【作者】 刘琼峰；

【导师】 段建南； 李明德；

【作者基本信息】 湖南农业大学 ， 土地资源利用与信息技术， 2013， 博士

【摘要】 城郊区在来自工业、农业、交通以及城市生活多重环境压力下,农田土壤重金属污染等环境质量恶化问题尤为复杂和严峻。城郊区农田土壤重金属的空间变异特征、影响因素、现状评价与风险评价研究是防控该区域农田土壤重金属污染生态风险的必要途径。本文以长沙城郊农田土壤重金属Pb、Cd为例开展相关研究。根据长沙市流域的分布情况,采集长沙城郊区近郊、中郊、远郊18个乡镇513个农田土壤样点,测定了农田土壤重金属Pb、Cd的含量及主要土壤理化性状指标(pH值、有机质、碱解氮、有效磷、速效钾、缓效钾)；以长沙市土地利用现状图为基础,在GIS平台下获取了土壤采样点的区位特征数据(土壤采样点离城镇的距离、离工矿建设用地的距离、离河流的距离、离农村居民点的距离)。采用ArcGIS缓冲分析、空间自相关分析和地统计分析模型等研究了长沙城郊农田土壤Pb、Cd的空间变异特征。采用传统回归模型(Ordinary least squares OLS)和地理加权回归模型(Geographically weighted regression GWR)分析比较了土壤Pb, Cd含量与影响因素(土壤理化特性、土壤区位特征)间的相关关系。运用单因子污染指数法对土壤Pb、Cd的质量状况进行了现状评价;采用Hakanson潜在生态危害指数法对农田土壤Pb、Cd进行了潜在生态风险评价；运用模糊综合评价法和层次分析法从农田土壤重金属Pb、Cd的含量、土壤理化特性、土壤区位特征三个方面选取评价指标,建立农田土壤重金属Pb、Cd的生态风险评价模型,对长沙城郊农田土壤Pb、Cd的生态风险进行分级和分区。长沙城郊农田土壤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、20-40km、40-60km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距离内对城郊区农田土壤Pb、Cd含量有较大影响；土壤Pb、Cd在较大范围内存在空间相关性,Pb和Cd的空间变异是由结构性因素和随机性因素共同作用引起的。土壤Pb、Cd的OLS模型和GWR模型估计结果表明,土壤Pb与Cd含量呈极显著正相关；土壤pH值、有机质、氮磷含量与土壤Pb、Cd含量的相关性显著；离城镇、河流、工矿建设用地的距离对于城郊农田土壤Pb、Cd含量也有一定影响,Pb、Cd的GWR模型拟合度较OLS模型高,残差不存在空间自相关,GWR模型在每个不同空间位置的采样点都有一组局部的参数估计来反映各影响因子与土壤Pb、Cd含量的相关程度,能更好地解释土壤Pb、Cd与影响因素变量的空间异质性。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。运用模糊综合评价法建立农田土壤Pb、Cd的生态风险评价模型,根据生态风险指数值(ERI)将生态风险等级划分为四级(Ⅰ级ERI>0.70、Ⅱ级0.6<ERI≤0.70、Ⅲ级0.4<ERI≤0.60、Ⅳ级ERI<0.40),22.9%的土壤样点生态风险指数为Ⅲ级和Ⅳ级水平,具有较高的生态风险,主要分布在河流、城镇、工矿建设用地和农村居民点的聚集区。根据各乡镇生态风险等级的差异,划分农田土壤重金属Pb、Cd的高、中、低生态风险区。该研究可为定量分析区域土壤重金属含量的空间结构与影响因素提供参考,为长沙城郊农田土壤重金属污染生态风险的防控提供理论依据。土壤Pb、Cd的“高-高”集聚区(土壤Pb或Cd含量高的区域被Pb或Cd含量高的其他区域所包围,区域土壤Pb或Cd含量水平较高,且空间差异程度较小)和离城镇、河流、工矿建设用地较近的农田是Pb、Cd污染风险防控的重点区域。农田土壤重金属的生态风险评价与分级、分区是建立农田土壤重金属生态风险防控体系的必要手段,可为农田土壤的科学合理利用提供技术支撑。更多还原

