Distribution And Risk Assessment Of Heavy Metals In Contaminated Soil And Groundwater

With the rapid advancement of industrialization and urbanization,industrial adjustment and optimization of urban layout,a large number of backward and obsolete smelters and chemical plants have been shut down or moved out one after another,leaving a large area of contaminated land-sites.How to do a good job in the investigation,evaluation and treatment of contaminated sites to achieve the purpose of reuse has become a topic that many scholars pay attention to and explore in recent years.In this paper,the distribution characteristics,ecological risks and health risks of heavy metals in the soil and groundwater of the former Hunan Ferroalloy Factory and its surrounding land were studied by means of ArcGIS spatial interpolation method,potential ecological risk assessment model and health risk assessment model(Refer to Technical Guidelines for Site Risk Assessment(HJ25.3-2014)),etc.,and suggestions for pollution control of the factory were put forward.The results are as follows:(1)There are many kinds of heavy metal pollution in the surface soil and the pollution range is large.The main heavy metals are total Cr,Cr(Ⅵ),Mn,Pb,Cd,Zn and Cu,with the average content of 1560 mg·kg-1,383 mg·kg-1,9429 mg·kg-1,1002 mg·kg-1 and 43.7 mg·kg-1.The over-standard rates of points are 70.2%,81.5%,46.2%,46.7%,23.1%,45.1%and 0%respectively,and the polluted areas are 458,880m2,526,720m2,285,440m2,302,080m2,159,360m2 and 278,400 respectively.In the middle and lower layers of soil,the main heavy metals are total Cr and Cr(hexavalent).With the increase of depth,the polluted areas are gradually reduced,with the proportions from 27.7%to 7.2%and 52.3%to 6.3%,respectively.(2)The potential ecological risk of six heavy metals in surface soil was evaluate,and the risk indexes were as follows Cd(184.9)>Pb(166.9)>Cr(60.4)>Zn(36.5)>Mn(16.0)>Cu(9.0).The comprehensive potential ecological risk assessment results show that the proportion of different risk intensities to the total site area is:strong ecological risk(44.1%)>medium ecological risk(20.5%)>strong ecological risk(18.5%)>slight ecological risk(17.9%).(3)There are many kinds of heavy metals in the surface soil with carcinogenic or non-carcinogenic risks.Cd and Cr(Ⅵ)have carcinogenic risks,among which Cr(Ⅵ)has a high carcinogenic risk of 79.49%,and the proportion of non-carcinogenic risks is Cr(Ⅵ)(88.2%)>total Cr(87.7%)>CD(66.7%)>Pb.The repair limits of Cd,total Cr,Cr(VI),Zn and Pb were calculated as 67.51 mg kg-1,264.03 mg kg-1,24.04 mg kg1,38,051.75 mg kg-1 and 207.04 mg kg-1 respectively.(4)Medium-heavy metals in groundwater are characterized by many kinds of pollution,high values and wide range.The main heavy metal ions are Cr6+,Mn4+,Cd2+,Pb2+ and Zn2+,with average values of 23.88 mg·L-1,105.6 mg·L-1,212.4 mg·L-1,140.6 mg·L-1 and 127 mg·L-1 respectively.The over-standard rates of points are 45.5%,65.9%,36.7%,31.8%and 43.2%respectively,and the polluted areas are 301440 m2,44160 m2,243840 m2,231680 m2 and 265600 m2 respectively.(5)The comprehensive evaluation results of groundwater environment show that the pollution indices of Cr6+,Mn4+,Cd2+,Pb2+ and Zn2+are 0.2～1090,0～14.73,0～145,0.1～74 and 0.2～11.8 respectively,and the comprehensive pollution indices are 0.240～791.80.The proportion of points with different pollution levels is:severe pollution(52.3%)>severe pollution(20.5%)>moderate pollution(11.4%)>clean(9.1%)>light pollution(6.8%).(6)There are many kinds of heavy metals in groundwater that have carcinogenic or non-carcinogenic risks.only Cr(Ⅵ)has carcinogenic risk,the proportion of which is "low risk" or higher is 59.1%,and the proportion of non-carcinogenic risks is Cr6+(43.2%)>Pb 2+(4.5%)>Cd2+(0%)=Zn2+(0%);The repair limits of Cr6+,Cd2+,Pb2+ and Zn2+were determined as 0.10 mg L-1,0.01 mg L-1,0.10 mg L-1 and 5.00 mg L-1,respectively.On the whole,the heavy metal pollution situation of Hunan Ferroalloy Factory is rather complicated,which is characterized by many kinds of heavy metal pollution,large pollution area,relative concentration and diffusion trend.The carcinogens in soil are mainly Cd and Cr(Ⅵ),but only Cr(Ⅵ)in groundwater.Suggestions:Areas with relatively concentrated heavy metals in the surface soil can be repaired by "digging and filling+ electro-remediation/solidification and stabilization+blocking technology";areas with relatively scattered contents and middle and lower layers of soil can be repaired flexibly by using exotic soil,bioremediation,blocking technology,etc.according to the properties of heavy metal ions.