The mobility of most radionuclides from soil to other organisms is predominantly via plant root uptake, which will be largely determined by physicochemical factors influencing the distribution of radionuclides between the solid and solution phases of soil. The uptake of most elements by plant roots occurs mainly from the soil solution.

The important interactions of any chemical species in solution, which can influence its mobility in soils and eventual root uptake, include: charge
interactions; complexation and precipitation reactions with other chemical species (e. g. organic and inorganic ligands); oxidation-reduction (redox) transformations; and specific interactions with soil components including soil biota. Soil factors influencing the mobility of some of these radionuclides are outlined in the following paragraph.

The extent of sorption in soil is described by the solid-liquid distribution coefficient (Kd).

Simple Kd-based models assume that the radionuclide on the solid phase is in equilibrium with that in solution. However, Kd can change with time as the sorption process ‘‘ages’’.

The Kd for a radionuclide may vary within various orders of magnitude depending on the combination of radionuclide and soil type.21 The use of a cofactor approach can decrease the variability of the ranges of Kd values associated with a soil type. For example, Kd is affected by the radiocaesium interception potential (RIP), K and NH4+ status for radiocaesium, the cation exchange capacity (CEC), Ca and Mg concentrations for radiostrontium, and the pH for heavy radionuclides.21

Caesium is strongly sorbed in soil by ion exchange, some of which is irrever­sible, or fixed, with fixation being influenced by clay mineralogy. A number of models relating the availability of radiocaesium to soil properties have been proposed, including increasing soil-plant transfer with increasing soil organic matter;22,23 decreasing soil-plant transfer with increasing soil solution potas­sium;24 a semi-mechanistic approach using soil clay and organic matter contents, exchangeable K status, pH and NH4+ concentration;25 and, more recently, the use of RIP of soils and exchangeable potassium concentration to predict caesium uptake.21 Characterisation of the soils in the areas of Japan which are receiving radionuclide deposition from Fukushima should enable reasonable predictions of the long term availability of radiocaesium to foodstuffs.

Recent comparisons of data21 showed that the Kd of strontium for sand, loam, clay and organic soil groups were similar, although the value for the sand group was significantly lower. For radiostrontium, the key soil characteristics determining sorption were CEC and calcium and magnesium concentrations.

The transuranic radionuclides, americium and plutonium, have relatively low mobility due to their strong tendency to sorb onto soil particles.26 Americium generally exists in the iii and/or iv valence state, whereas plutonium often exists in the iv state, but can be found in any of four oxidation states (iii, iv, v or vi) depending on the redox conditions of the soil system.