Distribution Characteristics And Sources Of Cd In Topsoil Of Xiaojiang River Valley

Cd pollution in the Xiaojiang River Valley is a prominent environmental problem in the upper reaches of the Yangtze River.However,the source of Cd pollution in the academic community is not clear at present.Some scholars speculate that it may be related to copper mining in the basin,but some scholars have found that Cd pollution is common in the middle and lower reaches of the Jinsha River including the Xiaojiang River Basin.Obviously,only mining activities can not give a reasonable explanation.In view of this dilemma,this paper starts with the spatial distribution investigation of Cd element in the river valley,combined with the main pollutants of copper mining（copper ore and tailings）and the Cd content analysis of local chemical fertilizers to carry out a comprehensive study of Cd pollution sources in Xiaojiang River Valley.The main understandings are as follows:（1）Cd pollution in topsoil is serious in the region.The content of Cd in floodplain surface soil in Xiaojiang River Valley ranged from 0.32 mg/kg to 5.92mg/kg,with an average value of 2.58mg/kg,which was 1.80 times of Yunnan soil background value and 26.33 times of China soil background value.The content of Cd in cultivated land ranged from 1.33 mg/kg to 11.37 mg/kg,with an average of 4.98 mg/kg,which was3.48 times higher than the background value of soil in Yunnan,and was much higher than the background value of soil in China.The average content of Cd in cultivated land was higher than that in floodplain.Geological cumulative index method and secondary phase and primary phase distribution ratio method（RSP）evaluation results showed that the surface soil Cd pollution in Xiaojiang River Valley was more serious,and the risk assessment coding method（RAC）evaluation results showed that the surface soil Cd in Xiaojiang River Valley was mainly high risk.（2）From the vertical distribution of the copper mining point to the downstream,the Cd content in the floodplain of Xiaojiang River Valley did not show a decreasing trend with the increase of the distance from the mining point,which did not conform to the migration and diffusion mode of Cd pollutants from the copper mining point to the downstream.On the contrary,the Cd content of cultivated land in the same river section is significantly higher than that in the beach below it,which was in line with the agricultural pollution model.（3）The Cd content of copper ore and tailings samples from mining sites and tailings ponds was generally low,which was lower than the background value of soil in Yunnan（1.43 mg/kg）.The content of Cd in local fertilizer was extremely high（especially phosphorus fertilizer）,which was tens of times higher than the background value of Yunnan soil.This indicates that copper ore and tailings can not constitute the main pollution source of Cd concentration conditions,and local fertilizer,especially high content of Cd phosphate fertilizer is more likely to become the main pollution source of Cd in Xiaojiang River Valley.（4）The Cu element was used as the indicator element of copper mining activities,and the available phosphorus（AP）was used as the indicator element of agricultural activities.Through the element test analysis,it was found that the correlation between Cd and AP in the topsoil of the Xiaojiang River Valley was much greater than that of Cu,indicating that agricultural activities contributed much to Cd pollution than copper mining.（5）The occurrence forms of Cd in topsoil in this region were mainly weak acid extraction state and oxidation extraction state,while the proportion of residual state was low.This shows that the surface soil Cd pollution in the region was less affected by copper mining,and was greatly affected by agricultural activities.In summary,Cd pollution in topsoil of Xiaojiang River Valley was mainly caused by the application of large amounts of phosphorus fertilizer.This study not only found an important clue for the diagnosis of Cd pollution sources in Xiaojiang River Valley,but also emphasized the multi-source pollution of heavy metals around industrial and mining enterprises.In the diagnosis of heavy metal pollution sources,we should not only focus on industrial and mining enterprises but also ignore other pollution sources.