城郊农田土壤Pb、Cd的空间变异与评价研究

为了研究城郊区域农田土壤重金属的空间分布特征与质量状况，采用ArcGIS缓冲分析和地统计分析模型分析了长沙城郊农田土壤Pb、Cd的空间变异规律；运用单因子评价法对土壤Pb、Cd的质量状况进行了现状评价；采用Hakanson法对农田土壤Pb、Cd进行了潜在生态风险评价。结果表明，土壤Pb、Cd含量大部分处于Ⅰ级水平（背景状况），但两种元素均存在不同程度的累积，尤其是Cd的累积程度更为突出。土壤Pb、Cd的平均含量在20km以内、20～40、40～60 km的缓冲区由近至远均呈下降趋势，表明城市人类活动在一定空间尺度内对城郊区农田土壤Pb、Cd含量有较大影响；通过半变异函数分析，土壤Pb、Cd在较大范围内存在空间相关性，Pb和Cd的空间变异均是由结构性因素和随机性因素共同作用引起的。Pb、Cd的单项累积指数和潜在生态危害系数均呈现出近郊>远郊>中郊的趋势，各区域土壤Cd的潜在生态危害趋势较土壤Pb更明显，城市近郊南北方向区域及远郊局部工矿聚集区是农田土壤Pb、Cd的累积程度和污染风险较高的区域

SPATIAL VARIABILITY CHARACTERISTICS AND ASSESSMENT OF SOIL PB AND CD IN SUBURB CROPLAND, CHANGSHA CITY,HUNAN,CHINA

To study the spatial distribution characteristics of soil heavy metals in suburb cropland of Changsha city,the buffer analysis and geostatistical analysis model were used to study the spatial variability characteristics of soil heavy metal contents(Cd and Pb).Single-factor assessment method was used to evaluate the quality status of Pb and Cd.While Hakanson method was used to assess the potential ecological risk degree.The results showed that the concentrations of Pb and Cd in largely sampling sites were at class I(below their average background values).There were some accumulations of Pb and Cd in soils,and the degree of accumulation of Cd was more serious than Pb.The average concentration of Pb and Cd in soils decreased with the distance in the buffer area at 3 ranges(i.e.less than 20 km,20 to 40 km and 40 to 60 km from urban area,respectively).It indicated that anthropic activity in urban has great influence on the concentrations of soil Pb and Cd in suburb cropland at certain spatial scale.The semivariograms of geostatistics analysis indicated the large-scale spatial autocorrelations for soil Pb and Cd.The spatial autocorrelation and random both contributed to the spatial structure of soil Pb and Cd.The results of single accumulation index and potential ecological hazard coefficient of soil Pb and Cd showed the tendency that near suburban area was more serious than middle suburban area and distant suburban area,while distant suburban area was more serious than middle suburban area.The potential ecological hazard tendency of heavy metal Cd was more serious than Pb in each area.The accumulation extent and pollution risk outstanding area was on south-north of near suburban and local industrial mining area of distant suburban.