Distribution Of Heavy Metals In Overlying Soil Of Ore-bearing Strata And Potential Ecological Risks

Carbonate rocks are widely distributed in Guizhou,accounting for 61.9%of the total area of the Guizhou province.Under natural conditions,the sources of heavy metals in soils are generally attributed to parent rocks.In the process of weathering and soil formation,limestone-dominated carbonate rocks are weathered,decomposed and precipitated with minerals rich in a variety of heavy metals.Residues and enrichments in soils often lead to high geochemical background of heavy metals in soils.Because of its unique rock assemblage sequence,the Qingxudong Formation has become the main ore-hosting stratum of lead-zinc deposits,and formed a unique lead-zinc metallogenic belt in the region.This kind of ore-bearing strata and rocks are often accompanied by high content of heavy metals,which increases the possibility of superimposition of heavy metals in the soil of this area.Heavy metals that enter the soil and enter the food chain through the enrichment of plants pose a potential threat to human health.Recognizing the source and enrichment regularity of heavy metals in the overlying soil of Qingxudong Formation is the key to prevent and control the pollution of heavy metals in soil.Taking the carbonate overlying soil of the Qingxudong Formation of the Lower Cambrian as an example,this paper tries to explore the influence of geological background on heavy metals in soil by analyzing the major elements and trace elements in surface soil and profile soil,and combining with the unique rock assemblage characteristics of the Qingxudong Formation,and identifies the study area by means of correlation analysis and principal component analysis.Sources of heavy metals in soil.The following conclusions are drawn:（1）The order of average contents of major elements in surface soil of Qingxudong Formation in Guizhou is SiO₂>Al₂O₃>Fe₂O₃>CaO>MgO>K₂O>TiO₂>Na₂O.The surface soil has obvious leaching characteristics of CaO,NaO,and rich in Al₂O₃ and Fe₂O₃.The main component of Qingxudong Formation rocks is CaO,and some of them contain high MgO content.（2）The average concentrations of As,Cd,Cr,Cu,Hg,Ni,Pb and Zn in surface soil were 41.83 mg kg^-1,3.51 mg kg^-1,77.98 mg kg^-1,37.17 mg kg^-1,0.51 mg kg^-1,45.27 mg kg^-1,311.30 mg kg^-1,621.18 mg kg^-1,respectively.Compared with the corresponding parent rocks,the characteristics of enriched elements in the soil of the study area are very obvious,especially elements Cd,Pb and Zn.The enrichment factor analysis showed that As,Cu,Ni and Cr were not enriched obviously or relatively small in the surface soil of the study area.Heavy metals Hg,Cd,Pb and Zn were the main enrichment elements in the surface soil.The effect of lead-zinc mineralization on heavy metals in soils shows that lead-zinc mineralization has superimposed accumulation.（3）The CIA values of Duyun Daping soil profile are all above 88,which is in a strong weathering stage.There are obvious processes of K,Mg and Fe removal during soil development,and with the increase of weathering degree,the leaching loss rate tends to be stable.The Cd,Pb and Zn of the four soil profiles in the study area were enriched to a certain extent,and their contents were obviously higher than the background values of Guizhou Province.Especially in the soil profiles of Duyun Daping,Duyun Duniu and Kaili,the distribution characteristics of Cd,Pb and Zn are similar,and the heavy metal elements are obviously enriched at the bottom.The accumulation of heavy metals Cd,Pb and Zn in the study area is mainly due to the release of some heavy metals from the weathering of rocks and the enrichment of heavy metals in the soil layer.（4）Through correlation analysis and principal component analysis,heavy metals as,Cr,Cu,Hg and Ni in soil mainly come from rock weathering,while Cd,Pb and Zn are mainly accumulated by the weathering of ore-bearing rocks.The activity analysis of heavy metals in soil profiles showed that the contents of Cd,Pb and Zn in soil decreased gradually with the deepening of weathering.From the speciation analysis of heavy metals in soil,it can be seen that Cd and Pb have a high proportion of available and potential available states,and the average contents of As,Cd,Pb and Zn are obviously higher than the screening values of soil risk.It is necessary to pay attention to the ecological risks brought by As,Cd,Pb and Zn.