A recent report on the Sava river in Serbia suggests that many of the river silts contain about 100 Bq kg of natural radioisotopes (Ra, Th, and U). According to the United Nations the normal concentration of uranium in soil ranges between 300 μg kg and 11.7 mg kg. It is well known that some plants, called hyperaccumulators, are able to absorb and concentrate metals within their tissues; iodine was first isolated from seaweed in France, which suggests that seaweed is an iodine hyperaccumulator.
Synthetic radioisotopes also can be detected in silt. Busby quotes a report on the plutonium activity in Welsh intertidal sediments by Garland et al. (1989), which suggests that the closer a site is to Sellafield, the higher is the concentration of plutonium in the silt. Some relationship between distance and activity can be seen in their data, when fitted to an exponential curve, but the scatter of the points is large (R = 0.3683).