中国西南典型地质背景区土壤重金属分布及生态风险特征

中国西南有22.3%的耕地重金属含量超标,区内广泛分布的峨眉山玄武岩和碳酸盐岩被认为是土壤中重金属的主要母质来源。目前,中国西南地区,尤其是峨眉山玄武岩分布区土壤重金属生态风险的研究程度仍有待提升,不同地质背景区(成土母岩)土壤中重金属含量、空间分布与生态风险缺乏对比。本文选择四川省典型地质背景区采集土壤样品,采用原子荧光光谱法、电感耦合等离子体质谱/发射光谱等方法测定重金属元素(As、Cd、Cr、Cu、Hg、Ni、Pb、Zn)含量和pH数据,结合地累积指数法和潜在生态风险指数法,研究了重金属元素的含量、空间分布特征和土壤重金属生态风险。结果表明:(1)玄武岩区土壤中Cd、Cr、Cu、Hg、Ni、Zn的含量高于碳酸盐岩区,也高于四川和全国背景值。各元素含量分别为四川背景值的3.25、1.08、5.08、1.72、1.55、1.63倍和全国背景值的2.60、1.40、6.87、1.47、1.87、1.91倍;(2)As、Cr、Pb的高含量区域与碳酸盐岩分布区对应,Cd、Cu、Hg、Ni、Zn的高含量区域与峨眉山玄武岩的空间分布对应;(3)地累积指数表明玄武岩分布区土壤中Cd、Cu、Ni、Zn污染程度高于碳酸盐岩区;(4)研究区内生态危害程度较高的元素为Cd、Cu和Hg;其在玄武岩分布区"强生态危害"及以上的比例比碳酸盐岩区分别高出22.4%、1.15%和26.0%。本研究揭示:(1)研究区内土壤中重金属元素的含量、分布及生态风险与地质背景密切相关;(2)产生这一规律的原因在于母岩中元素含量的差异、成土过程中元素的地球化学行为及元素次生富集等因素综合作用的结果;(3)研究区土壤酸碱度偏低(pH平均值为5.5),需预防土壤进一步酸化引起的重金属活化风险。

Distribution of Heavy Metals and Ecological Risk of Soils in the Typical Geological Background Region of Southwest China

BACKGROUND: 22.3% of the cultivated land in southwest China has excessive heavy metal content. The widely distributed Emeishan basalt and carbonate rocks in the region are considered to be the main source of heavy metals in the soil. At present, the level of research on the ecological risk of soil heavy metals in southwestern China, especially in the Emeishan basalt areas, still needs to be improved. The content, spatial distribution and ecological risk of heavy metals in soils of different geological backgrounds (earthogenic parent rocks) lack comparison.OBJECTIVES: To understand the content and spatial distribution characteristics of heavy metals in the soils from Emeishan basalt and carbonate rock areas.METHODS: Soil samples from the typical geological background areas of Sichuan were collected. AFS, ICP-MS and ICP-OES were used to determine the content of heavy metal elements (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) and pH values.The content and spatial distribution of the heavy metals in the soils, as well as the ecological risks were studied using the accumulation index and potential ecological risk index methods.RESULTS: The results showed that:(1) The content of Cd, Cr, Cu, Hg, Ni, and Zn in the soils of the basalt areas were generally higher than those of the non-basalt areas and the background values of Sichuan and China. The content of the above elements were respectively 3.25,1.08,5.08,1.72,1.55,1.63 times the Sichuan background values and 2.60,1.40,6.87,1.47,1.87,1.91 times background values of China. (2) The high content areas of As, Cr and Pb corresponded well to the carbonate rock areas, whereas the high content areas of Cd, Cu, Hg, Ni and Zn corresponded well to the Emeishan basalt areas. The corresponding relationship indicated that the spatial distribution of heavy metals in the soil was mainly determined by the geological background in the studied area. (3) The Geoaccumulation Index results indicated that the soil pollution degree of Cd, Cu, Ni and Zn in Emeishan basalt areas was obviously higher than those of the non-basalt areas; (4) Ecological hazard index results showed that Cd, Cu and Hg were the highest ecological hazard elements in the studied area. The proportions of "strong ecological hazard", "very strong ecological hazard" and "strong estecological hazard" of Cd, Cu and Hg in the basalt areas were respectively 22.4%, 1.15% and 26.0% higher than those in the non-basalt areas.CONCLUSIONS: The content, distribution and ecological risk of heavy metal elements in the soil in the study area are closely related to the geological background. Reasons include the differences in the element content in the parent rock, the geochemical behavior of the elements during the soil formation, and the secondary enrichment of the elements. The pH of the soil in the study area is low, and it is necessary to prevent the risk of heavy metal activation caused by further acidification of the soil.