Occurrence Forms And Accumulation Characteristics Of Soil Heavy Metals In Different Strata Of Northwestern Guizhou

Soil heavy metals in Northwest Guizhou exceed the standard seriously.Geological background is one of the reasons for heavy metals exceeding the standard.In order to study the accumulation of heavy metals in soil developed in different strata under the natural geological background in Northwest Guizhou,The rocks,developed topsoil and soil profiles of five s ets of strata（Carboniferous,Permian,Triassic,Jurassic and Cretaceous）in Weining and Hezhang counties in northwest Guizhou were selected as the research objects.Heavy metals and their speciation contents were measured experimentally to study the enric hment characteristics and speciation distribution differences of heavy metals in different strata rocks and their developed soils.Based on the measured data,the environmental impacts of soil heavy metals and their speciation were evaluated.The stepwise regression method was used to investigate the factors affecting the accumulation of heavy metals in soil.The following are the main conclusions:（1）In the Carboniferous and Cretaceous rocks,only Cd and As are higher than the average values of the crustal elements,and the Cretaceous has a relatively high super multiple,which is1.03 and 1.46 times that of the Carboniferous,respectively.In the Permian and Triassic,only Pb and Ni are not higher than the average values of crustal elements.The As and Pb in Jurassic rocks are higher than the average value of crustal elements,and the super multiple of As is higher,which is 6.12 times of the average value of crustal elements(1.8 mg·kg^-1).（2）The average content of all heavy metals in the developed Carboniferous topsoil was higher than the background value,and the multiple of Cd excess background value(0.66 mg·kg^-1)was the largest,which was 8.56 times of the background value.The content of Cd in the Carboniferous topsoil was significantly different from that in the Permian,Triassic,Jurassic and Cretaceous strata（P<0.05）.The highest average content of Zn,As and Pb is found in the Carboniferous,and the highest average content of Cr is found in the Triassic,which is 2.44 times of the background value(95.50 mg·kg^-1).The strata with the highest average Cu、Ni and Zn contents are Permian and Triassic respectively.Except for Cd,the Jurassic developed topsoil was lower than the background value.There are differences in the accumulation of the same heavy metals in soil profiles of different strata,as a whole,the content of the same heavy metals decreases with the increase of depth.（3）In the developed surface soils of different strata,Cd is mainly in the residual state,reducible state and acid-extracted state,Pb is mainly in the reducible state,and Cr,Cu,Ni and Zn are mainly in the residual state.The forms of Cr,Cu,Ni and Zn in the soil profile were consistent with those in the topsoil,show as the residual form accounted for the highest proportion.Although the proportion of residual Cd is the highest,the proportion of acid extracted Cd is the highest among all heavy metals,which is easily absorbed by plants and has high potential ecological risk.（4）The heavy metals As,Pb,Cr,Ni and Zn in the study area were unpolluted and slightly polluted.Cd has a higher ecological r isk than other heavy metals,Compared with the soil of other strata,the Carboniferous develop soil has a higher risk of heavy metals.（5）Based on the model and correlation analys is,rock heavy metals are the main factors affecting the soil heavy metal content.The content of heavy metals in the soil is also affected by the physical and chemical properties of the soil,but there are differences due to different types o f heavy metals.Under the natural geological background of Weining County and Hezhang County in northwestern Guizhou,heavy metals in the soil accumulate significantly.There are differences in the accumulation and speciation distribution of heavy metals in soils developed in different strata.The content of heavy metals in soils developed in Carboniferous is higher,and that in soils developed in Jurassic is low.Geological background is an important contributing factor to the excess of heavy metals in the soil of northwest Guizhou.