| With the rapid economic development in the Pearl River Delta region,the pollution trend is severe,and heavy metal pollution has attracted attention.In previous studies,the main focus was on pollution caused by mining disturbances,industrial wastewater discharge,polluted irrigation,agricultural fertilization,and atmospheric deposition.With the development of investigation and research,it has been gradually discovered that there are large areas of high abnormal values of heavy metals in the Quaternary sedimentary area of the Pearl River Delta.This is because a large number of carbonate rocks are distributed in the upper reaches of the Pearl River Delta.During the weathering of the carbonate rocks,a large amount of Cd elements are released.A large number of carbonate rocks are distributed in the upper reaches of the Pearl River Delta.During the process of weathering,a large amount of Cd is released.The soil developed from carbonates undergoes leaching of calcium during the process of acidification.With the migration of the river to the precipitation of alluvial deposits,and then the leaching of Cd,the same as the migration of the rivers to the precipitation of alluvial deposits,encountering the alkaline environment formed by the previous calcium deposits,Cd is adsorbed or precipitated,thereby accumulating A high background anomalous area brought about by alluvial parent material is formed.In this region,a large number of field sampling data showed the phenomenon of spatial mismatch of soil and crop pollution,that is,the soil exceeds the standard for crops but does not exceed the standard,or the soil does not exceed the standard for crops but exceeds the standard.my country’s current "Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard "(GB15618-2018)plays a small role in the evaluation and control of local farmland soil pollution,and cannot meet the current needs of soil environmental management.Based on the multi-target geochemical data set data in the Pearl River Delta,this paper explores the distribution rules and distribution factors of soil Cd in the Pearl River Delta.Then,combined with field sampling data in typical regions,geological types,soil properties and regional Cd geochemical anomalies,this paper proposes the management and control rules of Cd in farmland soils in the Pearl River Delta in different geological regions.This research conclusions are as follows:(1)In the Pearl River Delta region,from the perspective of geomorphological types,As,Cd,Cr,Cu,Hg,Ni,Pb,and Zn elements are mainly concentrated in alluvial plains and delta plains;in terms of geological types,they are mainly concentrated in the fourth System area;from the perspective of soil type,it is mainly enriched in de-fluctuated soil and fertile paddy soil regions.In the Pearl River Delta region,the deep surface layer and spatial distribution of Cd have extremely high consistency,indicating that geological factors are the main controlling factors.The areas with high Cd content have obvious distribution characteristics,mainly distributed in the alluvial plains of Beijiang and Xijiang with dense water systems,mainly in Foshan,Zhongshan,Zhuhai and the east of Huizhou.In addition to inheriting the characteristics of deep layers,surface Cd is also affected by human factors in the process of water system,topography and urbanization.(2)Cd element and other heavy metal elements have a positive correlation in spatial accumulation.The high-high accumulation area is mainly distributed in the Quaternary area,and the distribution space is obviously related to the river network,mainly sandwiched between the Xijiang and Beijiang.Between the plains.The low-low aggregation areas are mainly distributed in the granite area and the clastic rock area,while the non-significant areas are often distributed in the junction areas of different geological types,different landform types,and different soil types.The accumulation and dispersion of the eight heavy metals is controlled by the source of the natural background on the one hand,and on the other hand,it is also affected by the adsorption of the elements on the soil and the interaction between the elements.The spatial distribution of Cd,Cu and Zn is highly consistent.Cd is positively correlated with P,S,Mn,Ca O,Fe2O3 and p H in spatial aggregation.Among them,the correlation with P and Mn is strong,and the correlation with S is slightly weak.The soil type is the main factor controlling the distribution of these three elements.On the whole,Cu,Zn,Ca O,and Pb are the main controlling factors affecting the Cd content in the granite and Quaternary areas of the Pearl River Delta.The influence of these factors on the Quaternary area,the granite area,and the clastic rock area decreases sequentially,and the effects of each factor are not obvious in the clastic rock area.(3)Among the 177 soil-rice pairs collected in the quaternary area of the Pearl River Delta,6.78% of the rice Cd exceeded the standard and the soil Cd did not exceed the standard.In the quaternary area,attention should be paid to such pollution and in-depth research and investigation,and identify the influencing factors of rice enrichment,so as to better manage rice edible safety.After calculating by the species sensitivity distribution method(SSD)and the relationship between soil Cd content and rice Cd content,it is recommended to use 0.3 mg/kg as the soil risk screening value in the quaternary system when the p H is less than or equal to 6.5.Put forward different management and control suggestions for different geological types of the Pearl River Delta.In the Quaternary rock area,the soil safety threshold calculated by the field sample point data and suitable for the Quaternary area is used as the soil Cd pollution risk screening value for risk management and control.Classified management,strictly prevent soil acidification.In the granite and clastic rock areas,the "Soil Environmental Quality and Agricultural Land Soil Pollution Risk Control Standards"(GB 15618-2018)will continue to be used. |
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