2006 Volume 55 Issue 2 Pages 71-78
Concentrations of total U and Th in 82 Japanese agricultural soils (37 paddy field and 45 upland field soil samples) were determined by ICP-MS in order to estimate anthropogenic U contents. The geometric means of total U and Th in the paddy field soils were 2.75 mg kg-1 and 5.56 mg kg-1, respectively, while those in the upland field soils were 2.43 mg kg-1 and 5.17 mg kg-1, respectively. These values were within the range of reported values. The concentrations of Th and U in paddy field soils and upland field soils showed no differences between them, respectively. Concentration ratios of U/Th in paddy field and upland field soils were 0.53 and 0.49 on average, respectively, which were much higher than those in Japanese crust (0.28), non-agricultural fields (0.23) and river sediments (0.20) . These results imply that phosphatic fertilizers, which have high U concentrations, increased the total U concentrations of Japanese agricultural fields. Thus, using these U/Th ratios in non-agricultural areas, it is possible to calculate amounts of excess U due to the application of fertilizers. It was estimated that about 50% of total U in paddy field soils (range: 4-78%) and about 48% of total U in upland field soils (range: 4-74%) were originated from the phosphatic fertilizers.