Abstract:

There are many areas of high metal and metalloid pollution in soils, especially in agricultural land, where metal pollution is related to the quality and safety of agricultural products and to public health. In addition to human activities such as industrial and agricultural production, high soil metal content derived from geological background sources is also an important factor leading to excessive soil content of potentially toxic elements. Investigating and analyzing of the sources of soil metal pollution and identifying the main pathways of metals in regionally polluted soils are fundamentally important for preventing and controlling soil pollution and achieving the safe utilization of contaminated agricultural soils. However, there is relatively little research on the identification of the high-background geological characteristics of soil metal(loid)s. The soil pollution source analysis method was developed based on atmospheric pollution source analysis and can be summarized as the pollution source emission inventory, diffusion model method, and receptor model methods. Overall, there are differences in the contributions of pollution sources based on the different methods, but generally consistent pollution source information can be obtained using these methods. A major metal pollutant in Chinese soils is cadmium (Cd), along with pollutants such as copper (Cu), lead (Pb), mercury (Hg), and zinc (Zn). These elements are typically derived from industrial and agricultural activities. However, in some areas where human activities contribute less pollution, the high natural geological background sources are the major explanation for the exceedance of certain metals. At present, there are numerous studies on the occurrence, chemical forms, pollution characteristics, and risk assessment of metal elements in natural geological high background soils, and most work is focused on investigating and analyzing surface soils. There has been little investigation into the identification of high-background geological characteristics of soil metals and pollution tracing based on the distribution characteristics of metal elements at different depths of soil profiles. Moreover, from the perspective of soil environmental management and risk control, there is still no unified and quantifiable understanding of the relevant concepts and connotations of soil metals in areas with high geological background that can serve management. Therefore, in the investigation and tracing of the causes of soil metal pollution, if a simple and reliable method can be used to effectively identify whether there are geological high-background characteristics of metals, and further determine whether a soil metal exceeds the national standard due to geological high-background levels, this will help efficiently and accurately assess the sources of soil pollution and prevent and control pollution sources. The sources of metal elements in soil include natural and anthropogenic sources. Natural sources are mainly influenced by the parent material and soil formation processes, which determine the background environmental content of soil metals. Anthropogenic sources are mainly industrial and mining production activities, the use of agricultural inputs, and transportation systems. They enter agricultural soils mainly through atmospheric deposition, sewage irrigation, fertilization, and other pathways and are usually the main source of continuing accumulation and sometimes exceed the statutory limits of metals in soils. Only under the influence of natural sources, such as soil formation, do differences in the contents of various elements in different types of rocks result in differences in the parent materials of weathered soils, leading to further substantial differences in the elemental contents of different soil types. Soils of the same type developed from the same parent rock material may show small differences in the content of the same element at different depths in the soil profile. Thus, the metal content in different soil horizons is usually similar. Under high pollution inputs from human sources, whether through atmospheric deposition, sewage irrigation, fertilization, sludge farming, or other major pathways, metal inputs to agricultural land often initially accumulate in the surface soils. Based on the above assumptions and combined with the screening values of various metal elements in soil environmental quality standards, this study explored a simple and effective identification method for the geological high background characteristics of soil metals from the perspective of environmental risk control. Based on the analysis of the main sources of metals in agricultural soils and their distribution and accumulation characteristics under their influence, a “quadrant+zoning” method based on collaborative investigation and analysis of metal content in surface (0‒20 cm depth) and deep (100‒120 cm depth) soils was proposed. This method can intuitively, simply, and effectively identify the geological high-background characteristics of potentially toxic metals in regional soils and the main factors influencing the exceedance of national standards. The applicability of this method was investigated using the surrounding areas of a mining area in South China, the Pearl River Delta plain, and the western Guangdong basalt area as examples. The results showed that the main elements occurring at excessive levels in the soils around a mining area in northern Guangdong were Cd, Pb, and Zn, which were mainly sourced from external pollution inputs, and in some sites, soil Cd was also affected by high geological background levels. The major elements occurring at excessive levels were Cd, Hg, Pb, As, and Cu, which were present in the soils of the Pearl River Delta. Mercury was influenced mainly by external pollution inputs, whereas Cd was affected mainly by high geological background levels, and external pollution inputs. The major elements present at excessive concentrations were Cr and Ni in the soils of the basalt area in western Guangdong province, both due to the high geological background levels in the local basalt bedrock. This method can effectively and intuitively identify and assess the external inputs of pollutants, geological high-background effects, and their combined impacts. This can provide an important reference for investigating and tracing the sources of metal(loid) pollution in agricultural soils.

Key words: agricultural soils, metal(loid)s, analysis of pollution causes, high geological background levels, environmental risks

摘要：

当前土壤重金属污染问题依然严峻，尤其是农用地土壤重金属污染事关农产品质量安全和人类健康。除工农业生产等人为活动外，重金属地质高背景也是导致土壤重金属超标的重要因素。使用简便、可靠的方法有效识别是否存在重金属地质高背景特征，并进一步确定是否由地质高背景而导致土壤重金属超标，将有助于高效、准确研判土壤污染成因和污染源头防控。该研究从环境风险管控角度，对土壤重金属地质高背景特征的简便、有效识别方法进行了探讨，在分析农用地土壤重金属主要来源及其影响下重金属分布与累积特征的基础上，提出了基于表层（0－20 cm）和深层（100－120 cm）土壤重金属质量分数协同调查分析的“象限+分区”法，可较为直观、简便且有效识别区域土壤重金属地质高背景特征以及超标的主要成因，并以华南地区某矿区周边地区、珠三角平原区和粤西玄武岩地区等3个典型区域为例，开展了该方法的适用性研究。结果表明该方法可有效且直观地识别和研判污染外源输入影响、地质高背景影响以及两者叠加影响等情况，可为农用地土壤重金属污染源成因排查与溯源提供重要参考依据。

关键词: 农田土壤, 重金属, 污染成因分析, 地质高背景, 环境风险