Estimation of the amount of anthropogenic uranium in 112 Japanese agricultural soil samples due to application of phosphatic fertilizers

We determined concentrations of total Th and U in 112 Japanese agricultural soil samples (50 paddy field and 62 upland field soil samples) by ICP-MS after microwave digestion. The average concentrations of total Th and U in the paddy field soil samples were 5.7 mg kg-1 and 2.8 mg kg-1, respectively, while those in the upland field soil samples were 5.5 mg kg-1 and 2.6 mg kg-1, respectively. These Th and U concentrations showed no differences between paddy field and upland field soil samples. Concentration ratios of U/Th in paddy field and upland field soils were 0.53 and 0.52 on average, respectively, which were much higher than those in Japanese non-agricultural fields (0.23). The results implied that phosphatic fertilizers, which have high U concentrations, increased the total U concentration in the agricultural fields. Thus, using the natural U/Th ratio in non-agricultural areas, we estimated the excess amount of U. About 52% of total U in paddy field soils (ca. 1.5 mg kg-1 of U on average) and 50% of total U in upland field soils (ca. 1.3 mg kg-1 of U on average) were calculated as excess amounts of U. Thus using of phosphatic fertilizers in agricultural fields makes only a small contribution as external radiation to the general population