城乡结合带土壤重金属时空变异特征与源解析——以南京市柘塘镇为例

SPATIAL-TEMPORAL VARIATION AND SOURCE APPORTIONMENT OF SOIL HEAVY METALS IN PERI-URBAN AREA—A CASE STUDY OF ZHETANG TOWN,NANJING

为了解城乡结合带土壤重金属污染特征与规律,以南京市溧水区柘塘镇为例,就表层土壤中As、Cd、Cr、Cu、Hg、Ni、Pb和Zn等8种重金属的含量、时空变化特征及来源分别进行了研究。结果表明:柘塘镇8种重金属元素的平均含量为14.0、0.20、77.6、44.7、0.17、33.9、36.2和103.7 mg kg-1,空间分布以低于绿色标准为主,维持背景含量的区域较少。与2002年含量相比,绝大部分面积出现了8种重金属不同程度的富集,增长率分别为56.91%、61.06%、6.07%、60.15%、38.69%、19.94%、23.92%和56.17%。人类活动较为密集的地区富集程度较大,其中Cd、Cr、Cu、Ni和Zn的富集区基本均集中在城镇和工厂驻地;Hg和Pb的富集区多集中在交通干道,As元素的富集区主要集中在村镇区域。Cr、Cu、Ni和部分Zn可能来源于工业生产,Hg和部分Pb、Zn可能来源于交通运输以及禄口机场的运营,Cd和部分Zn可能来源于城市扩张和秦淮河渔业发展,As则可能来源于畜禽养殖、生活污水排放和污泥施用。南京城乡结合带正面临土壤重金属不断富集的问题,应摸清来源,尽早防治。

Peri-urban areas are not only a buffer protecting the zoology,environment,and climate of a city,but also a supplier of large amounts of food to city residents and a dump site for disposal of city waste and garbage as well. Recently,the peri-urban areas of this country are facing an increasingly serious problem of soil heavy metal pollution due to rapid development of urbanization and industrialization of the areas. Therefore,it is very urgent to conduct investigations of soil heavy metal pollution in the areas. A case study of Zhetang Town of Nanjing was carried out with a view to exploring status quo,spatio-temporal variation and source apportionment of the soil heavy metal pollution in the peri-urban area. A total of 109 soil samples were collected in 2012 and 21 in 2002 from the topsoil layer for analysis of As,Cd,Cr,Cu,Hg,Ni,Pb and Zn. With the aid of the GIS spatial analysis technology and related standards for soil heavy metal contents as reference,such as natural background values,green standard,Criteria Grade I and Ⅱ,status quo and spatial-temporal variation of the pollutions of the 8 soil heavy metals were analyzed. Probable sources of the soil heavy metals were also apportioned through Pearson＇s correlation and principal component analysis. Results show that continuous accumulation of the soil heavy metals accompanied urban expansion and industrial development of the city of Nanjing. The average content of As,Cd,Cr,Cu,Hg,Ni,Pb and Zn in Zhengtang Town was 14. 0,0. 20,77. 6,44. 7,0. 17,33. 9,36. 2 and 103. 7mg kg- 1,respectively. Most of the area was lower than the criteria of the Green Standard in heavy metals concentration,and only a very few regions maintained their background values. So,safety of agricultural produce was not a problem at the time of the study. The area of the soil with excessive heavy metals content is increasing with each passing day,gradually expanding from the urban outwards to surrounding farmlands and woodlands in the suburbs. Compared with 2002,2012 witnesses varying extent of heavy metal accumulation in most of the area. The concentration of As,Cd,Cr,Cu,Hg,Ni,Pb and Zn increased at a rate of 56. 91%,61. 06%,6. 07%,60. 15%,38. 69%,19. 94%,23. 92% and56. 17%,respectively. Areas intense in human activity like down town area,villages and buffer zones along traffic ways,were high in heavy metals concentration,while the concentrations declined gradually outwards,forming a downward gradient towards peripheral areas and rural areas. Cd,Cr,Cu,Ni and Zn were concentrated mostly in the urban area and around factory sites; Cd,along river banks; Hg and Pb alongside traffic arteries,and As in towns and villages. Cr,Cu,Ni and a part of Zn probably came from urbanization construction and discharge of waste water,slag and gases from industrial production,which kept on expanding in scale; Hg,Pb and part of Zn from heavy traffic on highways and operation of the Lukou Airport,where so many flights a day generated large volumes of exhaust from the jet engines and dust from tire abrasion; Cd and part of Zn from urban construction and expansion,and development of fishery in the Qinhuai River,which is an important economic sector of Zhetang town; As from livestock and poultry breeding,discharge of sewage and application of sludge as manuren. Through the case study of Zhetang Town,it is not difficult to find the characters and trends of the heavy metals pollution in the peri-urban areas of Nanjing. The soil environment of that region was deteriorating in the process of urbanization and industrialization. The existence of heavy metals in the soil is often accompanied by environmental pollution,of which its sources are complex but ultimately attributed to various intense human activities. It is essential and urgent for scientists to determine the status of soil heavy metals,where they come from and what kind of countermeasures should be taken as soon as possible to protect the ecological environment and assure steady,healthy and sustainable development of the peri-urban areas in the country.