Concentrations And Speciation Of Mercury In Soil And Sediments Affected By Bird Droppings In Tongli Wetland,East China

Mercury?Hg?is a kind of nerve poison that exists widely in the atmosphere,water and soil.Hg and its compounds are toxic and volatile,good mobility,it can enter the body by ingesting,skin contacting,inhaling,Hg vapor inhaling and so on,and enrichment,damage the human nervous system,reproductive system and immune system,cause acute or chronic Hg poisoning,especially MeHg.As the"source"and"sink"of Hg,it is of great significance to understand the source of wetland Hg.Wetland as an important part of the global ecosystem,because of its low topography,Hg generated by natural or human sources is easy to enter the wetland system through many channels,such as surface,underground runoff,atmospheric dry and wet deposition.A large number of birds are growing in wetlands,and the amount of waste is huge.Bird droppings as food residues that are not completely digested by animals,together with bioaccumulation and amplification of heavy metals by organisms,result in high contents of heavy metals in the bird droppings,especially for enrichment of Mn,Cd,Cu,Zn and Hg,A large number of bird droppings applied,will significantly aggravate the accumulation of Hg in the local soil and sediment.In this paper,the soil and sediment in the wetland of Tongli wetland were studied to quantitatively reveal the distribution of Hg in different media of the wetland:soil with bird droppings,soil without bird droppings,bird dropping sediments,wetland water and sediment.Hg morphology migration and transformation in soil and river sediments under the influence of bird droppings,and revealed the enrichment and morphological transformation of Hg in local soil and sediment by bird droppings.In this study,51 soil samples and 4 bird droppings samples were collected in Tongli wetland of Suzhou City,Jiangsu Province in winter and summer respectively.In winter,19 sediment samples were collected from river in Tongli wetland and 19 samples of river water were collected.Firstly,total mercury in soils,bird dropping,sediments and water bodies was determined by water bath-cold atomic fluorescence spectrometry.Secondly,the form of mercury was analyzed by an improved Tessier five step continuous extraction method and a dual channel fluorophotometer was used to determine the content of mercury.Finally,the land accumulation index method and the potential ecological risk assessment method are used to assess the mercury pollution and potential ecological risk in the soil and sediment.The main conclusions are as follows:?1?The total pH value of the Tongli Wetland Park in summer was 4.5,and the soil was acidic.The pH of OS is 4.9,while pH in WS is 4.0,of which the pH value of OS is significantly higher than that of WS.The average contents of N,C,H and S in the park soil were 3.69,38.07,10.97,0.52 g·kg^-1,respectively.In winter,the soil pH value of Tongli Wetland Park is 4.9,and it also has the characteristic of acid.The pH of OS was significantly higher than that of WS.The contents of N,C,H and S in the soil in the park were 2.72,24.37,9.93 and 0.69 g·kg^-1,respectively.The pH value in BS also shows acidity,which is 4.7,higher than the pH value of soil in summer,but lower than the pH value of winter soil.In summer,the content of Hg in different soils showed a certain difference.For OS,the average content of Hg was 0.134±0.039 mg·kg^-1,compared with WS(0.169±0.026 mg·kg^-1),the average content of WS was significantly higher than that of OS,which was 26.1%higher than that of OS.In winter,the same phenomenon was observed in different soil types.For OS,the average content of Hg was 0.104±0.032 mg·kg^-1,while that of WS was 0.194±0.155 mg·kg^-1.The average content of Hg in WS was significantly higher than OS?p=0.044<0.05?,higher than 86.5%.The concentration of Hg in bird droppings reached 0.182±0.020 mg·kg^-1,which was higher than that of Hg in OS and WS in summer and higher than that of Hg in OS in winter.?2?In summer,the average contents of F1,F2,F3,F4 and F5 of Hg in OS of Tongli Wetland Park were 0.003±0.000,0.006±0.002,0.022±0.005,0.022±0.003,0.082±0.037 mg·kg^-1,respectively.The average contents of Hg F1,F2,F3,F4 and F5 in the OS of wetland park in winter were 0.015±0.005,0.022±0.016,0.030±0.006,0.034±0.008 and 0.024±0.003 mg·kg^-1.The average contents of F1,F2,F3,F4 and F5 of Hg in bird droppings were 0.019±0.002,0.037±0.008,0.024±0.002,0.021±0.001 and 0.078±0.020 mg·kg^-1,respectively.?3?According to the correlation analysis between soil physico-chemical properties and soil THg,there are significant correlations between THg and N_tot and H??