【Abstract】 The agricultural soil contamination with heavy metals in the suburb area draws great attention because of its potential threat to food safety and detrimental effects on the ecosystem.The research about spatial distribution characteristics, the origins, and the status and ecological risk assessment of soil heavy metals have important significance for farmland ecological risk prevention and controlling in the region.In this study, a survey in agricultural soils of the suburb area of Changsha was conducted. A total of513surface soil samples were collected, and the concentrations of Pb and Cd, the soil physicochemical properties of sampling points were analyzed. The location characteristics of soil points were obtained from land-use map under GIS platform.The spatial distribution Characteristics of soil Pb and Cd in suburb cropland of Changsha city were analyzed by using the buffer analysis, spatial autocorrelation analysis and geostatistical analysis methods. The average concentration of Pb and Cd in soils decreased with the distance from near to far in the buffer area at3ranges (i.e. less than20kilometer,20to40kilometer and40to60kilometer 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 distance. The spatial autocorrelations for soil Pb and Cd was over a wide range. The spatial autocorrelation and random both contributed to the spatial structure of soil Pb and Cd.The origins of soil heavy metals in suburb interface are usually controlled by many factors, such as parent material, industry and agriculture activities, etc. To effectively decrease heavy metals pollution risks in suburb area and further establish reliable protection measures, it is quite necessary to understand their sources and spatial patterns. The conventional regression model (OLS) was usually used to analyze the relationship between soil heavy metals with their influential factors. However, OLS is only in a global or an average sense to estimate parameters, and it can not reflect spatial local variation or test spatial non-stationarity. Geographically weighted regression model (GWR) is a powerful tool for exploring spatial heterogeneity. The underlying idea of GWR is that parameters may be estimated anywhere in the study area given a dependent variable and a set of one or more independent variables which have been measured at places whose location is known. Not only can it test spatial non-stationafity, but also it can provide the corresponding solutions. As the local model, GWR model has been applied in researches about urban housing land prices and the spatial factors of economic development, but few to the origins and spatial structure of soil heavy metals. Typical influential factors of Pb and Cd were picked up from the points of soil property and location, such as soil pH, organic matter, alkali-hydro nitrogen, rapidly-available phosphorus, rapidly-available potassium, slowly available potassium, the distance from cropland to town, the distance from cropland to settlement, the distance from cropland to industrial construction land, and the distance form cropland to river. The conventional linear regression model(OLS) and spatial regression model(GWR) were applied to consider the relationship among the variables of influential factors and their spatial structure together. The results indicate that soil Pb was highly significantly positively related with Cd. The concentrations of soil pH, organic matter, alkali-hydro nitrogen and rapidly-available phosphorus were significantly positively related with the content of Pb and Cd. The distance from cropland to river, from cropland to town and from cropland to construction land also had some influence on the concentrations of Pb and Cd in agricultural soil of the suburb area of Changsha. The GWR models for Pb and Cd had a better goodness-of-fit than OLS models respectively as a result of GWR models indicated the same tendency of spatial correlation between the Pb and Cd measured values with their estimated values, and their residuals without spatial autocorrelation. GWR model indicated the dependency between soil Pb and Cd with their influence factor values by estimating local parameters of soil sampling point anywhere, and it can explain better their spatial heterogeneity.The quality status and potential ecological risk degree of Pb and Cd were evaluated by using single-factor assessment and Hakanson methods.. 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 results of single accumulation index and potential ecological hazard coefficient of soil Pb and Cd showed that the tendency which in 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.Fuzzy comprehensive evaluation and analytic hierarchy process were applied to establish ecological risk assessment models of Pb and Cd contamination, and evaluation indies (ERI) were set up based on Pb and Cd concentrations, soil physicochemical properties and soil location characteristics. The indies of soil physicochemical properties were soil pH, organic matter, alkali-hydro nitrogen and available phosphorus, while the indies of soil location characteristics were the distance from cropland to town, industrial construction site, river, and settlement. The ecological risk grade was divided to four ranks according to the values of ecological risk index, ERI>0.70,0.6<ERI≤0.70,0.4<ERI≤0.60, and ERI≤0.40were defined as calss I, II, III and IV respectively. About22.9%sampling points were distributed in class III and IV, most of high ecological risk soil samples were located at the surounding area of riverside, town, industrial construction site and settlement. The ecological risk regions of farmland soil Pb and Cd were divided into high, middle, and low ranks.The research could provide reference for the quantitative analysis of spatial structure and influence factors of soil heavy metals in the region. It played an important role in ecological risk prevention and controlling of soil heavy metals pollution. The high-high spatial cluster districts with high concentrations of Pb and Cd and the regions near town, river, and build where were the important regions for controlling pollution risk of Pb and Cd in agricultural soil of the suburb area of Changsha. Ecological risk assessment,classification and regionalization of heavy metals in agricultural soil had important significance for farmland ecological risk prevention and controlling system construction and soil utilization scientifically.更多还原