small,which has a strong potential for secondary release.The evaluation of the geo-accumulation index shows that the Ruxi River as a whole presents mild to moderate pollution of Cd,Zn and Hg.The ecological risk assessment shows that the prevention of Cd and Hg pollution is emphasized.The biological toxicity effect evaluation shows that Ni is 10%?75%May cause toxic effects to organisms,and Cd,Zn,Hg,Cu,Pb,Cr may have biological toxic effects less than 10%.The comprehensive effect coefficient indicates that the biological toxicity risk of heavy metals in the sediments of Ruxi River is low to medium-low.(2)The Hg in the water at the mouth of the Ruxi River is significantly lower than the natural unpolluted fresh water and the USEPA limit for preventing adverse chronic nutrition to aquatic organisms.In season,the concentration of DHg,PHg,RHg and THg is the highest in spring.Both TMe Hg and DMe Hg are the highest in summer.On the vertical profile,the concentration of THg gradually decreases with the increase of water depth in spring and autumn.In summer,it first decreases and then increases.In winter,there is no obvious concentration gradient change.The concentration of TMe Hg in summer,spring and autumn showed an increasing trend with the increase of water depth.The concentration of TMe Hg in winter did not show a significant gradient difference like THg.(3)The concentration of THg in the sediments of the Ruxi River estuary is within the range of global soil mercury.There was no significant difference in the THg concentration in each sample(ANOVA test,p>0.05).At the same time,there was no significant difference in the four seasons(ANOVA test,p>0.05),and the winter was slightly higher than other seasons.In the vertical section,the THg concentration showed that except for the sample point directly intersecting with the main stream at 8 cm,the maximum value of the other sample points appeared in the surface or subsurface layer,and showed a trend of decreasing with increasing depth in the vertical direction.The concentration of TMe Hg in the sediments of the estuary is close to the concentration of mercury in bare soil in the fluctuation zone of the Three Gorges Reservoir.The TMe Hg concentration at the S7 sample site,which is affected by both the upstream inflow and the main stream water backflow,was significantly higher than the other sample sites(ANOVA test,p<0.05).The seasonal performance is summer>autumn>winter>spring,and there are significant differences between the four seasons(ANOVA test,p<0.05),among which the S7 sample TMe Hg has obvious seasonal differences.The concentration of TMe Hg in the vertical section basically shows a trend of decreasing with the increase of depth.Except for the maximum TMe Hg concentration at the sample point which directly intersecting with the main stream,the maximum value of the TMe Hg concentration appears on the surface of the sediment.(4)There is no obvious seasonal distribution of THg in the sediments of the Ruxi River Estuary,but Me Hg presents an obvious seasonal distribution:autumn is higher than summer,higher than winter,and finally higher than spring,THg,Me Hg and bioavailable mercury are basically in the vertical profile.Shows a tendency to decrease with increasing depth.Both DHg and DMe Hg in the pore water of the sediment generally decrease with the increase in depth in the vertical profile.The maximum values of DHg and DMe Hg in the season appear in the surface layer of 0?2cm,and are much higher than the concentration of DHg and DMe Hg in the overlying water.,The seasonal distribution trend is autumn is higher than summer,higher than spring,and finally higher than winter.The distribution coefficient of inorganic mercury at the solid/liquid interface of the sediments in the Ruxi River estuary is from high to low in order of winter is higher than spring,higher than summer,and finally higher than autumn.The distribution coefficient of methylmercury at the solid/liquid interface of the sediments in the Ruxi River estuary from high to low is in the order of winter is higher than autumn,higher than summer,and finally higher than spring.And the methylmercury in the pore water of the sediment was significantly correlated with the methylmercury in the sediment(r=0.737,p<0.001,n=23).The diffusion flux of DHg and DMe Hg in the pore water of estuary sediments has seasonal changes.The diffusion flux of DHg is highest in autumn and lowest in winter.The diffusion flux of methylmercury increases with the increase of DMe Hg concentration in pore water.The diffusion flux of methylmercury is the largest in summer and the smallest in winter.(5)The methylmercury content in the sediment pore water is proportional to the bioavailability of mercury(r=0.21,p<0.05,n=23),and there is a positive correlation with the corresponding SO₄^2-(r=0.23,p<0.05,n=23),and was significantly negatively correlated with AVS(r=-0.35,p<0.01,n=23).The maximum values of sulfate ion content and Me Hg content in the pore water of sediments are located on the surface,and the content Decrease as the depth increases.At the same time,the methylmercury content in the pore water has a significant positive correlation with the content of active iron(r=0.16,p<0.01,n=23),and a significant negative correlation with the Fe(?)content(r=-0.38,p<0.01,n=23),indicating that the biogeochemical cycles of iron and sulfur in the sediment ecosystem jointly affect the methylation process of inorganic mercury under the action of microorganisms,but the specific internal influence mechanism needs further study.(6)The methylation rate and demethylation rate of the sediments at the mouth of the Ruxi River were significantly higher at 30°C than at 12°C,which is consistent with the results of the field survey.The methylmercury content in summer and autumn is significantly higher than that in winter and spring.Consistent.At the same time,after the general fungicide chloramphenicol,methanogen inhibitor ESA and Na₂Mo O₄ were added to the culture sediments in the Ruxi River Estuary,the sediments all showed a significant decrease in demethylation rate.After adding the broad-spectrum bacteriostatic agent chloramphenicol to the sediment,the methylation rate in the sediment also showed a significant decrease.After adding ESA,HFO,Na NO₃,C₆H₁₂O₆and Na₂SO₄,the methylation rate in the sediments all increased to a certain extent,and the addition of Na₂SO₄ increased the methylation rate constant by about 2 times.After adding Na₂Mo O₄ and HFO,the methylation rate did not increase but decreased by about55%.After adding Na₂SO₄ and HFO,the methylation rate increased by nearly 3 times,indicating that SRB and Fe RM have active treatments.The highest rate of methylation.Comprehensive experimental results show that the key microorganisms controlling the methylation process in the cultured sediments of the Ruxi River Estuary are sulfate-reducing bacteria.