Environmental Geochemical Behaviors Of Trace Elements In Soils Of The Northeastern Qinghai-tibet Plateau

Trace elements have attracted global attention due to wide sources,various forms and complex migration and transformation mechanisams.Compared with the other areas of the Qinghai-Tibet Plateau,the northeastern region is affected by relatively more entensive anthropogenic factors.However,information on environmental geochemical behaviors of trace elements in soil of this region is relatively scarce.This study collected 127 soil samples and 127 plant samples in the study area.This study researched the distribution,sources,and speciation of target trace elements in soils of the study area as well as the effects of different climatic conditions on soil physio-chemical properties and environmental geochemical behaviors of trace elements in soils of the study area.The conclusions of the study were shown as the followings:?1?The soil in the study area was mainly alkaline loam.The average concentrations of target heavy metals in soil samples of the study area ranged from0.2?Hg?to 500.5?Cr?mg/kg.The average concentrations of Pb,Cd,Cr,and Hg reached 7.2,9.1,6.9,and 8.0 times compared with background values,respectively.Concentrations of total rare earth elements?REEs?ranged from 55.3 to 306.8 mg/kg with average value of 178.6 mg/kg.REEs were greatly affected by the soil composition.Average geo-accumulation index values ranged from-0.9?Tm?to 3.1?Pb?while average enrichment factor values of heavy metals in topsoils ranged from1.0?Co?to 8.4?Hg?,exhibiting significant spatial variation.Several methods were used to evaluate the soil pollution by trace elements.Values of modified contamination degree ranged from 0.7 to 32.3 with the average of 2.2.Average modified contamination degree values of trace elements in different areas ranged from1.0 in background area to 7.0 in mining area.Trace elements including Cr,Hg,Zn,Pb and Cd were the main pollution contributors for sites with high or more serious pollution levels.The pollution load index values in all sampling sites ranged from 0.8to 2.3 and pollution in industry and mining area was more serious than that in the remaining areas based on pollution load index evaluation.Nemerow pollution index values of trace elements in soils ranged from 1.1 to 378.3 to show that background area underwent low to moderate pollution while the remaining areas mainly exhibited moderate to high pollution.Potential ecological risk index values of trace elements in soil ranged from 75.3 to 14 253.2 while Hg and Cr served as the dominant ecological risk contributors.The average potential ecological risk index values of different areas varied from 113.6 in background area to 3166.6 in mining area while industrial and mining areas exhibited very high ecological risks.The average concentrations of trace elements in wild plants growing in different areas ranged from 0.0?Hg?to 412.9?Cr?mg/kg.Average concentrations of Co,Mo,Cr,Ni,and Cu in plants from industrial area were higher than those from the remaining areas.Average concentrations of Cd,Zn,and Pb in plants from mining area were higher than those from the remaining areas.The bioaccumulation factor values of heavy metals ranged from 0.0 to 17.9.The average bioaccumulation factor values of Pb,Co,Sb and V were all under 0.3 in different areas while those of Mo,Cd,Cu,Zn,Sn,Hg and Ni in different areas ranged from 0.5 to 1.6.The average concentrations of REEs in plants followed the order of Tm<Lu<Tb<Ho<Eu<Yb<Er<Dy<Gd<Sm<Pr<Sc<Y<Nd<La<Ce.The bioaccumulation factor values of all REEs ranged from 0.0 to 0.3,showing that it was relatively difficult for REEs to transfer between soils and plants.?2?The residual fraction percentage of V,Cr,Cu,Sn,Sb and Hg in soils exceeded50%on average,suggesting that it was not easy for these metals to become mobile in soils of the study area.V might exhibit some bioavailability and toxicity while Hg might pose some potential ecological risk.The non-residual fractions were the main components for Co,Ni,Zn,Mo,Cd and Pb.Heavy metals including Co and Ni in soils of the study area were generally sensitive to environmental changes and could readily migrate and transform.Mo and Cd had potential ecological risk to environment.Pb had high potential ecological risk under reducing conditions.Pd,Mo,Co,Hg,Zn,and V posed high ecological risks in 23.5%,29.4%,17.6%,5.9%,5.9%,and 5.9%of sampling sites,respectively.The residual fractions were the main components for REEs which showed relatively low bioavailability,toxicity,and potential ecological risk.Results of different approaches including Pearson correlation analysis,Principal Component Analysis and Cluster Analysis Pb showed that significant positive relationship existed among Cu,Zn,Cd and Sn,suggesting these trace elements might originate from the same or similar source.The results of positive matrix factorization showed that the average contribution rate of trace elements from natural sources and anthropogenic sources were 66.3%and 33.7%,respectively.The²⁰⁶Pb/²⁰⁷Pb values of study area ranged from 1.166 to 1.224.Binary model results showed that less than 53.9%of Pb in soils of the study area was contributed by anthropogenic activities.The average anthropogenic contribution proportion for Pb was 7.6%in the agricultural and pastoral area,23.3%in the industrial area,27.6%in the mining area,and 13.4%for the salt-lake area.Anthropogenic contributions in the industrial and mining area were higher in comparison with the other areas.?3?The effects of different environmental factors on the environmental geochemical behaviors of Pb and Cr in soils of the northeastern Qinghai-Tibet Plateau were also investigated under laboratory conditions.The results showed that the soil p H,soil texture,cation exchange capacity and micro-morphology were influenced by freeze-thaw to a certain extent.However,the soil texture and functional groups did not change after freeze-thaw.After freeze-thaw process,the distribution and speciaiton of Pb and Cr in soil were generally unchanged.However,the content of various fractions changed to some extent.Leaching had a certain effect on Pb and Cr in soil while the leaching rates of Pb and Cr in soil were not high.