长株潭典型污染区稻田剖面土壤重金属的累积及形态特征

为探究长株潭地区不同污染源影响下稻田土壤中重金属的分布特征，对该区有机肥施用区、沼液灌溉区、铅锌矿污染区以及工业污染区剖面土壤中重金属的总量和形态进行分析，采用单项污染指数和综合污染指数进行风险评价，并利用已知污染源区的土壤重金属形态特征对未知污染源区的土壤进行污染源解析. 结果表明:①研究区表层(0~10 cm)土壤中Cd、Pb、Cu、Zn、Cr和Ni的平均含量分别为0.8、54.5、43.6、158.6、106.2和23.3 mg/kg. 相对于长株潭地区土壤背景值，表层土壤中Cd、Pb和Zn均有累积，且Cd平均含量超过《土壤环境质量 农用地土壤污染风险管控标准(试行)》(GB 15618—2018)中农用地土壤污染风险筛选值；随着土壤深度增加，Cd、Pb、Zn和Cr在次表层(10~20 cm)土壤也有累积，其平均含量分别比当地背景值高出6.9%、19.8%、101.4%和67.0%；在深层(80~100 cm)土壤中，Zn和Cr的平均含量仍分别比当地背景值高出30.4%和48.4%. ② Cd、Pb的单项污染指数高值集中在表层土壤，部分采样点呈轻微或轻度累积，少数采样点呈中度累积；Zn以轻微累积和中度累积为主，极少数采样点为重度累积，个别采样点存在污染风险；综合污染指数显示，研究区内表层土壤为重度或中度累积. ③有机肥施用区TOC含量与可氧化态Cu、Zn含量呈显著或极显著相关；沼液灌溉区TOC含量则与可氧化态Cu、Zn含量均呈极显著相关；铅锌矿污染区可交换态Pb含量与可交换态Cd、Zn含量均呈极显著相关；而工业污染区则只有可交换态Pb含量与可交换态Cd含量呈极显著相关. 研究显示，污染源的不同造成长株潭地区稻田土壤中不同重金属的累积程度及其主要赋存形态间的相关性存在明显差异，因此，可以借助土壤中重金属的累积程度、主要赋存形态及其相关性对土壤中重金属的主要来源进行初步识别，为源头防治污染土壤提供理论依据. 

Accumulation and Speciation Characteristics of Heavy Metals in Paddy Soil Profiles in Typical Polluted Areas of Chang-Zhu-Tan Region

In this study, the concentrations and species of heavy metals in soil profiles of paddy fields in Chang-Zhu-Tan region in Hunan province, China were investigated. The soil profiles represented typical polluted areas in this region affected by manure application, biogas slurry irrigation, lead-zinc mining, and industrial activities. Single factor pollution index and combined pollution index were used to evaluate the pollution status. Based on the speciation characteristics of heavy metals in the soil profiles of known pollution sources, the possible major sources of pollution were identified for the areas where the sources were still unknown. The results show that: (1) In the surface soils (0-10 cm), the average contents of Cd, Pb, Cu, Zn, Cr and Ni were 0.8, 54.5, 43.6, 158.6, 106.2 and 23.3 mg/kg, respectively. Compared with the background values in the Chang-Zhu-Tan region, there was obvious accumulation of Cd, Pb and Zn in the surface soil. The average content of Cd exceeded the risk screening values of Soil Environmental Quality Standards for Agricultural Soils (GB 15618-2018). The accumulation of Cd, Pb, Zn and Cr extended to subsurface soils (10-20 cm), and the average content of Cd, Pb, Zn and Cr was 6.9%, 19.8%, 101.4% and 67.0% higher than the local soil background values, respectively. Accumulation of Zn and Cr reached deeper layers than other metals. In deep soils (80-100 cm), the average content of Zn reached 78.7 and 96.3 mg/kg, which were 30.4% and 48.4% higher than the background value, respectively. (2) The high values of the single factor pollution index of Cd and Pb were mainly found in the surface soils, which indicated mild or slight Cd accumulation, and moderate accumulation in a few cases; Zn showed mainly mild and moderate accumulations, with small amount of heavy accumulation. The combined pollution index showed that the surface soils in the study area were basically moderately or heavily accumulated. (3) There was a significant correlation between the TOC content and the oxidizable Cu and Zn contents in the soil profiles in the manure application and biogas slurry irrigated areas. The contents of exchangeable Pb, Cd and Zn were significantly correlated in the soil profiles near the lead-zinc contaminated areas. However, in industrial polluted areas, only exchangeable Pb and Cd contents showed significant correlation. The current study showed that the differences in pollution sources resulted in different accumulation characteristics of heavy metal and different correlations among fractions of heavy metals in the paddy soils of the Chang-Zhu-Tan area. Thus, the main pollution sources of heavy metals in the agricultural soils could be evaluated through the heavy metal accumulation characteristics, their speciation and correlations. These results provide a theoretical basis for the pollution prevention and control from the sources.