Spatial And Temporal Distribution Characteristics Of Soil Heavy Metal Pollution In A Shipyard In Guangzhou And Analysis Of Pollution Sources

With the implementation of the policy of "back two into three" and "back city into garden",old factories have gradually withdrawn from the urban areas,resulting in a large number of industrial legacy and abandoned sites.The soil of these abandoned sites is often polluted by heavy metals to different degrees,and if not properly treated,the redevelopment and use of these sites seriously threatens human health and ecological safety.As an important gathering place of shipbuilding industry in China,many plots left over from the relocation of shipbuilding bases in Guangdong also face the demand for reuse.In this paper,the land left after the relocation of a shipyard in Guangzhou was taken as the research object to investigate the pollution characteristics of heavy metals(Pb,As,Zn,Hg,Ni,Cd,Cu,Cr)in the soil,so as to explore the pollution sources.Based on this,risk assessment was carried out and suggestions on risk control were put forward.This study can provide some references for the treatment and accurate restoration of contaminated sites in shipbuilding industry,which has important theoretical and practical significance.The main conclusions are as follows:(1)The content of five heavy metals(Pb,As,Zn,Hg,Ni)in some soil samples exceeded the screening value for " Class I land" in the "Soil Pollution Risk Control Standard for Soil Environmental Quality Construction Land"(GB 36600-2018).The over-standard rates were8.66%,1.99%,1.98%,0.74% and 0.25%,respectively.The variation of the coefficient of variation ranged from 49.24% to 684.1%,which belonged to the high variation level.These results indicate that the distribution of heavy metals in the soil of the study site is uneven and strongly influenced by human activities.(2)The inverse distance weighting method in Arc GIS 10.5 was used to explore the horizontal distribution of soil heavy metals in the study area,and the results showed that the spatial distribution of eight heavy metals in the topsoil showed a "patchy aggregation" feature.In the investigation results of soil samples with excessive heavy metal content,it is found that the samples with Pb and Zn exceeding the screening value have a wide distribution area and high multiple of exceeding the standard.The soil samples containing high concentration of Pb were mainly distributed in the parts of the hull and the ship berth,and the highest concentration reached 8993.43 mg/kg,which was 22.5 times of the soil screening value.The distribution range of samples with Zn content exceeding the screening value was the second,mainly concentrated in the pretreatment area of steel,with the highest concentration of 66019 mg/kg,which was 4.3 times of the screening value of soil.In addition,the content of As,Hg and Ni samples in the soil was less than 2 times of exceeding the standard,and they were mainly sporadically distributed.The areas with excessive Hg and As contents are distributed in the coating workshop,while Ni is distributed in the pipe processing workshop.(3)In the soil point profile analysis of Pb,Hg,Zn,Ni and As content exceeding the standard,it was found that the heavy metal concentration in the surface soil was relatively high,but its content decreased rapidly with the increase of soil profile depth,reaching a depth of about m,close to the corresponding heavy metal content in the soil of low value area.This phenomenon indicates that heavy metals in the surface soil have been obviously imported superposition,so it is inferred that shipbuilding activities may be an important factor leading to heavy metal pollution in the soil.Interestingly,the contents of Zn,As,Hg and Pb show an obvious enrichment effect at 1.5 ～ 2.0 m depth in the ship berth area,armor section area and pipe processing workshop.(4)The precise analysis of pollution sources by correlation analysis,cluster analysis and principal component analysis showed that Cd and Cr originated from natural input.Pb,Cu,As and Hg showed a tendency of compound pollution in the soils of each region,and their pollution was mainly related to the anti-fouling paint fragments and metal particles produced in the painting and welding processes.Coal stacking and use are the main sources of Ni in soil.As an isolated element,Zn mainly comes from Zn-containing dust and residue generated in steel pretreatment,nail fitting and machine fitting.(5)The evaluation results of single factor pollution index,Nemerow comprehensive pollution index and geological accumulation index showed that 12.25% of the collected soil samples were at the level of severe pollution,6.12% were at the level of moderate pollution,and 12.24% were at the level of light clean pollution.At the same time,Zn,Pb,As,Hg and Ni presented different levels of pollution in the soil at 0-2.0 m depth,in which Ni and Zn reached the level of mild and moderate pollution,respectively,while Pb and As showed the level of severe pollution.The comprehensive evaluation results of potential ecological risk index showed that the ecological hazards in the study area were mainly slight(RI < 150),and Pb and Hg in individual samples showed moderate ecological risk level.(6)The health risk assessment of heavy metals was carried out,and the unacceptable risk of Pb and As in soil to human health was determined.The non-carcinogenic risk of Pb was the most significant,while As exhibited a higher carcinogenic risk.Especially in the surface layer of soil,the level of pollution decreased with the increase of soil depth.Combined with the characteristics of pollutants and the actual characteristics of the plot,the remediation technologies were screened,and the soil remediation was initially determined to be carried out using a combination of drenching,ex-situ solidification stabilization and barrier landfill techniques.