| Uranium is one of the main natural pollutants that affect the environment because of its radiation and chemical toxicity.Granite is the main carrier of radionuclides such as U and Th in nature for its content of U and Th is much higher than other common rock types.A great many studies have shown that in the process of strong weathering of granite,most parts of the Uranium have been released and migrated from the rock body.Studies abroad have shown that the soil and sediments rich in organic matter have a cumulative enrichment effect on uranium,making the uranium content in organic-rich soil or sediments in wetlands as high as 0.3%-0.4%,which is interpreted as an potential environmental risk,and the source of uranium is believed to be mainly from the weathering of granitic rocks.Nanling area is the center of the big Southern China granite province,and thus has created the world-renowned Nanling W-REE-U polymetallic metallogenic belt,where many high uranium(>9×10-6)granite bodies and associated uranium deposits are distributed,or ion adsorption REE deposits are developed within the large outcrops of the granite bodies,which is represented by Dabu rock mass in Ganxian county,Jiangxi province.Up to now,no special research on the potential environmental risks for large-area exposure of high-uranium granite bodies in Nanling area is reported.Funded by science and technology research project of Jiangxi Education Department“Study on supergene activation of uranium of high-uranium granite and its environmental risk in Nanling area--a case study of Dabu REE deposit in Ganxian county?project no.GJJ160595?”,the paper selects Dabu REE mining area?Jiangwozi ore block?as the research area,through detailed field geological investigation and systematical collection of samples of fresh and weathering granite,alluvial sediments/soil from heap leaching field,surface water and sediments,as well as rice,and analyzes the main components,trace and rare earth elements of the samples,aims to study the rule,mechanism and fate of the mobilized uranium in the weathered high-uranium granite by mass balance research methods of mobile components,and also to evaluate the pollution degree of radionuclides?238U and 232Th?in Dabu REE mining area by single factor index method and geological accumulation index method.And thus,it is of great theoretical and practical significance to reveal the rule and mechanism of uranium release and migration and its environmental risk of extensively exposed high-uranium granite rock mass in Nanling area under natural and artificial?mining?conditions.The main results and conclusions of this paper are as follows:?1?the average U content of the REE-bearing granitic mass in Dabuhu,Ganxian is up to 21.6×10-6,and the Th/U ratio is1.35?n=25?,which suggests it is a typical high uranium rock mass(U>9×10-6,Th/U<3);?2?Relative to the bedrock,the uranium mass change rate of weathered granite samples is averaged-64.37%,suggests that most of the uranium?nearly two-thirds?are released,but there is no evidence to support that U is accumulated in lower soil of the weathering crust nor in the soil/sediments in the valley,so we think most of the released uranium might have leached into the Tao Jiang river by atmospheric precipitation as uranyl complex cation(UO22+);?3?the average content of U and Th in alluvium from the heap leaching site is 11.6×10-6 and 33.1×10-6,respectively,which is significantly higher than that in weathered granite(9.14×10-6 and 26.9×10-6,respectively)and that in soil or sediment without affected by mining(8.21×10-6 and 20.4×10-6,respectively).The results of evaluation by single factor index method and geological accumulation index method also showed that the soil or sediment near the heap leaching site was slightly contaminated by radionuclides.All these indicate that the impact of mining activity of ion-type REE ore on the environment in terms of radioactive pollution is also significant;?4?the average U content of paddy soil is as high as 8.09×10-6,which is about twice as high as the background value of Jiangxi soil,and even significantly higher than that of the paddy soil in unmined areas of a uranium ore deposit in Jiangxi,indicating the significant influence of high-uranium granite on U background of soil.?5?the U content of rice(3.80×10-9-47.0×10-6,averaged 21.4×10-9)is 32 times as high as the U background value of rice in Jiangxi province,and 11 times as high as that of rice in a uranium mining area in Jiangxi province.It might be a warning of food safety problems.However,the number of rice samples is small?5 pieces?,and the analysis results have not been further verified.Therefore,it is suggested to further evaluate the food safety of rice produced around Dabu rock mass. |
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