Preliminary results of spatial distribution of uranium and thorium in soil profiles near a uranium industrial site, Guangdong province, China

Four soil profiles were collected from locations with different distances (5, 50, 250 and 1000 m) from a uranium mill tailings dam, Guangdong province, China, to investigate the pollution status of the soil in mining/milling-related areas based on the contents of uranium (U) and thorium (Th), thus to understand the impacts of uranium industrial activities to the surroundings. The U and Th concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS) after conventional HF-HNO₃-HClO₄ digestion procedures. The results indicate that the soils within 50 m from uranium tailing were severely contaminated; those in 250 and 1000 m soil samples were observed to be with local background level even though these in 250 m had slightly higher U/Th ratios. Uranium concentrations varied from 5.50 ± 0.27 to 160.55 ± 8.03 mg/kg, with maximum values recorded in an intermediate layer of the 5-m distance soil profile. In comparison, the concentration of Th ranged from 6.02 ± 0.30 to 84.71 ± 4.24 mg/kg, with maximum values observed in the top layer of the 1000-m distance soil profile. The U/Th ratio varied from 0.15 to 11.99 compared with 0.20, 0.22 and 0.26 of the average for Guangdong province, national China and the world, respectively. The mean U/Th of four soil profiles showed a reduction with distance from the uranium mill tailing dam, suggesting the relatively large magnitude of uranium elevation in soils within limited distances.