The PH should be measured. Study on the present situation and distribution characteristics of cadmium pollution in sewage irrigated farmland soil. College of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing10083; 2. The total cadmium content of heavy metal in farmland soil irrigated by sewage in towns along the river in Shenyang suburb was analyzed, the cadmium pollution in soil was evaluated, and the distribution characteristics of cadmium along the river, horizontally and vertically were discussed. The results showed that the cadmium content in farmland soil ranged from 0115 ~ 8123 mg kg, with an average value of1175 mg kg. The total cadmium content in soil is evaluated by the second standard value of soil environmental quality standard, and the average cadmium pollution index is 5 195, which is serious pollution. According to the standard of soil background value, the average cadmium pollution index is 5 195, which is 8 139 times higher than the local background value level. Sewage irrigation caused heavy metal cadmium pollution in this area, and the pollution degree was very serious. From upstream to downstream, the cadmium content in coastal soil gradually decreased. In the horizontal distribution, the farther away from the river, the cadmium content tends to decrease gradually; Date of acceptance: 20062 12229 About the author: Wang Yun (1982-), female, from Jingzhou, Hubei, with a master's degree. Sewage irrigation has been widely used in China, especially in the water-deficient areas in the north, where the content of nitrogen and phosphorus in industrial and urban sewage is high. In six sampling points, the cadmium content in the subsoil of 40 ~ 60 cm is higher than the background value of soil cadmium in this area (0 1 19mgP Wang Yun et al.: Study on the present situation and distribution characteristics of cadmium pollution in sewage irrigated farmland 7 3? 0? 8 1994-2008 China Academic Journals Electronic Publishing House. All rights reserved. Ki.net kg), which shows that under the condition of long-term sewage irrigation, the continuous infiltration of sewage irrigation not only caused the pollution of the surface soil in this irrigation area, but also the pollution has developed vertically, which should be paid attention to. Fig. 2 Total Cadmium Content in Soils of Different Profiles along the River 3 Conclusion (1) The analysis results of total cadmium content in topsoil of farmland in some towns along the river in Shenyang suburbs show that the range of cadmium content in the sewage irrigation area is 0115 ~ 8123 mg pkg, with an average value of1/kloc-. The total cadmium content in soil is evaluated by the second standard value of soil environmental quality standard, and the average cadmium pollution index is 5 195, which is a serious pollution. Based on the local soil background value, the average cadmium pollution index is 9 139, which is 8 139 times higher than the local background value level. Sewage irrigation has caused the cumulative pollution of cadmium in the surface soil, and the pollution degree is serious. (2) Distribution characteristics of soil pollution: From upstream to downstream, the cadmium content in soil shows a downward trend; In the horizontal distribution, the farther away from the river, the cadmium content in the soil tends to decrease gradually; In vertical distribution, cadmium pollution is mainly concentrated in the surface layer and has migrated downward, but the migration speed is slow. The discussion is that this river receives urban sewage every winter and spring for half a year. After the urban sewage enters the river, some main pollutants remain and deposit, and the water quality is seriously polluted, which makes it lose the function of natural water body and become the urban sewage channel. At the beginning of irrigation period, external water is introduced, and under the impact of rapids, the sediment in the river rises. At this time, dozens of pumping stations along the coast pumped water to irrigate the fields, and sewage flowed into the fields to be discharged. With the decline of suspended solids and the adsorption of organic and inorganic colloids on the soil surface, a large number of heavy metals are deposited on the surface layer of 0 ~ 20 cm. The retention rate of heavy metal cadmium is 85% ~ 95%, which has a great accumulation in soil. After years of sewage irrigation, a considerable amount of cadmium will accumulate in the bottom and surface soil of irrigation canals, and once it is soaked in acidic water, it is easy to release cadmium ions. Cadmium is a heavy metal in soil, which mainly exists in acid-soluble state and exchangeable state, with high activity and easy to be absorbed by crops. 80% of cadmium in soil combines with the dominant inorganic ligand Cl- to form soluble CdCl2 and CdCl- ions, which are easily absorbed by crops such as rice, resulting in the serious cadmium content in rice exceeding the standard. In the study of the vertical distribution of soil cadmium pollution, it was also found that the cadmium content in the lower soil was higher than that in the upper soil in 2 #, 3 #, 4 # and 5 # profile layers. This shows that there may be the possibility that cadmium pollution in groundwater may lead to the increase of cadmium content in lower soil on some profiles, but this phenomenon needs further confirmation. According to the results of this study, although sewage irrigation has stopped at present, the pollution degree of heavy metal cadmium in coastal soil is still very serious. It is very difficult to repair cadmium pollution in soil in a short time. Crops and groundwater in this area will be polluted, which seriously threatens people's health in this area, so the harm of sewage irrigation still exists. References: [1] Chen Zhujun, Zhou Jianbin. Application of sewage irrigation in Israeli agriculture [J]. Agricultural environmental protection, 200 1, 20 (6): 462 ~ 464. [2] Liuli. Analysis of the influence of sewage irrigation on rice crops in Xiaoling River [J]. Urban and rural environment in Liaoning Province 19 (1): 43 ~ 46. [3] Huang Zheng, Wangjialing. Mutagenicity detection and GC-MS analysis of urban sewage irrigation wastewater [J]. Journal of Huazhong University of Science and Technology (Medical Edition), 1992, (2 1): 25 ~ 26 1 [4] Wu. Study on background values of soil elements in Liaoning Province [M]. Beijing: China Environmental Science Press, 1994. Wu, Zhang Xuexun. Ecological study on cadmium pollution in Zhangshi irrigation area of Shenyang [J] 9 (1): 2 1 ~ 26. [6] Chen Huayong, Ouyang Jianping, Ma Zhendong. Present situation of cadmium pollution in farmland near Daye Nonferrous Smelter and its control countermeasures [J]. Soil, 2003, (1): 76 ~ 82.74 China Environmental Monitoring No.23.