=0.528,0.578,p<0.01?,THg is correlated with C_tot,while there are no statistically significant correlations between THg and pH and Stot.In the summer,the C/N ratio in the soil is concentrated in the range of 7.5-12.5,which is higher than that in winter?within the range of 8.0-10.0?.Because the Ardeid birds belong to birds in summer,during the summer,birds emit large quantities of droppings into the soil,resulting in higher C/N ratio.As time goes on,bird droppings begin to transform organic matter and humus in winter,which lead to the ratio of C/N decreased.The Hg/C ratio in winter is higher than that in summer Hg/C,because the increase of Hg concentration in winter soil is caused by leaching and migration of summer and autumn,and the absorption of organic matter and humus.Over time,the form of Hg in the soil with bird droppings began to transform into Hg in bird droppings,especially the F1 and F2.Among them,in winter,either the soil with bird droppings or the soil without bird droppings reduced the content of the residual state,while the content of Hg in the alkali-soluble state and organic-bound state showed an upward trend.Indicating that bird droppings can aggravate the activity of soil mercury in summer,especially after long-term fresh bird droppings is converted into humus and organic matter,the effective Hg concentration rises significantly.?4?The average content of TOC in river sediment in Tongli Wetland Park was 0.02±0.01g·kg^-1,the average content of N_tot was 1.33±0.33g·kg^-1,the average content of Ctot was11.43±3.58g·kg^-1,the average content of H was 6.88±1.21g·kg^-1,the average content of Stot was1.09±0.32g·kg^-1,the value of pH was 6.7±0.5.Through analysis of Hg concentrations in river water of Tongli wetland,the average concentration of Hg in river water was 0.075±0.071?g·L^-1.The average concentration of Hg in river sediment was 0.182±0.054 mg·kg^-1,of which the concentration of Hg in river sediment with bird droppings was 0.179±0.009 mg·kg^-1,and the concentration of Hg in river sediment without bird droppings was 0.186±0.012 mg·kg^-1.In general,the form of Hg in sediments are in decreasing order of F5>F4>F3>F2>F1,of which the concentration of F5 was 0.054±0.007 mg·kg^-1 and the concentration of F4 was 0.047±0.005mg·kg^-1,the concentration of F3 was 0.043±0.007 mg·kg^-1,the concentration of F2 was 0.027±0.007 mg·kg^-1,and the concentration of F1 was 0.012±0.003 mg·kg^-1.The five morphological variation coefficients of Hg in the sediments showed moderate intensity variation.?5?It will increase N,C,and TOC levels in local soils for entering of bird droppings.Over time,and with microbial activity,nutrient elements convert to organic matter and humus,which would increase the adsorption of Hg in water and soil,and increase the concentration of Hg.The introduction of bird droppings affects the morphology of Hg in river sediment due to the lower pH value in bird droppings and higher nutrient elements such as TOC,N and H,resulting in increased F1 and F2 concentration in the sediments of region with bird droppings.?6?The pollution status of Hg in soil of Tongli wetland were analyzed by the method of geoaccumulation index.In summer,the I_geo of wetland is less than 1 and there was no pollution.The I_geo of both WS and OS were less than 1,and they also showed no pollution.In winter,the I_geoeo of wetland is less than 1 and there was no pollution.Comparing WS and OS,the I_geo of WS were significantly higher than those of OS.There were three samples in WS and the I_geo values were0.842,1.039 and 1.098 respectively,which reached the slight pollution and moderate pollution.On the whole,the I_geo of the sediments was less than 1,and there was no obvious pollution.Except that the potential ecological risk index of Hg in soil with bird droppings was less than 30 in winter,the average index of other soils and sediments was more than 30 and less than 60,which is medium risk.However,there were some samples of region with bird droppings over 120 in summer,which needed attention.