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3.4.6 Biological samples

Some species of flora and fauna have the ability to concentrate naturally occurring or artificial radionuclides. Iodine, for example, is known to concentrate in certain algae and shellfish, while caesium can exhibit an enhanced uptake in plants like lichens, heather, fir and spruce, as well as mushrooms. It should be noted that in general, radionuclides have stable sister isotopes which are common in nature and are taken up to varying degrees by biota. Natural processes of plant or animal uptake have evolved which ought not to be affected by the nuclear properties of the element. This results in a broad and mainly still uninvestigated field of promising use as bio-indicators and, moreover, for bio-remediation.

Some bio-indicators have been identified, as shown in Table 3.29, which does not claim to be exhaustive.

Bio-indicator Radionuclide
Algae, shellfish, peat deposits Iodine
Heather Caesium
Snail shell, fish bone Strontium
Mushrooms, fir, spruce Caesium
Mycorhiza plants Caesium
Thyme Caesium in Mediterranean regions
Lichen Caesium in boreal ecosystems
Honey Caesium
Milk Caesium, strontium and iodine
Seaweed Ruthenium, technicium
Sheep droppings Caesium

Table 3.29 Example bio-indicators for some key radioelements

Due to the fact that most risk- and dose-based regulations are concerned with potential future land use that may differ from the current land use, vegetation samples are unsuitable for demonstrating compliance with regulations. There is a relationship between radionuclide concentrations in plants and those in soil (the soil-to-plant transfer factor is used in many models to develop DCGLs) and the plant concentration could be used as a surrogate measurement of the soil concentration. In most cases, a measurement of the soil itself as the parameter of interest is more appropriate and introduces less uncertainty in the result.

In some situations the vegetative cover is not considered part of the surface soil sample and is removed in the field. For agricultural scenarios where external exposure is not the primary concern, soil particles greater than 2 mm (0.08 in.) are sometimes not considered as part of the sample. Foreign material (e.g., plant roots, glass, metal, or concrete) is also then not considered part of the sample, but should be reviewed on a site-specific basis. It is important that the sample collection procedure clearly indicate what is and what is not considered part of the sample.