Migration Pattern And Risk Evaluation Of Heavy Metals In Soils Around Long-Term Coal Gangue Stockpiles

In order to investigate the distribution characteristics and potential risks of heavy metals in the surrounding soil of long-term coal gangue stockpiles,this study took the prevailing wind direction soil around the coal gangue stockpile area of Ningxia Zaoquan coal mine as the research object,explored the distribution characteristics and distribution state of heavy metal elements in the soil,evaluated the pollution status of its heavy metal elements in the study area,and provided scientific basis for the prevention and control of heavy metal pollution in the soil of the stockpile area.The main conclusions are as follows.(1)The contents of Cu,Zn,Pb,Cr and Mn in the surface soil(0-20 cm)and deep soil(20-40 cm)of the study area were measured,and it was found that the average values of Zn,Pb and Mn in the deep soil were higher than those in the surface soil,and the contents of Zn,Pb,Mn and Cr all showed a decreasing trend as they moved away from the stockpile area.In the distribution of the fugitive forms of each heavy metal in the soil,the exchangeable states of Cu and Pd in the surface soil showed a decreasing trend with distance,the percentage of exchangeable states in the deep soil was slightly higher than that in the surface soil,and the percentage of Zn,Cr and Mn in the residue state was higher in both the surface and deep soil.The correlation analysis and cluster analysis of heavy metals in the study area were consistent.pH showed correlation with Zn,Pb and Mn in the correlation analysis between soil physicochemical indicators and total heavy metals.In the correlation analysis between different fugitive forms of heavy metals and physicochemical indexes,the organic and residue states of Cu were correlated with TN to a certain extent,and the exchangeable states of Pb and Cr were significantly negatively correlated with pH,while the remaining fugitive forms of heavy metals were not significantly correlated with physicochemical indexes.(2)The single factor index evaluation method,the Nemero integrated pollution method,the potential ecological risk evaluation method and the RAC evaluation method were used to evaluate the heavy metal pollution of soil in the prevailing wind direction in the stockpile area.The results showed that Cu,Zn,Mn and Cr in the single factor pollution index and comprehensive pollution index evaluation study area soils were higher in the deep soil than in the surface soil,and Mn showed moderate pollution level in the deep soil.The results of the potential ecological risk evaluation method showed that the five heavy metals in the surface soil were in the low potential ecological risk range at different distances from the soil,and the potential ecological risk showed a certain degree of decreasing trend as the distance from the stockpile area increased.The results of RAC evaluation showed that the ecological risk levels in different depths of soil were Pb>Cu> Mn > Cr > Zn,and Pb in different depths of soil had higher Pb has a higher bioavailability and risk level in different depths.(3)In the dynamic leaching experiment,with the increase of leaching time,the pH of the leachate showed a trend of increasing and then decreasing and leveling off,and the EC showed a trend of gradually decreasing.The cumulative release pattern of heavy metals showed a rapid precipitation followed by a smooth pattern.The modified Elovich equation in the kinetic fitting results can better explain the release law of heavy metals in coal gangue than the quadratic regression equation.