Superposition Pollution Effect Of Heavy Metals On Soil Associated With A Geochemical Anomaly

The southwestern carbonate area centered on Guizhou is a typical heavy metal geochemical anomaly area with an area of about 150,000 square kilometers.The superposition of industrial and mining sources,agricultural sources and geological sources in northwestern Guizhou Province,has caused serious pollution of heavy metals in the soil and agricultural products were found over-limit,which is a prominent environmental problem in the region.We collected typical heavy metal industrial pollution superimposed small watershed（MS）,taking unpolluted small watershed（HS）as a control,comparing the horizontal,morphological and vertical distribution of heavy metals（Cd,As,Pb,Cu,Zn）in soil under three different land use methods:agricultural land,forest land,and wasteland.Analysis of superposition effect of heavy metal pollution in soil,provide scientific basis for soil pollution remediation and environmental management.The results showed that:（1）In MS polluted small watershed,the average values of surface soil Cd,As,Pb,Cu,and Zn are 8.14 mg·kg^-1,45.07 mg·kg^-1,263.97 mg·kg^-1,41.36 mg·kg^-1,716.98 mg·kg^-1,which are higher than the soil background value of 1.30-12.33 times in the northwest of Guizhou Province.The variation coefficient of the five heavy metals is between 25.94-85.07%,which is a moderat spatial variability.The high value of heavy metals mainly appears in the northeast of the pollution source,the spatial distribution is regular;in the control small watershed HS,the average values of the top soil Cd,As,Pb,Cu,and Zn are 6.55 mg·kg^-1,30.78 mg·kg^-1,67.36mg·kg^-1,44.58 mg·Kg^-1,263.6 5mg·kg^-1,and are higher than 2.60 to 4.84 times of the soil background value in northwestern Guizhou Province.The variation coefficient of the five kinds of heavy metals are between 20.56-38.56%,which is a moderat spatial variability and the spatial distribution is relatively uniform.（2）In MS polluted small watershed,The heavy metal Cd in the exchangeable state accounted for 31.16%,the reproducible ratio was 40.84%,the oxidizable state accounted for 9.05%,and the residual state accounted for 19.16%,Soil Pb has the highest proportion of reproducible state,soil Zn has the highest proportion of oxidizable state,and soil As and Cu residue have the highest proportion;in the control small watershed HS,the heavy metal Cd in the exchangeable state accounted for38.50%,the reproducible ratio was 32.04%,the oxidizable state accounted for 3.88%,and the residual state accounted for 24.29%,the soil Pb,Zn,As,Cu residue have the highest proportion;MS small watershed soil heavy metal activity is strong,pollution accumulation is obvious,most of the HS small watershed the heavy metal activity is weak,and the abnormal heavy metal content is more likely caused by the high geological background.（3）In MS polluted small watershed,the soil heavy metals Cd,As,Pb,and Cu all showed the highest content of agricultural land,and soil Zn showed the highest content of wasteland;in the control small watershed HS,the soil heavy metals As,Cu,and Zn all showed the highest content of wasteland,and soil Pb showed the highest content of agricultural land,the accumulative characteristics of heavy metals in soil differ greatly in different land use methods.（4）In MS polluted small watershed,the five heavy metals are the highest in the soil surface layer,and their contents exceed the background value of northwest Guizhou in Guizhou Province,and the over-standard multiple is between 1.41-15.19times,showing the characteristics of superposition of soil surface pollution;in the control small watershed HS,Soil Cu and Zn are the highest at 80-100 cm,and their contents exceed the background value of northwest Guizhou in Guizhou Province,and the over-standard multiples are 1.55 and 4.31 times,showing high geological background.The accumulation of heavy metals in the soil of contaminated area is obvious,and the heavy metal activity is strong.The soil in the bottom layer of the control area has a high content of heavy metals and weak migration.The heavy metals in the northwestern Guizhou soil generally show the distribution characteristics of geological high back and pollution superposition,which have strong environmental ecological risks.