Evaluation Of Heavy Metal Contamination In Soils Around Lead-Zinc Mining Areas In Northwest Guizhou Based On Land Use

Lead and zinc mines in China are mainly distributed in the northwest region of Guizhou.Since the 17th century,the soil surrounding mines has been polluted seriously caused by“three waste”in smelting（local zinc smelting）and transportation of lead and zinc ores.While soil heavy metals are enriched by natural weathering,they are also affected by anthropogenic activities such as mining and industrial emissions,which is likely to cause the"activation"of soil heavy metals to change their chemical morphology.Under the influence of the superposition of the intrinsic and extrinsic factors,the causes of heavy metal pollution and the ecological risk are unclear in the contaminated soil of lead-zinc mine,which is not conducive to the control of the source of pollution and the scientific prevention and control of environmental ecological risk,which is more detrimental to the rational utilization of regional land resources.Researches show significant differences in soil heavy metal contamination levels under different land-use modes,but their patterns,spatial differences and potential risks remain unclear.Based on this paper,the zinc refining area-Hezhang County is a typical area in northwest Guizhou province.The heavy metal contamination of soil under different land-use modes is the main research line.Systematic research on soil heavy metals,sediment morphology,source and spatial differentiation is carried out.With a view to controlling and repairing soil heavy metal pollution in lead-zinc mining area,implement agricultural land classification management,strengthen pollution source supervision,do well to prevent soil pollution for reference.The main findings are as follows:（1）The pollution status of heavy metals（Cd,Cr,Cu,Ni,Pb,Zn）in the soil of different land use types shows:Cd:Heath>Arable Land>Grassland>Woodland>Silt;Cr:Silt>Arable Land>Wasteland>Grassland>Woodland;Cu:Arable Land>Silt>Wasteland>Grassland>Woodland;Ni:Silt>Wasteland>Arable Land>Grassland,Woodland;Pb:Wasteland>Arable Land>Grassland>Woodland>Silt;Zn:Wasteland>Arable land>Grassland>Woodland>Silt.The changes of Cd,Cu and Ni content were not obvious and the trend was stable.Zn is complex.As the depth of soil profile increasing,the watershed 1 wasteland section（0-90 cm）has a gradual increasing trend of inflection points of 70-80 cm;The cultivated land section（0-60 cm）has a decreasing trend and an inflection point of 30-40 cm;The variation of the section（0-240cm）in watershed 2 is not obvious;The cross section（0-200 cm）has a decreasing trend,with an inflection point at 120-140 cm.（2）The chemical morphological recovery rate of heavy metals in different soils is between 84.17%～102.87%.Heavy metal elements（Cd,Cr,Cu,Ni,Pb,Zn）mainly have stable residue state（F4）.In addition to the residue state（F4）,the weak acid extraction state（F1）,reducible state（F2）and oxidable state（F3）are distributed differently in different geographical classes:Pb’s F2 and F3 are mainly deposited in arable soil,Cr F1-based,Ni,Cu F3-based,the distribution of Cd elements in cultivated land in the two watersheds is different.Basins 1 are dominated by F2 and F3.Mainly F1 and F2 in watershed 2;Pb elements in woodland soil mainly F2 and F3,Zn mainly in the form of F1,F2,F3 enrichment,Ni elements are mainly enriched in F3,Cr and Cu are distributed differently in two watershed woodlands:Cr is enriched in watershed 1 by F1,in watershed 2 by F2,and in watershed 2 by Cu by F3;Bioactive components of Pb and Zn elements（F1+F2+F3）enriched in grassland soil,The Cu and Ni elements are mainly based on the F3 distribution.Cd mainly F2,Cr in grassland soil mainly F1 distribution;Morphological proportions in sludge and wasteland soils showed that Pb and Zn elements were enriched with different bioactive components（F1+F2+F3）in both land uses.As the depth of soil profile increases,the chemical proportional distribution of heavy metal elements is characterized by:（F1+F2+F3）The morphological distribution in the cross-sectional soils is more obvious than in the surface soils,and there are four elements:Cd,Cr,Pb and Zn.（3）The spatial distribution of heavy metal elements in soil showed that the distribution of Cd,Pb and Zn in soil was similar.The three high-value areas are mainly located around the lead-zinc mine area,in the central zone of basin 1 and in the southwest part of basin 2.While Cr,Cu,Ni elements are influenced by land-use type and human factors,there are different spatial distributions.Multivariate statistical analysis of soil heavy metals in the study area,including correlation analysis and principal component analysis（PCA）,clustering analysis found that there is a significant correlation between the heavy metal elements Cd-Pb-Zn and similar pollution sources in the study area.（4）The results of evaluations using the Geological Cumulative Index(I_geo),Secondary Phase versus Native Comparison Value（RSP）and Potential Ecological Risk Index（PERI）were slightly different.The results of the I_geo evaluation show that all species in Basin 1 are at pollution-free levels,but Cd elements are lowest at 50-60 cm across sections;Basins 2The main polluting elements in soil are Pb and Zn,which are more obvious in cultivated land,heath land and grassland,while woodland is the safest species.The Pb I_geo is highest at 140-160cm in the Basin 2 section.The RSP results show that there are different levels of pollution of Cd,Pb,Zn elements in cultivated land,grassland and wasteland soil.Only mild contamination levels of Pb elements were present in the soil of 2 arable land in the basin.PERI showed that heavy metal elements were at a slight risk of Grade I in the basin.The spatial distribution of the potential risk of heavy metal elements is consistent with the characteristics of the element content.The results of the evaluation are slightly different due to different evaluation levels and focus.Therefore,when planning for land use in an area,it is necessary to understand the status of land use in the region and consider the status of heavy metal pollution such as soil heavy metals,morphology and biotoxicity.