中文摘要：

包头白云鄂博矿区进行铁矿、稀土矿开采已近60a，稀土矿中伴生的232Th未利用而暴露于环境中，可能引起一定的环境风险，因此对该矿区周边表层土壤中w（232Th）及其分布特征进行采样监测．结果表明：土壤中W（232Th）为3．43—59．09mg／kg，平均值为12．79mg／kg，高于世界平均值（7．50mg／kg）和我国全国平均值（9．88mg／kg），说明包头白云鄂博矿区周边土壤中232Th的分布受到了矿物开采活动的影响；其主要来源是矿区东、西两侧的2个尾矿堆，矿区周边表层土壤中232Th的分布主要受风力作用和人为活动等因素的影响．土壤剖面0—20cm区间W（232Th）平均值与五分法土壤表层中的W（232Th）接近，矿区南部（16号采样点）和北部（12号采样点）土壤中232Th的分布未受白云鄂博矿区的影响，其剖面上W（232Th）变化不明显，接近全国平均值；矿区东南部（17号采样点）底层土壤W（232Th）较低，表层土壤W（232Th）较高，并且在10．5～17．5cm区间出现w（232Th）剧增的趋势，W（232Th）由14．70mg／kg升至19．54mg／kg，表明土壤中232Th的分布明显受白云鄂博矿区的影响．

英文摘要：

The Bayan Obo deposit, situated near Baotou, Inner Mongolia, is the largest repository of rare earth resources in the world. Bayan Obo mining district has been mined nearly 60 years for iron and rare earth elements. Thorium and rare earth elements associated with minerals, were unused and heaped in two tailings which may have an adverse effect on human and animal in the environment due to their exposures. The distribution of thorium in the soil was investigated around Bayan Obo iron mining zone. Twenty-three soil samples were collected in the vicinity of Bayan Obo mining area. After digested by HNO3 , HC104 and HF mixture in Teflon beaker, the isotope of 232Th in samples was determined using Inductively Coupled Plasma Mass Spectrometry. The w（232Th） in soil samples ranged from 3.43 to 59.09 mg/kg with average of 12.79 mg/kg, which was apparently higher than the average values of the world （7.50 mg/kg） and of China （9.88 mg/kg）. This indicated that the mining of the Bayan Obo, especially the two tailings may contribute to the level of thorium. Thevariations of 232Th were not obvious in vertical profiles of soil core 16 and 12, further suggesting that soils near the sampling sites of 16 and 12 were not affected by mining activities. In comparison, the distribution of 232Th in the soil profile around 17 sampling sites may be affected by mining activities in Bayan Obo. The contents of w（232Th） were low in the lower layer and high in the upper layer of the soil profile. Moreover, the w （232 Th ） steeply increased in the 10. 5-17.5 cm interval.